From Wikiversity
Jump to: navigation, search
This ultraviolet-wavelength image mosaic, taken by NASA's Galaxy Evolution Explorer (GALEX), shows a comet-like "tail" stretching 13 light years across space. Credit: NASA.

A nomy (Latin nomia) is a "system of laws governing or [the] sum of knowledge regarding a (specified) field."[1] Nomology is the "science of physical and logical laws."[1]

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night,[1] succeeds even in its smallest measurement, astronomy is the name of the effort and the result.

Many of the terms and concepts of astronomy have been introduced to most students at or by a secondary level. Astronomical theory and fundamentals are worked on during courses at the university undergraduate level. Ultimately, this learning resource engages the student with the state of the art. Enjoy the challenge!

To serve both as an introduction to the field of astronomy and to connect together observational and theoretical astronomy, it is a lecture and an article.

Students interested in a course setting may enjoy introduction to astronomy or principles of radiation astronomy of which this lecture/article is a part.

Sensing a body in the sky[edit]

This is a visual image of the Sun with some sunspots visible. The two small spots in the middle have about the same diameter as our planet Earth. Credit: NASA.

Each of the many senses possessed by intelligent life forms on Earth adds information about the objects or entities being sensed and concluded about that

  1. transit at an apparent constant rate,
  2. continue occupation of a position for longer periods, or
  3. apparently change course or position in the sky.


This is an aerial view of the Barringer Meteor Crater about 69 km east of Flagstaff, Arizona. Credit:D. Roddy, U.S. Geological Survey.
This image shows a late-summer rainstorm in Denmark. The nearly black color of the cloud's base indicates the foreground cloud is probably cumulonimbus. Credit: Malene Thyssen.
Here at low tide in Sandwich Bay birds are gathering. Credit: Nick Smith.
These images show the libration of the Moon over a single lunar month. Credit: Tomruen.
This is a panorama photograph taken during a lightning storm over Bucharest, Romania. Credit: Catalin.Fatu.

Most people associate astronomy with the sense of seeing, what can be termed visual astronomy. Like other areas within astronomy, contributions come from amateurs, unpaid, and professionals, paid. Usually amateur astronomers build visual telescopes to study and enjoy the objects or entities in the night or daytime sky.

While it seldom seems that listening to an entity or object in the sky is beneficial, application of the science of acoustics to studying the Sun provides insight into the Sun's interior.

Occasionally, objects fall from the sky. When and where this occurs, depending on the energy dumped into the atmosphere and the impact on the crust of the Earth, life forms nearby hear it, feel the vibrations from it, and recoil if the intensity is too high.

Some birds celebrate the sunrise and sunset in song.

Even with no other sense than touch, the passage of the Sun overhead is detectable. How?

Smell presents a greater challenge, especially if it's the only sense you have. Think about it. Can you figure out a way with a keen sense of smell, some intelligent being may deduce the passage of the solar terminator on the Earth's surface? And, that the Sun is overhead or travels overhead. Hint: it's sometimes edible.

Some creatures hunt and eat at night, while others do the same during the day.

In addition to the Sun, the Moon effects life forms on Earth such as those along the shores of bodies of water through the production of tides.

Intelligent biological life forms performing astronomy with unaided senses may be conducting 'bioastronomy', or an intelligent life form using a technologically enhanced sensor system either on the surface of a planet or on board a probe in space searching for habitable environments, extremophiles, or evidence of prebiotic chemicals may be conducting astrobiology. Usually, both terms 'bioastronomy' and 'astrobiology' refer to the latter.

Natural objects in the sky[edit]

The Aurora Borealis, or Northern Lights, shines above Bear Lake, Alaska. Credit: Senior Airman Joshua Strang, United States Air Force.
Mount Redoubt in Alaska erupted on April 21, 1990. The mushroom-shaped plume rose from avalanches of hot debris that cascaded down the north flank. Credit: R. Clucas, USGS.

There are many other natural objects, entities, bodies, or phenomena that occur in the sky. Some of these may occur frequently: the Sun passes overhead every day, so does the Moon either during the day or at night, a variety of clouds pass across the sky and sometimes completely fill the sky for days, occasionally a few go in the opposite direction across the sky or in different directions.

Usually when clouds fill the sky and associated with some of these clouds is lightning, a phenomenon that moves so quickly it’s difficult to think of it as an object or entity with a body.

Slower moving phenomena are often thought of as objects that appear to be falling or at least passing by.

Depending on what natural events are occurring in your neighborhood, some objects are being thrown into the sky: dust clouds, ash clouds, rocks, and bombs.

Objects falling from the sky[edit]

Volcanic bombs are thrown into the sky and travel some distance before returning to the ground. This bomb is in the Craters of the Moon National Monument and Preserve, Idaho, USA. Credit: National Park Service.
The Williamette Meteorite is on display at the American Museum of Natural History in New York City. Credit: Dante Alighieri.

Common bodies falling from the sky are rain drops, snow flakes, and hail. Less often meteors which are stones and chunks of metal.

Depending on your location, it doesn’t snow every day, or rain, or fog over, but in many locations snow can occur for months intermittently to be followed by some rainy days; then to be followed by days of snow falling again. If you or your local farmers plant seeds shortly after the snowfalls turn to rainfalls and harvest the grain or other crops before the snowfalls return, you are aware of the seasons.

All of the objects, entities, phenomena, or bodies sensed in the sky portrayed or described so far, especially when they occur near your position, or others, are more noticeable, dramatic, and attention-getting than many of the other astronomical occurrences that occur at night.

With the possible exception of predicting when spring is coming and winter is coming, there is no reason to care about the sky unless things come from it unexpectedly that cause harm, or you are curious. Do you agree?

Sources manifesting in the sky[edit]

Images of a variety of radiation sources manifesting in the sky are displayed in this lecture. See if you can find them:

  1. a source reflecting orange radiation,
  2. a source emitting green radiation,
  3. a source emitting infrared radiation,
  4. a source emitting X-radiation,
  5. one source and object reflecting ultraviolet radiation,
  6. seven sources and five objects emitting visual radiation,
  7. seven sources and objects reflecting visual radiation, and
  8. two sources and objects emitting ultraviolet radiation.

Entities controlling the sky[edit]

While it is arguable that any entities are controlling the sky, entities may be assigned as a first approximation in the theory of cause and effect. Three comet-like objects occur in lecture images. An entity that produces comet-like objects may exist. The Sun emits visual radiation that may reflect off a comet's tail. The coronal cloud in close proximity to the Sun also emits X-rays that produce visual fluorescence from gases in a comet's coma and tail.

The Sun moves across the sky during the day time only. An entity or two may be responsible for this.

A storm cloud blocks the day time sky while releasing rain. Perhaps a different entity may be assigned for each of these effects, or one for all. Many people like gravity as an entity. Is gravity the entity or force behind the falling of the rain or the releasing of the rain?

The Moon also crosses the sky occasionally sometimes in the daylight other times at night. The Moon doesn't always reflect uniformly during its travels. A shadow often blocks some of the reflection. Which entity is the cause for this, or is it an object, or perhaps a source of shadow? Is there a shadow radiation?

Although the entity Thor (also called Jupiter in other cultures) is an entity assigned to throwing lightning bolts, is Thor throwing the lightning bolts over Bucharest, Romania, in the image in this lecture? Is gravity causing the lightning to fall from the clouds? Is the entity of spacetime responsible for the lightning?

In the Mount Redoubt volcanic eruption in Alaska, is the entity of electromagnetism hanging the cloud of ash over the volcano?

Theoretical astronomy tries to assess which entities may be responsible for controlling the sky away from a pleasant fair weather day. And, in turn seeks to explore those that may allow us to control the sky for a few more fair weather days.


Main source: Astrochemistry
Comet West is photographed on March 6, 2006. Credit: Jlsmicro.

In the efforts to understand our own origins and the origin of life on Earth, comes the necessity of exploring the sky and the [outer space] space beyond. To our surprise chemicals occur in the sky above, fall from the sky, and occur in outer space. Astrochemistry uses laboratory chemistry to understand some of the spectroscopy obtained through the use of telescopes.


This is a graph of the global mean atmospheric water vapor superimposed on an outline of the Earth. Credit: NASA.

Being outside in the day light to look upward when the Sun is off to the East or West, you may see that the [diffuse sky radiation] sky is blue depending on the weather. If the wind is blowing and the temperature is above freezing, you may be buffeted by an invisible force. The atmosphere of Earth, assuming that's the planet's solid surface you're bipedally standing on, is composed of small particles called molecules. While the wind may be physically assaulting you, it's actually these molecules, lots of them, that are contacting your skin. The main reason you're holding your shape is that your internal fluids and solids are pushing back.

These molecules in many instances are in turn made up of atoms of chemical elements. At your geographical location, specified in [geographic coordinate system] latitude and longitude, this gaseous envelope extends upward. The atmosphere of Earth changes with altitude. At high enough altitude the composition changes significantly, as does the temperature and pressure. If you've packed the appropriate instruments on board for the trip upward, you may have noticed something else has changed.

Interplanetary medium[edit]

The Zodiacal Light is over the Faulkes Telescope, Haleakala, Maui. Credit: 808caver.

Chemical ions above the Earth's atmosphere, moving at very high speeds and at concentrations up to 100 particles per cm3 (centimeter cubed, a unit of volume) constitute the interplanetary medium.

The Zodiacal light is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the Sun along the ecliptic or zodiac.[2] It is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders it invisible. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for major part[3] of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the gegenschein. The dust forms a thick pancake-shaped cloud in the Solar System collectively known as the zodiacal cloud, which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 micrometres in diameter, with most mass around 150 micrometres.[4]


Main sources: Plasmas/Ions and Ions
The International Space Station is featured in this image photographed by an STS-134 crew member on the space shuttle Endeavour after the station and shuttle began their post-undocking relative separation. Credit: NASA.

Between the surface and various altitudes there is an electric field. It changes with altitude from about 150 volts per meter to lower values at higher altitude. In fair weather, it is relatively constant, in turbulent weather it is accompanied by ions. At greater altitude these chemical species continue to increase in concentration. To continue upward you may need protective clothing and appropriate breathing apparati. The air pressure is lowering as is the ambient temperature.

Upon reaching the top of the mesosphere, the temperature starts to rise, but air pressure continues to fall. This is the beginning of the ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer. The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.

At these altitudes and somewhat hostile environmental conditions, you may prefer to check into the International Space Station for a bit of food and comfort. Is the International Space Station made of chemicals?

From the ground below, or with spectrometers on platforms at higher altitude, including satellites, ion species and concentrations are measured. Into the exosphere or outer space, temperature rises from around 1,500°C (centigrade) to upwards of 100,000 K (kelvin).


Main sources: Chemicals/Materials and Materials
This image is a photo of the Nimrud lens in the British museum.
This is an image of a biconvex lens. Credit: Tamasflex.

Efforts to magnify objects in the sky probably began with the use of crystal lenses. "Lens-shaped crystals have long been known from Bronze Age contexts"[5]. These are "usually recognized as short-focus magnifying lenses."[5]

The slightly oval lens [40 x 35 mm] has been roughly ground and has a focal point about 110 millimetres (4.5 in) from the flat side.[6][7]

Today, lenses are typically made of glass or transparent plastic. This glass is usually about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals and hence does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries and can follow surface tension, which imposes a microscopically smooth surface.


Main sources: Rocks/Meteorites and Meteorites
This image is a cross-section of the Laguna Manantiales meteorite showing Widmanstätten patterns. Credit: Aram Dulyan.

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. ... Most meteorites derive from small astronomical objects called meteoroids ... When a meteoroid enters the atmosphere, frictional, pressure, and chemical interactions with the atmospheric gasses cause the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star.

Meteorites have been found on the Moon[8][9] and Mars.[10]

Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite and taenite called plessite can be found.


This is the dome of the Zeiss telescope at Merate Astronomical Observatory, Merate (LC), Italy. Credit: CAV.

The domes of observatories, such as in the image at right, and the objects inside used to observe and control these observatories are made of chemicals.

For intelligent life forms to survive in conditions above the Earth's atmosphere or even within that atmosphere, often requires protection. This protective gear or mechanisms are made of chemicals.


The relative absorption of an infrared laser. In the red line's profile you can see the hyperfine-structure of the first excited level of rubidium. Credit: Clemens Adolphs.

Spectroscopy is the study of the interaction between matter and radiated energy.[11][12] The concept comprises any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is often represented by a spectrum, a plot of the response of interest as a function of wavelength or frequency. Spectrometry and spectrography are terms used to refer to the measurement of radiation intensity as a function of wavelength and are often used to describe experimental spectroscopic methods. Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers.


Main sources: Locations/Geography and Geography
This image shows the pyramids of Giza. Credit: [Liberato].

Each individual or small group of astronomical knowledge recorders among the hominids after some effort has realized that they are on the surface of a planet, a spheroidal object, further, when any of them study the sky, they are only sensing a small portion relative to their location (such as backyard astronomy). To combine what they've learned, when they get together to discuss it or record it, they have to locate that knowledge where that knowledge occurred.

Locations on Earth[edit]

Stonehenge is a Neolithic monument that may have functioned as a celestial observatory.[13] Credit: .
National Observatory of Athens, Greece, is on top of the Nymphs' Hill.

Astronomy is performed all around the surface of the Earth by location. Astronomy on Earth is subject to local geography. Astronomical observatories on Earth occur at specific locations.

The shapes and sizes, as well as functions, of particular observatories have changed over time, as have their altitude.

To locate themselves on the surface of the Earth, various grid systems are devised using special points of reference. Each local center of civilization that realized the need for locating itself produced a system. One such system, the geographic coordinate system has based its locational grid on a line passing near the Royal Observatory, Greenwich (near London in the United Kingdom UK). Another on French Institut Géographique National (IGN) maps still uses a longitude meridian passing through Paris, along with longitude from Greenwich.


The Canada-France-Hawaii Telescope is located at the Mauna Kea Observatory in Hawai'i. Credit: Fabian_RRRR.

"The Canada-France-Hawaii Telescope (CFHT) is a 3.6 m optical-infrared telescope located on the summit of Mauna Kea on the island of Hawaii."[14] The Canada-France-Hawaii Telescope: the CFHT is "at an altitude of 4,204 meters". "Mauna Kea last erupted 4,000 to 6,000 years ago [~7,000 b2k].

The Mauna Kea Observatories are used for scientific research across the electromagnetic spectrum from visible light to radio, and comprise the largest such facility in the world.

Extreme locations[edit]

Four antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) gaze up at the star-filled night sky. Credit: ESO/José Francisco Salgado (
Lunokhod 2 carries aboard an X-ray telescope for observing solar X-rays from the lunar surface. Credit: NASA.
The large white arrow indicates Luna 21. The smaller white arrows indicate the rovers tracks. The black arrow indicates the crater where it picked up its fatal load of lunar dust. The grey dot lower left of the crater is thought to be the rover itself. Credit: NASA.

The Atacama Large Millimeter/submillimeter Array (ALMA) is being constructed at an altitude of 5000 m on the Chajnantor plateau in the Atacama Desert of Chile. This is one of the driest places on Earth and this dryness, combined with the thin atmosphere at high altitude, offers superb conditions for observing the Universe at millimetre and submillimetre wavelengths. At these long wavelengths, astronomers can probe, for example, molecular clouds, which are dense regions of gas and dust where new stars are born when a cloud collapses under its own gravity. Currently, the Universe remains relatively unexplored at submillimetre wavelengths, so astronomers expect to uncover many new secrets about star formation, as well as the origins of galaxies and planets.

ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The project is scheduled to be fully operational by the end of 2012. The array has been fully operational since March 2013.

At times the location of the observatory is even more dramatically different as in the situation with Lunokhod-2 which is an X-ray observatory (X-ray telescope), carried to the Moon by Luna 21 to observe solar X-rays. Luna 21 landed on the Moon on January 15, 1973, at 22:35:00 UTC, latitude 25°51' N, longitude 30°27' E. Less than 3 hr later Lunokhod 2 disembarked onto the lunar surface at 01:14 UTC on January 16, 1973. While still near the Luna 21 platform Lunokhod 2 carried out solar X-ray studies.[15]


Main sources: History/Prehistory and Prehistory
This photograph is of an astrolabe quadrant from England of 1388. Credit: PHGCOM.

The prehistory period dates from around 7 x 106 b2k to about 7,000 b2k.

The Hominidae have apparently been on Earth for around seven million years, at least somewhere in Africa and possibly elsewhere. Fortunately and deliberately, many of these have worked out ways to record knowledge about the objects or entities in the sky. Astronomy as a science has a long history.

Ancient history[edit]

This is a small section of the glyphs carved into La Mojarra Stela 1 with the Mayan Long Count calender on the left. Credit: Madman2001.

The ancient history period dates from around 8,000 to 3,000 b2k.

"Sumerian astronomers were the first to keep written records of what they learned about the heavens."[16] A form of writing known as cuneiform emerged among the Sumerians around 3500–3000 BC [5500-5000 b2k].

The modern scientific discipline of astronomy focuses on reproducibility and physical theory to explain and describe observations. Many ancient observers produced remarkably reproducible calendars such as the Mesoamerican Long Count calendar which apparently goes from August 11, 3114 [Before the Common Era] BCE, to October 13, 4772, and beyond.

Historians consider, that "[the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."[17] Star names later categorized in the twenty-eight mansions have been found on oracle bones unearthed at Anyang, dating back to the middle Shang Dynasty (Chinese Bronze Age), and the mansion (xiù:宿) system's nucleus seems to have taken shape by the time of the ruler Wu Ding (1339-1281 BC, [3339-3281 b2k]).[18]

It is believed that the first pretelescopic astronomers were the Chinese due to conclusive evidence such as the Gan Shi Xing Jing (the oldest recorded star catalog which was produced during the 5th century BCE).

Early history[edit]

Main sources: History/Early and Early history
This is an image of the Dunhuang map from the Tang Dynasty of the North Polar region. Constellations of the three schools are distinguished with different colors: white, black and yellow for stars of Wu Xian, Gan De and Shi Shen respectively. Credit: Laurascudder, from: Brian J. Ford (1993). Images of Science: A History of Scientific Illustration, Oxford University Press. ISBN 0195209834.

The early history period dates from around 3,000 to 2,000 b2k.

"About 280 B.C. [2280 b2k], ... Aristarchus of Samos proposed the hypothesis that the Sun is at rest, while the Earth and the planets rotate about the Sun."[19] "Aristarchus also figured out how to measure the distances to the Sun and the Moon and their sizes."[20]

The celestial sphere may have been produced very early: from the Wikipedia article on Chinese astronomy: According to records, the first celestial globe was made by Geng Shou-chang (耿壽昌) between 70 BC and 50 BC. In the Ming Dynasty, the celestial globe at that time was a huge globe, showing the 28 mansions, celestial equator and ecliptic. None of them have survived.

The Dunhuang map from the Tang Dynasty of the North Polar region at right is thought to date from the reign of Emperor Zhongzong of Tang (705–710). Constellations of the three schools are distinguished with different colors: white, black and yellow for stars of Wu Xian, Gan De and Shi Shen respectively. The whole set of star maps contains 1,300 stars.

The divisions of history for astronomy are sincerely based on the belief that the trained astronomers of today are different from the observers of antiquity. One fundamental differentiator is technology.

A big boon to visual astronomy is the telescope. " first known practical telescopes were invented in the Netherlands at the beginning of the 1600s (the 17th century), using glass lenses. And, on this instrument of astronomy, there is this: "There are indeed ancient tablets that mention astronomers' lenses supported by a golden tube to enlarge the pupil, and in Nineveh a rock crystal [Nimrud lens] lens was found (Pettinato 1998). Maybe one day a new archaeological excavation will find a Babylonian telescope for the first time."[21]


Main source: Archeoastronomy
The rising Sun illuminates the inner chamber of Newgrange, Ireland, only at the winter solstice. Credit: Richard Gallagher.

Today, astronomical history is usually divided into three parts: archaeoastronomy, historical astronomy, and the history of astronomy. These divisions are sincerely based on the belief that the trained astronomers of today are different from the observers of antiquity.

Archaeoastronomy (also spelled archeoastronomy) is the study of how people in the past "have understood the phenomena in the sky how they used phenomena in the sky and what role the sky played in their cultures."[22] Clive Ruggles argues it is misleading to consider archaeoastronomy to be the study of ancient astronomy, as modern astronomy is a scientific discipline, while archaeoastronomy considers other cultures' symbolically rich cultural interpretations of phenomena in the sky.[23][24]

Historical astronomy[edit]

The astronomer Clyde Tombaugh is the discoverer of Pluto. Credit: NASA.

Historical astronomy is the science of analysing historic astronomical data. The American Astronomical Society (AAS), established in 1899, states that its Historical Astronomy Division "...shall exist for the purpose of advancing interest in topics relating to the historical nature of astronomy. By historical astronomy we include the history of astronomy; what has come to be known as archaeoastronomy; and the application of historical records to modern astrophysical problems."[25] Historical and ancient observations are used to track theoretically long term trends, such as eclipse patterns and the velocity of nebular clouds.[26] Conversely, utilizing known and well documented phenomenological activity, historical astronomers apply computer models to verify the validity of ancient observations, as well as dating such observations and documents which would otherwise be unknown.

History of astronomy[edit]

The Jantar Mantar is a collection of architectural astronomical instruments, built by Maharaja Jai Singh II at his then new capital of Jaipur between 1727 and 1733. Credit: Knowledge Seeker.

Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, and astrological practices of pre-history: vestiges of these are still found in astrology, a discipline long interwoven with public and governmental astronomy, and not completely disentangled from it until a few centuries ago in the Western World. In some cultures astronomical data was used for astrological prognostication. Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.


Main source: Mathematics
The first synchrotron function can be used to represent the spectrum (intensity vs. wavelength, usually normalized to 1.0) of synchrotron radiation produced by a plasma. Credit: Dijon.

Although most of the mathematics needed to understand the information acquired through astronomical observation comes from physics, there are special needs upon situations that intertwine mathematics with phenomena that may not yet have sufficient physics to explain the observations.

Fixed point in the sky[edit]

By choosing an equal day/night position among the fixed objects in the night sky, the observer can measure equatorial coordinates: declination (Dec) and right ascension (RA).

The observations require precise measurement and adaptations to the movements of the Earth, especially when and where, for a time, an object or entity is available.

With the creation of a geographical grid, an observer needs to be able to fix a point in the sky. From many observations within a period of stability, an observer notices that patterns of visual objects or entities in the night sky repeat. Further, a choice is available: is the Earth moving or are the star patterns moving? Depending on latitude, the observer may have noticed that the days vary in length and the pattern of variation repeats after some number of days and nights. By choosing an equal day/night position among the fixed objects in the night sky, the observer can measure equatorial coordinates: declination (Dec) and right ascension (RA).

Once these can be determined, the apparent absolute positions of objects or entities are available in a communicable form. The repeat pattern of (day/night)s allows the observer to calculate the RA and Dec at any point during the cycle for a new object, or approximations are made using RA and Dec for recognized objects.

Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Description of the relations between axial tilt (or obliquity), rotation axis, plane of orbit, celestial equator and ecliptic.

Independent of the choice made (Earth moves or not), the pattern of objects is the same for days or nights of the repeating length once a year. The vernal equinox is a day/night of equal length and the same pattern of objects in the night sky. The autumnal equinox is the other equal length day/night with its own pattern of objects in the night sky.

The projection of the Earth's equator and poles of rotation, or if the observer hasn't concluded as yet that it's the Earth that's rotating, the circulating pattern of stars in ever smaller circles heading in specific directions, is the celestial sphere.


Main source: Orbits
ISEE-3 is inserted into a "halo" orbit on June 10, 1982. Credit: NASA.

Historically, the apparent motions of the planets were first understood geometrically (and without regard to gravity) in terms of epicycles, which are the sums of numerous circular motions.[27] Theories of this kind predicted paths of the planets moderately well, until Johannes Kepler was able to show that the motions of planets were in fact (at least approximately) elliptical motions.[28]

In the geocentric model of the solar system, the celestial spheres model was originally used to explain the apparent motion of the planets in the sky in terms of perfect spheres or rings, but after the planets' motions were more accurately measured, theoretical mechanisms such as deferent and epicycles were added. Although it was capable of accurately predicting the planets' position in the sky, more and more epicycles were required over time, and the model became more and more unwieldy.

In theoretical astronomy, whether the Earth moves or not, serving as a fixed point with which to measure movements by objects or entities, or there is a solar system with the Sun near its center, is a matter of simplicity and calculational accuracy. From the Wikipedia article Copernican heliocentrism: Copernicus's theory provided a strikingly simple explanation for the apparent retrograde motions of the planets—namely as parallactic displacements resulting from the Earth's motion around the Sun—an important consideration in Johannes Kepler's conviction that the theory was substantially correct.[29] "[Kepler] knew that the tables constructed from the heliocentric theory were more accurate than those of Ptolemy"[29] with the Earth at the center. Using a computer, this means that for competing programs, one written for each theory, the heliocentric program finishes first (for a mutually specified high degree of accuracy).

Logical laws[edit]

Main source: Logical laws
The diagram illustrates Kepler's three laws using two planetary orbits. Credit: Hankwang.

Kepler's laws of planetary motion, Kepler's laws:

  1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
  2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.[30]
  3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

The diagram at the right illustrates Kepler's three laws of planetary orbits: (1) The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1. (2) The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2. (3) The total orbit times for planet 1 and planet 2 have a ratio a13/2 : a23/2.


Main source: Physics
Here the Meissner effect is demonstrated by the levitation of a magnet above a liquid nitrogen cooled superconductor. Credit: Mai-Linh Doan.

Physics is a "science that deals with matter and energy and their interactions"[1] and forces and fields such as gravitation, electric and magnetic fields, and the weak and strong nuclear forces.

Time and conditions[edit]

The photograph of various lamps illustrates the effect of color temperature differences (left to right): (1) Compact Fluorescent: General Electric, 13 watt, 6500 K (2) Incandescent: Sylvania 60-Watt Extra Soft White (3) Compact Fluorescent: Bright Effects, 15 watts, 2644 K, and (4) Compact Fluorescent: Sylvania, 14 watts, 3000 K. Credit: Ramjar.

In physics, a key value on the time axis for collecting physical data is the starting time. Incandescents reach full brightness a fraction of a second after being switched on.

Eventually it occurred to many of the intelligent life forms on Earth that in addition to where the observations of the natural objects or entities in the sky are taken, also when and how the observations are taken is important. The results of observations change with time, temperature, and other atmospheric conditions.

Many of the early laws begin to make sense and increase understanding of the phenomena observed when coupled to experimental and theoretical studies performed in a laboratory here on Earth under controlled conditions. Astronomy benefits from physics.

Laboratory conditions are often expressed in terms of standard temperature and pressure.

Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar),[31] An unofficial, but commonly used standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs free energy of a reaction.

"Standard conditions for gases: Temperature, 273.15 K [...] and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued."[31]

NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (59.00 °F, 15.00 °C) and 101.325 kPa.[32]

Line of sight[edit]

This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

Def. a straight line along which an observer has a clear view is called line of sight.

In the section on 'senses' above is a demonstration of the principle of 'line of sight'; i.e., "a line from an observer's eye to a distant point toward which [the observer] is looking"[1]. In the image on the left of rain beneath a dark cloud, there is a highway with a vehicle on it. The vehicle is further away from the observer than the right turn onto a side road. Is the blue sky behind the dark cloud? Is the line of trees in the background further away than the dark cloud? Many objects in this image and the others can be layered relative to the observer (some are closer by inspection than others). These layers or strata are strata along the line of sight. The principle of line of sight can be used to make deductions about the relative locations (or positions) of objects from the observer's perspective.

By observing many of the wandering lights in the night sky, an occasional occultation of the light of one astronomical object may occur by the intervention of another along a closer astronomical stratum. On April 25, 1838, an occultation of Mercury by the Moon occurred when Mercury was visible to the unaided eye after sunset.[33] An occultation of Venus by the Moon occurred "on the afternoon of October 14", 1874.[33] An earlier such occultation "occurred on May 23, 1587, and is thus recorded by [Tycho Brahe] in his Historia Celestis"[33]. "Thomas Street, in his Astronomia Carolina (A.D. 1661), mentions three occultations by Venus, being two occasions when the planet covered Regulus, and once when there was an occultation of Mars by Venus."[33] "[Thomas Street] describes [the occultation of Mars by Venus] as follows: "1590,. Oct. 2nd, 16h. 24s. Michael Mœstlin observed ♂ eclipsed by ♀.""[33]


Cartesian coordinate system with a circle of radius 2 centered at the origin marked in red. The equation of a circle is (x - a)2 + (y - b)2 = r2 where a and b are the coordinates of the center (a, b) and r is the radius. Credit: 345Kai.

A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis of the system, and the point where they meet is its origin, usually at ordered pair (0,0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.


Main source: Distances
Distance along a path is compared in this diagram with displacement. Credit: .

Def. the amount of space between two points, usually geographical points, usually (but not necessarily) measured along a straight line is called distance.

Distance (or farness) is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria (e.g. "two counties over"). In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, and provides a concrete way of describing what it means for elements of some space to be "close to" or "far away from" each other.


The universe within 1 billion light-years (307 Mpc) of Earth is shown to contain the local superclusters, galaxy filaments and voids. Credit: Richard Powell.

In set theory, emptiness is symbolized by the empty set: a set that contains no elements

Def. the state of being devoid of content; containing nothing is called empty.

Free space, a perfect vacuum is expressed in the classical physics model. Vacuum state is a perfect vacuum based on the quantum mechanical model. In mathematical physics, the homogeneous equation may correspond to a physical theory formulated in empty space are disambiguations for "empty space".

In astronomy, voids are the empty spaces between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.[34]


Main sources: Physics/Forces and Forces

Def. a physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn) is called force.

Def. a force associated with nuclear decay is called the weak nuclear force.

Def. a fundamental force that is associated with the strong bonds is called the strong nuclear force.


Main sources: Physics/Energy, Energies, and Energy

Def. a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent is called energy.


Main sources: Regions/Fields, Fields, and Field
Surface magnetic field of Tau Scorpii is reconstructed by means of Zeeman–Doppler imaging. Credit: Pascalou petit.

Def. a region affected by a particular force is called a field.

Def. a region of space around a charged particle, or between two voltages; it exerts a force on charged objects in its vicinity is called an electric field.

Def. a condition in the space around a magnet or electric current in which there is a detectable magnetic force and two magnetic poles are present is called a magnetic field.


Def. the fundamental force of attraction that exists between all particles with mass in the universe is called gravitation.


Def. the basic structural component of the universe [that] usually has mass and volume is called matter.

In physics, mass, more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

  1. exerted by an object (active gravitational mass), or
  2. experienced by an object (passive gravitational force)

when interacting with a second object. The SI unit of mass is the kilogram (kg).


Main source: Measurements
A typical tape measure with both metric and US units is shown to measure two US pennies. Credit: Stilfehler.

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the meter, second or degree Celsius.

Physical units[edit]

Main sources: Physics/Units and Physical units

Def. "exactly 149,597,870.691 kilometres" is called one AU.[35]

Def. "86,400 SI seconds (s)" is called 1 day (d).[35]

Def. "1.9891 x 1030 kg" is called the mass of the Sun.[35]

Def. "9,460,730,472,580.8 km" is called the light-year (ly).[35]


In nuclear physics and nuclear chemistry, nuclear fission refers to either a nuclear reaction or a radioactive decay process in which the [atomic nucleus] nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), and releasing a very large amount of energy, even by the energetic standards of radioactive decay. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.[36][37] Most fissions are binary fissions, but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus.


Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. Fusion is the process that powers active stars, the [Teller–Ulam design] hydrogen bomb and some experimental devices examining fusion power for electrical generation. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to nucleosynthesis, the creation of the heavy elements during events such as supernovas.


Main source: Radiation
A lead castle is built to shield a radioactive sample. Credit: Changlc.
The electromagnetic spectrum. The red line indicates the room temperature thermal energy. Credit: Opensource Handbook of Nanoscience and Nanotechnology.

Def. an action or process of throwing or sending out a traveling ray in a line, beam, or stream of small cross section is called radiation, from radiation astronomy.

The term radiation is often used to refer to the ray itself.

Radiation comes in many forms and energies.

Notation: let various International System of Units, SI prefixes, occur before the unit of energy, the electronvolt, abbreviated as eV.

For example, PeV denotes 1015 eV.

Cosmic rays may be upwards of a ZeV (1021 eV). Ultra high energy neutrons are around an EeV (1018 eV). But, X-rays only range up to about 120 keV, while the visible (visual) range is around 2 eV.

Astronomy likely started with visual astronomy. Visual refers to that portion of the electromagnetic spectrum called the visible spectrum. Probing the sky with additional portions of this spectrum is difficult as the atmosphere absorbs over many portions.

This has produced fields of observational astronomy based on some portions of the electromagnetic spectrum:

  1. Gamma-ray astronomy,
  2. X-ray astronomy,
  3. Ultraviolet astronomy,
  4. Infrared astronomy, and
  5. Radio astronomy.

Planetary sciences[edit]

An ultraviolet image of the planet Venus is taken on February 26, 1979, by the Pioneer Venus Orbiter. Credit: NASA.

Planetary science (rarely planetology) is the scientific study of planets (including Earth), moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from astronomy and earth science,[38] but which now incorporates many disciplines, including planetary geology (together with geochemistry and geophysics), atmospheric sciences, oceanography, hydrology, theoretical planetary science, glaciology, and exoplanetology.[38] Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

Classical planets[edit]

In antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun (and closest to the Earth); all easily visible without a telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn.

Apparently 5102 b2k (before the year 2000.0), -3102 or 3102 BC, is the historical year assigned to a Hindu table of planets that does not include the classical planet Venus.[39] "Vénus seule ne s'y trouvait pas."[39] "Venus alone is not found there."[40]

Planetary astronomy[edit]

A true color image of Ganymede is acquired by the Galileo spacecraft on June 26, 1996. Credit: NASA/JPL.

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."[41] "[I]nterplanetary space ... is a stormy and sometimes very violent environment permeated by energetic particles and radation constantly emanating from the Sun."[41]

Each of the astronomical objects that constitute planetary science emits, reflects, or fluoresces radiation that is observed and analyzed.

"The spectrum of gaseous methane at 77 K in the 1.1-2.6 µm region [is] a benchmark for planetary astronomy".[42]


Main source: Sciences
This image demonstrates obstacles to observation (the Singapore skyline) and one atmospheric object: haze. Credit: SpLoT.
An eclipse is when the light of the Sun or Moon is blocked such as this annular eclipse on October 3, 2005, observed at Medina del Campo, Valladolid, España. Credit: Locutus Borg.

The body of knowledge acquired is usually given a name. Astronomology would be "the science of physical and logical laws" about "any natural body in the sky", but this is cumbersome. Astrology could be the doctrine, theory, and science of these bodies and use to be, but its focus has changed. 'Astronomy' has also come to mean the science acquired.

The science of astronomy consists of three fundamental parts

  1. the physical and logical laws,
  2. any natural body (objects and entities), and
  3. the sky (often the visible sky).

The section on geography describes some of the concerns regarding the sky in reference to the location of the observer (skygazing). The section on physics discusses another aspect, the atmosphere (and its absorption) between the observer and the natural body. Other aspects of the atmosphere include line of sight (e.g. obstacles), illumination during observation (such as city lights), and using the atmosphere as a detector.

The second part is the objects and entities of interest themselves. Here on the surface of the Earth when sensing upward a number of many kinds of natural bodies are detectable:

  1. clouds,
  2. haze,
  3. auroras,
  4. meteors (some liquid, some not),
  5. discs (such as the Sun and the Moon), and
  6. apparent points of origin for radiation.

The first part of 'physical and logical laws' is customarily determined by application of the scientific method. Although a variety of such methods of inquiry exist, the fundamental principle is reproducibility.

When you receive your clay tablet copy of someone else's publication of their results, you should be provided with sufficient information and guidelines to go into your laboratory, create and control the same conditions described and produce the same result three to five times (with some minor variation between each trial that should also be described).

For astronomy the significant difference is the lack of a laboratory for controlled conditions. While conditions can be controlled considerably within the observatory, there is still the necessity of letting the radiation in (but not the meteors).

Harm from the sky[edit]

In Oban, U.K., the tide has gone out. Credit: Angelia Streluk.
The volcanic eruption from Mount Pinatubo deposits a snowlike blanket of tephra on June 15, 1991. Credit: R.P. Hoblitt, USGS.

The section above is about ‘sensing a body in the sky’, or, more generally, sensing a body, an entity, an object, a phenomenon, or a thing, in the sky. While exploration and discovery is a lot of fun and expressing curiosity is enjoyable, when things or phenomena come from the sky that cause harm to anyone at your location or other intelligent life forms at other locations, can we explore and express curiosity so as to maybe reduce the probability of harm or possibly prevent the harm?

When it begins to get colder, we can put on more clothing to keep warm, build a shelter, start a fire, or spend time in a cave where the temperature is more stable.

On a warm day when it rains, we can use an umbrella, a rain coat, or get inside.

With no obstructions of the Sun between its locations as it passes overhead and your skin, there can be burning or tanning. Wearing light weight clothing may protect against the burning as can smearing certain chemicals on your skin.

For obtaining food, let’s say you like to go fishing. You come down to the shore with your gear, the water is gone, and your boat is sitting on land. Is there a way to predict this, prevent it, or adapt to it?

An ashfall occurs from a nearby volcano, before you can leave the area or maybe even go to work. Can this possibly harmful event be prevented, predicted, or adapted to?

To explore, discover, predict, adapt to, or prevent, begins with knowledge. Intelligent life forms can think, deduct, use logic.

By studying some clouds from vantage points that are relatively higher than others may come the awareness that some of these clouds are very close to the surface of the Earth. The same is true when a cloud obscures a landmark.

Noticing natural bombs that fall from the sky are similar in composition to those closer to a nearby volcano encourages the deduction that these bombs originated there or very near there.

The association of most lightning with darker clouds, rain usually occurring after lightning and thunder, and that some clouds are lower than some high points of ground or land indicate that these are very close to the ground. Taking a hike on a trail only to walk through fog which at a lower altitude looks like a cloud helps to conclude that many of the objects in the sky are close by. Further, these same objects block the Sun and the Moon.

From traveling around or conferencing with others, reading written text on various media, writing down what you observe and communicating these to others, you can find out that aurora almost always occur with greater frequency near the poles of the Earth where the Sun never rises very high in the sky.

With communication and knowledge comes the realization that many of the things, phenomena, objects, entities, or bodies in the sky are local weather or Earth-based geology in action. Most of the harm that befalls intelligent life on Earth appears to be local to the Earth itself. Do you disagree?

Phenomena above the Earth[edit]

Here at Réunion is an example that some of those white puffy objects in the sky may be quite close by. Credit: B.navez.
Cirrus clouds never seem to touch any mountain. Yet sunrise reveals they are closer to the ground than the Sun. Credit: Simon Eugster.

While there are still objects between here and the Moon or the Sun, many of those in the sky are not the focus of astronomy. But, how far away are the highest clouds? Are there clouds higher up between us here on the Earth’s surface and the Moon or the Sun? To begin to answer these questions may require history (has anyone recorded some event that may be helpful), technology (finding a way to get higher or observe better), geography (we’ve used some geography already by hiking up above some clouds), physics (we used the concept of relative position, clouds obscuring the Sun, has the Sun ever passed between the Earth and the Moon?), mathematics (we used the concept of order: we’re on the ground and the Moon is in the sky), chemistry (studying the composition of bombs from the sky), and greater efforts of exploration.

Observing a solar eclipse tells you that when both objects (the Sun and the Moon) are in the sky at the same time, close to each other, the Moon is between you and the Sun.

What causes the shadow to cross the Moon over a single month? Where is the Sun when this happens?

The Moon is dark at two times under different circumstances, what are they?

Part of knowledge is terminology to communicate observations or events to others. The Sun passing overhead or approximately overhead may be called a transit. Does the Sun stop during its transit or does it seem to move steadily or continuously? Does the Sun move northward for part of the day then southward for the rest? Does the Sun rise in the east one day then rise in the west on the next?

Have you ever seen the Sun revolve around the Moon, where revolve means go around the Moon in an orbit?


This is a photograph taken in 1910 during the passage of Halley's comet. Credit: The Yerkes Observatory.
A photo of the planet Mars is taken in Straßwalchen (Austria) on September 19, 2003, shortly after its closest approach. Credit: Rochus Hess,
The Earth and Moon is imaged by the Mars Global Surveyor on May 8, 2003, at 12:59:58 UTC.
The Chicxulub impact crater is outlined. Credit: NASA/JPL-Caltech, modified by David Fuchs.
The telescope photograph of the Great Andromeda Nebula is taken around 1899. Credit: Isaac Roberts.

Theoretical astronomy provides a definition (Def.) of an object in astronomy:

Def. a natural object in the sky especially at night is called an astronomical object.

A celestial object is any astronomical object except the Earth.

Some objects seem to wander around in the night sky relative to many of the visual points of light. At least one occasionally is present in the early morning before sunrise as the Morning Star and after sunset as the Evening Star, the planet Venus. These wanderers and related objects are subjects for observational astronomy and some are meteors.

Others only make an appearance after decades, sometimes spectacularly.

The 1910 approach, which came into naked-eye view around 10 April[43] and came to perihelion on 20 April,[43] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.[44] Furthermore, the comet made a relatively close approach of 0.15AU,[43] making it a spectacular sight. Indeed, on 19 May, the Earth actually passed through the tail of the comet.[45][46] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[47] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet."[48] His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[49] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.[48]

"It is quite possible that [faint streamers preceding the main tail and lying nearly in the prolonged radius vector] may have touched the Earth, probably between May 19.0 and May 19.5, [1910,] but the Earth must have passed considerably to the south of the main portion of the tail [of Halley's comet]."[50]

Of the other planets of the solar system, Mercury, Mars, Jupiter, Saturn, Uranus, and Neptune, none has apparently produced as much drama and excitement recently on Earth among some of the intelligent life forms as Halley's comet.

Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, 55,758,006 km (0.372719 AU; 34,646,400 mi), magnitude −2.88, on 27 August 2003 at 9:51:13 UT

But asteroid impacts, though rare, occur once in a while, over very large areas, at aperiodic intervals such as the Chicxulub crater. Most scientists agree that this impact is the cause of the Cretatious-Tertiary Extinction, 65 million years ago (Ma), that marked the sudden extinction of the dinosaurs and the majority of life then on Earth. This shaded relief image of Mexico's Yucatan Peninsula shows a subtle, but unmistakable, indication of the Chicxulub impact crater.

Still much further away from the Earth than the Sun or Neptune are the many stars and nebulae that make up the Milky Way. Beyond the confines of our galaxy is the Andromeda Galaxy.

Of the Local Group, its "two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[51] making [Andromeda] one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[52] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[53] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[54]

The various objects and entities that are observed and studied that do not appear to cause us harm also engender separate areas within astronomy, some of which are

  1. Extragalactic astronomy,
  2. Galactic astronomy,
  3. Physical cosmology,
  4. Planetary sciences,
  5. Solar astronomy, and
  6. Stellar astronomy.


First Public Image is from GOES 14 taken with the Solar X-ray Imager (SXI). Credit: NWS Internet Services Team of the NOAA/Space Weather Prediction Center.

Def. a natural source usually of radiation in the sky especially at night is called an astronomical source.

The image at right is the first public image taken by the solar X-ray imager (SXI) aboard the GOES 14 satellite. These geostationary operational environmental satellites (GOES) monitor the Sun’s X-rays for the early detection of solar flares, coronal mass ejections (CMEs), and other phenomena that impact the geospace environment. This early warning is important because travelling solar disturbances affect not only the safety of humans in high-altitude missions, such as human spaceflight, but also military and commercial satellite communications. In addition, CMEs can damage long-distance electric power grids, causing extensive power blackouts.

The GOES satellites circle the Earth in geosynchronous orbits.

In addition to sources of radiation, there are sources of objects such as the Oort clouds, the Kuiper belts, and the asteroid belt. These may have been formed at the beginning of the solar system or be a product or partial product of the solar binary consisting of the Sun and Jupiter. Such a solar binary may serve to establish an upper limit for interstellar cometary capture.


This image is a portrait of Johannes Kepler. Credit: Dr. Manuel.
This is an image of the title page of Johannes Kepler's Rudolphine Tables (1627). Credit: Johannes Kepler.


1.a: an "independent, separate, or self-contained [astronomical] existence",
1.b: "the [astronomical] existence of a [person, place, or] thing as contrasted with its attributes", or
2. "some [astronomical] thing that has separate and distinct existence and objective or conceptual reality",[1]

is called an astronomical entity.

This article and each of the articles focusing on any entity from astronomy is itself an astronomical entity. In terms of meaning and generalness: 'being' > 'body' > 'something' or 'thing' > 'entity'.[55] But, for information processing, astronomical 'being', 'body', 'something' or 'thing' are also astronomical entities.

Each section in this article mentions or shows images of astronomical entities:

  1. Barringer Meteor Crater,
  2. Sun,
  3. Tides,
  4. Moon,
  5. Earth (as a planet),
  6. Aurora Borealis,
  7. Lightning,
  8. Clouds,
  9. Meteors,
  10. Astronomical objects,
  11. Astronomical sources,
  12. Astronomers and Observers,
  13. Planets,
  14. Nebulae,
  15. Galaxies,
  16. Calendars, Star charts, Constellations,
  17. Telescopes and other technology,
  18. Observatories,
  19. Physical astronomy and Astrophysics,
  20. Astrodesy, Astrognosy, and Astrometry,
  21. dominant groups, orbits and logical laws,
  22. Chemicals, Materials, and ions, etc.

At right beneath a portrait of Johannes Kepler is a copy of the title page of Johannes Kepler's Rudolphine Tables (1627). It is regarded as the most accurate and comprehensive star catalogue and planetary tables published up until that time. It contained the positions of over 1000 stars and directions for locating the planets within our solar system. Kepler finished the work in 1623 and dedicated it to his patron, the Emperor Rudolf II, but actually published it in 1627. The table's findings support Kepler's laws and the theory of a heliocentric astronomy.


Main source: Technology
This is a schematic of a Keplerian refracting telescope which uses two different sizes of planoconvex lenses. Credit: .

Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures.

Refracting telescopes[edit]

This woodcut illustration is of a 45 m focal length Keplerian air telescope built by Johannes Hevelius. From his book Machina coelestis (first part), published in 1673. Credit: .

The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[56] It uses a [plano]convex lens as the eyepiece instead of Galileo's [double] concave one. The advantage of this arrangement is [that] the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. ... All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye could collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.

A range of sizes for planoconvex crystalline lenses has been discovered since about 1600 BC (3600 b2k),[5] where b2k is notation for 'before the year 2000'. Glass biconvex lenses have been discovered from 43-50 AD (1957-1950 b2k).[5]

While finding an ancient tube telescope would be wonderful, air telescopes are a possibility.

Reflecting telescopes[edit]

The Space Shuttle Discovery's Cargo Bay and Crew Module, and the Earth's horizon are reflected in the helmet visor of one of the space walking astronauts on STS-103. Credt: .
The NASA logo on Bldg. 703 at the Dryden Aircraft Operations Facility in Palmdale, California, is reflected in the 2.5 m primary mirror of the SOFIA observatory's telescope. Credit: .
The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a state-of-the-art CCD. Credit: .

Another type of telescope uses a planoconcave lens with a mirror coating on the concave surface. This mirror configuration reflects and focusses the incoming light at an internal additional mirror system in the field of view that directs the focussed light externally for viewing. One configuration directs the light through a circular hole in the planoconcave lens.

While ancient planoconvex lenses have been discovered with gold foil attached to the plane surface so that the lens may reflect light somewhat like an astronauts helmet, it seems at present unlikely that ancient observers developed reflecting telescopes. Although a planoconvex lens with a circular hole in the center is known.[5]

An extreme example of a reflecting telescope is demonstrated by the grazing incidence X-ray telescope (XRT) of the Swift satellite that focuses X-rays onto a state-of-the-art charge-coupled device (CCD), in red at the focal point of the grazing incidence mirrors (in black at the right).

Optical observatories[edit]

The telescope at the Aldershot obsevatory shown here is an 8-inch refractor constructed in 1891 by Grubb. Credit: .

Historically, observatories [are] as simple as [using or placing stably] an astronomical sextant (for measuring the distance between stars) or Stonehenge (which has some alignments on astronomical phenomena). ... Most optical telescopes are housed within a dome or similar structure, to protect the delicate instruments from the elements. Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes.


This is an image of a real X-ray detector. The instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. Credit: .

A visual, optical telescope itself, together with the astronomer, various detectors and accompanying computers, plus necessary hardware to move the telescope and the dome to keep the telescope aimed at a specific astronomical object or entity are often all housed inside an observatory.

Detectors such as the X-ray detector at right collect individual X-rays (photons of X-ray light), count them, discern the energy or wavelength, or how fast they are detected. The detector and telescope system can be designed to yield temporal, spatial, or spectral information.

A technique called wavelength dispersive X-ray spectroscopy (WDS), is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging X-ray and the crystal's lattice spacings are related by Bragg's law where the detector] counts only [X]-rays of a single wavelength. Many elements emit or fluoresce specific wavelengths of X-rays which in turn allow their identification.


Main source: Clocks
This is a sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.
This image shows another sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.
This chart shows the increasing accuracy of NIST (formerly NBS) atomic clocks. Credit: National Institutes of Standards and Technology (NIST), USA.
The FOCS 1 is a continuous cold caesium fountain atomic clock in Switzerland. Credit: METAS.

The image at right shows a sun dial from Ai Khanoum, Afghanistan, dated to the 3rd century BCE, ~2300 b2k. The image at left is also from Ai Khanoum, Afghanistan, showing its workings.

"[T]he earliest known sundial [is] from an Egyptian burial dated in the fifteenth century B.C. Sometimes called a shadow clock, or an L-board because of its shape [with] relatively crude performance."[57] A "[f]ragment of a late Egyptian sundial [from] about 3000 B.C." exists.[57]

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region[58] of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The FOCS 1 continuous cold cesium fountain atomic clock started operating in 2004 at an uncertainty of one second in 30 million years. The clock is in Switzerland.

Motion calibrators[edit]

Main source: Motion calibrators

POA CALFOS is the improved Post Operational Archive version of the Faint Object Spectrograph (FOS) calibration pipeline ... The current version corrects for image motion problems that have led to significant wavelength scale uncertainties in the FOS data archive. The improvements in the calibration enhance the scientific value of the data in the FOS archive, making it a more homogeneous and reliable resource.

Lofting technology[edit]

Main source: Lofting technology
Carried aloft on a Nike-Black Brant VC sounding rocket, the microcalorimeter arrays observed the diffuse soft X-ray emission from a large solid angle at high galactic latitude. Credit: NASA/Wallops.
The MeV Auroral X-ray Imaging and Spectroscopy experiment (MAXIS) is carried aloft by a balloon.

Additional technology used to benefit astronomy includes sounding rockets which may carry gamma-ray, X-ray, ultraviolet, and infrared detectors to high altitude to view individual sources and the background for each wavelength band observed. Balloons are used as a long-duration facility above 99 % of the Earth's atmosphere. The MeV Auroral X-ray Imaging and Spectroscopy experiment (MAXIS) is carried aloft by a balloon for a 450 h flight from McMurdo Station, Antarctica. The MAXIS flight detected an auroral X-ray event possibly associated with the solar wind as it interacted with the upper atmosphere between January 22nd and 26th, 2000.[59]

With the advent of lofting technology comes the possibility of placing an observatory as a free floating yet when necessary either a geostationary, rotating, or fixed form in orbit.


Lift-off of the Thor Able Star launch vehicle. Credit: US Air Force/Navy.
Pictured here is the Solrad 3 X-ray astronomy observatory atop the satellite stack being fitted with a nose cone. Credit: US Navy.

Lofting an observing system into an orbit around the Earth requires designing and testing for survival of the rocket trip upward and the orbiting technique (usually a second stage for orbital insertion). At left is an early X-ray observatory (Solrad 3), the spherical silver ball with antenna, atop a stack of satellites, being fitted with a nose cone to reduce atmospheric drag and to protect the satellites.

Once the satellite is securely aboard the second stage, the lofting rocket is fueled (when liquid fuel is used), and the launch commences. At right is the Thor Able Star rocket being launched by the US Air Force from Cape Canaveral, Florida, USA.

Solrad 3 is operated by the US Naval Research Laboratory beginning with its launch on June 29, 1961, through to the end of its mission on March 6, 1963. Although Solrad 3 did not successfully separate from the satellite immediately below it in the stack (Injun 1), it successfully returned solar X-ray data until late in 1961. It is not expected to re-enter the Earth's atmosphere for ~900 years.


Main source: Computers
This image is of the large astrolabe made by Gualterus Arsenius in 1569. Credit: Rama.

The astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy.

Some form of an "astrolabe" may have been in use by the third millennium BC.[60]


Main sources: Information/Printing and Printing
This photograph of a printing press reminds us that writing descriptions down preserves knowledge for the future. Credit: MatthiasKabel.

The invention of the printing press (an early example is shown at right) made it possible for scientists and politicians to communicate their ideas with ease, leading to the Age of Enlightenment; an example of technology as a cultural force.


Main source: Hypotheses
  1. Observers have been watching the skies and recording what they saw for more than 40,000 years.

See also[edit]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Philip B. Gove, ed (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts: G. & C. Merriam Company. pp. 1221. 
  2. Internet Encyclopedia of Science Accessed April 2010
  3. Reach, W. T. (1997). "The structured zodiacal light: IRAS, COBE, and ISO observations", page 1 (in Introduction)
  4. Bernhard Peucker-Ehrenbrink and Birger Schmitz (2001). Accretion of Extraterrestrial Matter Throughout Earth's History. Springer. pp. 66–67. ISBN 0-306-46689-9. 
  5. 5.0 5.1 5.2 5.3 5.4 Dimitris Plantzos (July 1997). "Crystals and Lenses in the Graeco-Roman World". American Journal of Archaeology 101 (3): 451-64. Retrieved 2011-10-17. 
  6. Austen Henry Layard (1853). Discoveries in the ruins of Nineveh and Babylon: with travels in Armenia. G.P. Putnam and Co. pp. 197–8,674. 
  7. D. Brewster (1852). "On an account of a rock-crystal lens and decomposed glass found in Niniveh" (in German). Die Fortschritte der Physik (Deutsche Physikalische Gesellschaft). 
  8. McSween Jr., Harry Y. (1976). "A new type of chondritic meteorite found in lunar soil". Earth and Planetary Science Letters 31 (2): 193–9. doi:10.1016/0012-821X(76)90211-9. 
  9. Rubin, Alan E. (1997). "The Hadley Rille enstatite chondrite and its agglutinate-like rim: Impact melting during accretion to the Moon". Meteoritics & Planetary Science 32 (1): 135–41. doi:10.1111/j.1945-5100.1997.tb01248.x. 
  10. Opportunity Rover Finds an Iron Meteorite on Mars. JPL. January 19, 2005. Retrieved 2006-12-12. 
  11. Crouch, Stanley; Skoog, Douglas A. (2007). Principles of instrumental analysis. Australia: Thomson Brooks/Cole. ISBN 0-495-01201-7. 
  12. . doi:10.1351/pac198658121737. 
  13. GS Hawkins (1966). Stonehenge Decoded. ISBN 978-0880291477. 
  14. Paul Murdin (November 2000). Paul Murdin. ed. Canada-France-Hawaii Telescope, In: Encyclopedia of Astronomy and Astrophysics. Bristol: Institute of Physics. doi:10.1888/0333750888/4166. Bibcode: 2000eaa..bookE4166.. 
  15. Kocharov, G. E.; Viktorov, S. V.; Victorov, S. V.; Chesnokov, V. I. (1975). "Investigation of solar X-rays from the lunar surface, carried out on Lunokhod-2". Space Research 15: 633-6. 
  16. Betsy Maestro (2004). The Story of Clocks and Calendars. New York: HarperCollins. pp. 48. ISBN 0-688-14548-5. 
  17. Needham, Volume 3, p.171
  18. Needham, Volume 3, p.242
  19. B. L. van der Waerden (1974). "The Earliest Form of the Epicycle Theory". Journal for the History of Astronomy 5: 175-85. 
  20. D. Koutsoyiannis and A. N. Angelakis (2003). Hydrologic and Hydraulic Science and Technology in Ancient Greece, In: Encyclopedia of Water Science. New York: Marcel Dekker, Inc.. pp. 415-7. doi:10.1081/E-EWS 120016393. Retrieved 2011-10-26. 
  21. Giulio Magli (2009). When the method is lacking, In: Mysteries and Discoveries of Archaeoastronomy from Giza to Easter Island. Rome, Italy: Copernicus Books. pp. 97-116. doi:10.1007/978-0-387-76566-2_5. ISBN 978-0-387-76564-8. Retrieved 2011-10-15. 
  22. Sinclair, R.M. (2006). "The Nature of Archaeoastronomy". In Todd W. Bostwick and Bryan Bates. Viewing the Sky Through Past and Present Cultures; Selected Papers from the Oxford VII International Conference on Archaeoastronomy. Pueblo Grande Museum Anthropological Papers. 15. City of Phoenix Parks and Recreation Department. pp. 13–26. ISBN 1-882572-38-6. 
  23. Ruggles, C.L.N. (2005). Ancient Astronomy. ABC-Clio. ISBN 1-85109-477-6. 
  24. Ruggles, C.L.N. (1999). Astronomy in Prehistoric Britain and Ireland. Yale University Press. ISBN 0-300-07814-5. 
  25. Historical Astronomy Division. 
  26. Historical eclipses and Earth's rotation. 
  27. Encyclopaedia Britannica, 1968, vol. 2, p. 645
  28. M Caspar, Kepler (1959, Abelard-Schuman), at pp.131–140; A Koyré, The Astronomical Revolution: Copernicus, Kepler, Borelli (1973, Methuen), pp. 277–279
  29. 29.0 29.1 Christopher M. Linton (2004). From Eudoxus to Einstein—A History of Mathematical Astronomy. Cambridge: Cambridge University Press. ISBN 978-0-521-82750-8. 
  30. Bryant, Jeff; Pavlyk, Oleksandr. "Kepler's Second Law", Wolfram Demonstrations Project. Retrieved December 27, 2009.
  31. 31.0 31.1 Alan D. McNaught, Andrew Wilkinson (1997). Compendium of Chemical Terminology, The Gold Book (2nd ed.). Blackwell Science. ISBN 0-86542-684-8. 
  32. Natural gas – Standard reference conditions (ISO 13443). Geneva, Switzerland: International Organization for Standardization. 1996. 
  33. 33.0 33.1 33.2 33.3 33.4 Samuel J. Johnson (1874). "Occultations of and by Venus". Astronomical register 12: 268-70. 
  34. U. Lindner, J. Einasto, M. Einasto, W. Freudling, K. Fricke, E. Tago (1995). The Structure of Supervoids I: Void Hierarchy in the Northern Local Supervoid "The structure of supervoids. I. Void hierarchy in the Northern Local Supervoid". Astron. Astrophys. 301: 329. The Structure of Supervoids I: Void Hierarchy in the Northern Local Supervoid. 
  35. 35.0 35.1 35.2 35.3 P. K. Seidelmann (1976). Measuring the Universe The IAU and astronomical units. The International Astronomical Union. Retrieved 2011-11-27. 
  36. Arora, M. G.; Singh, M. (1994). Nuclear Chemistry. Anmol Publications. p. 202. ISBN 81-261-1763-X. Retrieved 2011-04-02. 
  37. Saha, Gopal (2010). Fundamentals of Nuclear Pharmacy (Sixth ed.). Springer Science+Business Media. p. 11. ISBN 1-4419-5859-2. Retrieved 2011-04-02. 
  38. 38.0 38.1 Stuart Ross Taylor (29 July 2004). "Why can't planets be like stars?". Nature 430 (6999): 509. doi:10.1038/430509a. PMID 15282586. 
  39. 39.0 39.1 Jean Baptiste Joseph Delambre (1817). Histoire de l'astronomie ancienne. Paris: Courcier. pp. 639. Retrieved 2012-01-13. 
  40. Immanuel Velikovsky (January 1965). Worlds in Collision. New York: Dell Publishing Co., Inc.. pp. 401. Retrieved 2012-01-13. 
  41. 41.0 41.1 Theodore E. Madey, Robert E. Johnson, Thom M. Orlando (March 2002). "Far-out surface science: radiation-induced surface processes in the solar system". Surface Science 500 (1-3): 838-58. doi:10.1016/S0039-6028(01)01556-4. Retrieved 2012-02-09. 
  42. A. R. W. McKellar (November 1989). "The spectrum of gaseous methane at 77 K in the 1.1-2.6 μm region: a benchmark for planetary astronomy". Canadian Journal of Physics 67 (11): 1027-35. doi:10.1139/p89-180. Retrieved 2012-02-09. 
  43. 43.0 43.1 43.2 D. K. Yeomans (1998). Great Comets in History. Jet Propulsion Laboratory. Retrieved 15 March 2007. 
  44. D. A. Mendis (1988). "A Postencounter view of comets". Annual Review of Astronomy and Astrophysics 26 (1): 11–49. doi:10.1146/annurev.aa.26.090188.000303. 
  45. Ian Ridpath (1985). Through the comet’s tail. Revised extracts from A Comet Called Halley by Ian Ridpath, published by Cambridge University Press in 1985. Retrieved 2011-06-19. 
  46. Brian Nunnally (May 16, 2011). This Week in Science History: Halley’s Comet. pfizer: ThinkScience Now. Retrieved 2011-06-19. 
  47. "Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet". The New York Times. 8 February 1910. Retrieved 15 November 2009. 
  48. 48.0 48.1 "Ten Notable Apocalypses That (Obviously) Didn't Happen". Smithsonian magazine. 2009. Retrieved 14 November 2009. 
  49. Interesting Facts About Comets. Universe Today. 2009. Retrieved 15 January 2009. 
  50. Heber D. Curtis (June 1910). "Photographs of Halley's Comet made at the Lick Observatory". Publications of the Astronomical Society of the Pacific 22 (132): 117-30. 
  51. Abraham Loeb, Mark J. Reid, Andreas Brunthaler, and Heino Falcke (November 2005). "Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33". The Astrophysical Journal 633 (2): 894-8. doi:10.1086/491644. Retrieved 2011-11-14. 
  52. The Grand Collision, from the series: The Sky At Night, airdate: November 5, 2007
  53. Cox, T. J.; Loeb, A. (2008). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society 386 (1): 461–474. doi:10.1111/j.1365-2966.2008.13048.x. 
  54. Cain, F. (2007). When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?. Retrieved 2007-05-16. 
  55. Peter Mark Roget (1969). Lester V. Berrey and Gorton Carruth. ed. Roget's International Thesaurus, third edition. New York: Thomas Y. Crowell Company. pp. 1258. 
  56. AH Tunnacliffe, JG Hirst (1996). Optics. Kent, England. pp. 233–7. ISBN 0-900099-15-1. 
  57. 57.0 57.1 Winthrop W. Dolan (1975). A Choice of Sundials. S. Greene Press. pp. 146. ISBN 0828902100. Retrieved 2012-10-24. 
  58. Dennis McCarthy, P. Kenneth Seidelmann (2009). TIME from Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH. ch. 10 & 11. 
  59. R. M. Millan, R. P. Lin, D. M. Smith, K. R. Lorentzen, and M. P. McCarthy (December 2002). "X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer". Geophysical Research Letters 29 (24): 2194-7. doi:10.1029/2002GL015922. Retrieved 2011-10-26. 
  60. David Brown (2000). Cuneiform Monographs 18: Mesopotamian Planetary Astronomy-Astrology. Groningen: Styx Publications. pp. 113-20. Retrieved 2011-11-01. 

Further reading[edit]

  • James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. ISBN 0691004021. OCLC 39108765. 
  • Kaufmann, W. J. (1994). Universe. W H Freeman. ISBN 0-7167-2379-4. 
  • Smith, E.V.P.; Jacobs, K.C.; Zeilik, M.; Gregory, S.A. (1997). Introductory Astronomy and Astrophysics. Thomson Learning. ISBN 0-03-006228-4. 

External links[edit]

{{Astronomy resources}}{{Principles of radiation astronomy}}