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The star cluster M4 in the constellation Scorpius
A 1498 illustration of skygazing

Skygazing, or more literally gazing at the sky, is an activity for leisure or with an interest in amateur astronomy. Astronomical observations are generally made with the naked eye or with basic optical aids. Simple naked-eye observations of the sky can reveal a great deal about the basics of astronomy and give a better understanding of the cosmos, while instruments, such as telescopes, are used to study deep space. Many different celestial objects can be viewed while skygazing during both night and daytime.

Naked eye sky gazing

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Diurnal (daytime) observation

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The extreme brightness of the Sun saturates the sky during the day and prevents naked eye observation of less luminous objects, with the exception of the Moon and occasionally Venus.

Looking directly at the Sun can be very damaging for your eyes. Not even sunglasses will prevent eye damage, so special filtered glasses fitted with materials such as metalized PET film are essential. These can also be used to observe solar eclipses. Projecting an image of the Sun on to a surface using a piece of card with a pinhole can also be very effective, and safe. NEVER look directly at the Sun through a telescope or binoculars as this will cause instant and permanent blindness. Beware that some solar filters supplied with cheaper telescopes are not safe enough. Only filters clearly identified as complying with current safety standards should be used. [1] and [2]


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Photo taken during the French 1999 eclipse.

One of the Sun's most spectacular phenomena is a partial or total eclipse.

Solar eclipses only occur when the Moon is in the New Moon phase (which occurs every 29.5 days), however, eclipses cannot always be seen in these periods, because the Moon goes "above" or "below" the Earth as it orbits. The Moon's orbit is tilted by about 5.2° away from the plane of the Earth's orbit around the Sun. During this period, the Earth, Sun, and Moon are aligned only during certain times of the year.

During eclipses, from the vantage point on Earth, viewers witness the Moon slowly moving partially or totally past the Sun, (one recent example being in France on August 11, 1999). The diameter of the Moon and Sun appear almost equal, although in reality the Sun is much bigger, it is also much further than the Moon and therefore appears smaller. This allows the Moon to totally block out the Sun during a solar eclipse. Sometimes a ring made by the Sun shining past the outside of the circumference of the Moon is also visible; this is a special type of total eclipse, called an annular eclipse.

Conditions of trajectories for the solar eclipse

In the total eclipse zone, it is possible to see the most brilliant stars in the daytime, and especially Mercury, which is usually difficult to observe because it is always very close to the Sun.

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Sunspots are difficult to see with the naked eye but can be safely viewed using projection as described above. They are effectively solar disturbances on the Sun's surface and can change over time as conditions within the Sun change, somewhat like storms on the Sun. By observing the Sun over time the appearance of sunspots changes due to both the Sun's rotation and each storm's evolution.

Aurora Viewing
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Viewing Aurorae is a typical sky watching activity especially at higher magnetic latitudes.

Cometary Viewing
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Comets can be easy to locate and photograph and an excellent reason to sky watch.

Atmospheric Optical Phenomena

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Phenomena linked to the Sun and Moon: Other interesting observations in relation to the Sun and Moon (even Venus at times) can be made, although their observation depends on the particular atmospheric conditions. These can be easier to observe since they do not always require any eye protection.

A phenomenon that can be seen mainly in winter and at altitude, it takes the form of a big luminous club, a little extended in its width and centered on the Sun. It is produced by the refraction of the solar rays through a fine and uniform layer of high altitude clouds, the cirrostratus. Often requires eye protection, as the Sun is within the field of view.
A double rainbow
Visible during or after a rainy period with a partially clear sky, it is a bow of white light refracted into all the visible colures of the spectrum by solar rays passing through raindrops. In optimal conditions, a second less luminous bow, with reversed colures, can be observed (see picture at right), the space between the two being slightly darker than the rest of the sky.
Produced like the solar halo but by different clouds and occurring more frequently, these are two luminous spots, often in diffracted colors like in a rainbow, situated on both sides of the Sun at similar distances very close to the club of the associated halo.

Nocturnal (nighttime) observation

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With good viewing conditions, during a dark and moonless night, it is possible to distinguish about 3000 stars in the celestial sphere.

A waxing gibbous (increasing) Moon, as seen from Earth's Southern Hemisphere

The queen of the night - it is our only natural satellite. Naked eye observation allows us to track its changes over the monthly cycle.

  • Phases of the Moon
The following diagram explains how the Moon's phases are produced by the relative positions of the Moon, the Earth and the Sun in space.
Orbit of the Moon and phases seen from the Earth
Its light coming only from the reflection of the Sun's light on its surface, the Moon will present the appearance a thin crescent at twilight or at dawn when it will be located between the Earth and the Sun; a half disc for half of the night when it will be the same distance from the Sun as our planet and finally a complete disc all night long when it will be opposite the Sun in relation to the Earth. The sight of the setting crescent Moon against blue sky at twilight is well worth staying up late for.
The effect of the path of its illuminating solar rays can also be seen: in its first rising phase or its final setting phase, when it is only a crescent, one can often see the dark part of its disk weakly illuminated, permitting us to distinguish the form of the complete disc. This is due to the solar rays falling on the Earth reflecting toward the Moon and illuminating the side that is in darkness. This long route makes for a weak light reaching us, but it is sufficient to distinguish it.
Chart showing the seas and the main craters of the Moon
Phases of a lunar eclipse
  • Lunar craters
These are pits or depressions in the surface of the Moon, produced by great impacts of gigantic meteoroids which mostly took place billions of years ago. They range in size from huge walled plains more than a hundred miles across to microscopic pits. The smallest craters which can be glimpsed through ordinary binoculars are about twenty miles across. These craters are most common in the light-colored Lunar highlands. They are named after historical figures, mostly scientists.
Of different and darker composition than the rest of the surface, the Maria (singular "mare", Latin for sea) are composed of basalt. These flat areas of ancient frozen lava form the familiar pattern of dark spots which can be seen with the naked eye. They are given the names of fanciful bodies of water since early observers believed them to be literal seas. Charts of the Moon are available to aid the observer in identifying lunar features.
  • Lunar Eclipses
Following the same principle as the solar eclipses, the lunar eclipses only take place when the Moon is full and that the Earth is positioned between the Moon and the Sun. The diameter of the shadow of our planet being a lot bigger than the one of our satellite, these take place more frequently and have the same appearance wherever the viewer is located on Earth. At the time of the total phase, the Moon remains visible and has an orange color that is due to refracted solar rays tinted by the terrestrial atmosphere.
  • Lunar halo
Invoked by the same meteorological phenomenon as the solar halo, this presents itself however as the appearance of a luminous disc of a more vivid diffuse form with a reduced diameter than its solar equivalent.


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Over a period of days a viewer will notice a few stars move relative to the others. They are actually planets. To distinguish a planet from a star, it is helpful to know that stars twinkle while planets have a steadier light, the stars being much further away and so presenting as a point rather than the disc of a planet. Once you have found a planet, it is interesting to know more about it and even to the naked eye, much can be discovered. Indeed, all visible planets have some features unique to them:

  • Mercury is rarely visible from Earth since its orbit keeps it very close to the Sun.
    Crescent Moon and Venus
  • Venus, also known as the "Shepherd's star", is white in color, and appears brighter than any object in the sky after the Sun and Moon. Visible near twilight and dawn, like Mercury, Venus is an interior planet (one whose orbit lies between the Sun and the Earth) and appears to follow the Sun relative to our perspective (its maximum elongation is 47°). Its apparent magnitude varies depending on its phase (very much like the Moon) as well as its distance from Earth.
  • Mars is not unusually bright, but it can be distinguished from other celestial bodies by its reddish light. A casual observer (over the period of several days) may notice that it seems to make a U-turn (stops and then moves in retrograde): this is a property Mars shares with all the outer planets, and is owed to Earth's relative movement around the Sun. In the case of Mars, this takes place roughly every two years, with the period of greatest change in its apparent motion taking place over two months.
  • Jupiter is pale in color. Although bright enough to be confused with Venus, a viewer can recognize it instantly: if one observes the equivalent of Venus in the middle of the night, it is Jupiter.
  • Saturn is considerably less bright than Jupiter (Uranus and Neptune are dimmer still).

Besides the planets, there are many other celestial curiosities:

Milky Way

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The Milky way is imaged from within.

The Milky Way is a denser region of stars in comparison to the rest of the sky. The image above represents the plane of our galaxy as viewed from within.

Stay one night in a place far from the bright lights of big cities so that your eyes get used to darkness, relax and wait. You will see a gigantic irregular milky band crossing the sky. This appearance gives its name since Greek antiquity. Scanning the myriad of stars that constitute the Milky Way is one of the greatest spectacles of sky gazing.


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An early nineteenth century punch card for locating the stars of Ursa Major.

These are not strictly speaking celestial objects since they constitute a grouping of stars making the shape of a figure, in general that of an animal or a mythological being, this nomenclature dating back to the Ancient Greek times for the stars seen in the Northern Hemisphere. Charts are available that show a complete view of what stars are visible at any given time on Earth. Astronomy 101 is based on these star charts, enabling the reader to navigate amongst the stars, using the Polaris star as a celestial north-pointing compass to be able to find the brightest of the stars: the Andromeda galaxy or the most luminous star of the sky (Sirius of Canis Major) for example. Punch cards with holes for finding and aligning the stars of the constellations were once popular for star gazing. This nineteenth century punched star chart by British engraver Sidney Hall (right photo) is an example of one used for star gazing in the northern hemisphere.

During extended sky gazing you will occasionally notice streaks of light crossing the sky very quickly: "shooting stars". These are meteoroids that often weigh less than a gram but ignite when heated up by friction as they penetrate into the denser terrestrial atmosphere. One can see several dozen in one night. Some nights are especially favorable for their observation because the Earth, in its orbit, regularly crosses clouds of meteoroids well known to astronomers (see "meteor shower" for the dates).

Other phenomena are also visible to the naked eye, such as the comets, interesting and sometimes magnificent like Halley's comet as seen in 1910. Also galaxies, star clusters and nebulae are visible, but only appear as small milky patches, save for the remarkable Pleiades in the constellation of Taurus where one can distinguish the different stars.

Binocular gazing

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M 42 and M 43: the nebula of Orion.

Binoculars are very useful when you wish to observe bright, large astronomical objects. Thanks to them it is possible to see lunar craters. In spite of the distance between us and the Moon, one can observe the relief of these craters along the terminator, the separation line between the illuminated and darkened parts of the Moon. Lunar features are emphasized in this zone where sunlight strikes at a low angle and casts long shadows. This spectacle is a good start to sky gazing due to its ease.

Binoculars are good for the observation of large nebulas and the occasional bright comet. The reason is due to their very nature: the binocular enlarges images and adds brightness compared to naked eye views. Very extended objects can be seen in their entirety due to the wide field of view (which may not be the case with a telescope and its greater magnification) and with improved clarity and contrast compared to the naked eye. The Orion nebula is one of the most luminous and one of the easiest to locate. It is situated in the constellation of Orion, a constellation visible in winter, big and easily recognizable with its rectangular form and a short row of three bright stars forming the belt of Orion. One can also observe the cluster called the Pleiades, a stellar group composed of over fifteen stars which can be found by extending one of the diagonals of the rectangle of Orion toward the Northwest. Also visible in the late summer, fall and winter is another striking spectacle which lies beyond our own Milky way galaxy, the Andromeda galaxy. Though faintly visible to the naked eye, locating this object requires one to know how to identify the main constellations (see Locations of the constellations). The constellation Andromeda is situated under Cassiopeia in relation to the pole star. While viewing the Beta star of Andromeda in the binoculars, one ascends very slightly toward Cassiopeia until one sights a first small star, then one ascends again very slightly until one sees a fuzzy patch of light which is the heart of the Andromeda galaxy. If sky conditions are good, you may also see a surrounding very diffuse oval that represents the arms of the galaxy. This vast collection of stars is located 2.5 million light years away! It is one of the more distant objects that can be seen with common binoculars.

With experience, so long as the binoculars are held steadily and with decent atmospheric conditions, viewers endowed with good vision will be able to discern the four Galilean moons of Jupiter, even with simple 8x35.

Choice of binoculars

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Their features are determined by two numbers: the first number indicates the magnification, the second the diameter of the lenses in the front, or aperture. Large apertures are recommended because they will collect more light and so reveal fainter objects, and give (for the same magnification) a larger field of view. Thus, while a birdwatcher might prefer a compact 8x35 binocular, a sky gazer will do better with a larger 10x50 glass.

Another number to consider is the exit pupil, which is the ratio of the objective lens diameter to the magnification. For optimal night viewing, the exit pupil should be greater than or equal to the diameter of the viewer's pupil under viewing conditions. For instance, assume the pupil enlarges to 5mm under given lighting conditions. 10x50 optics with an exit pupil of 5.0 would be preferable to 8x35's with an exit pupil of 4.375.

Observations with a refracting telescope

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A refracting telescope is an instrument containing at least two lenses which focus light into an image at the focal plane. An eyepiece situated at the focus functions as a magnifying glass which works by permitting the eye of the observer to focus on this image at very close range, causing it to appear magnified. A good refracting telescope can be an instrument that one retains all his or her life, even after the acquisition of a bigger telescope.

The usually small aperture and general precision of refracting telescopes makes them well suited to the observation of objects which are bright and detailed, such as the Moon and planets. Even a small model of 60 mm diameter will reveal some detail on planets, and much detail on the Moon.

Jupiter is an ideal target for the first-timer equipped with a refracting telescope. Its observation lets us see the four main companions of the planet which are the Galilean moons as well as some details of the surface of the planet. It shows how much astronomical observation is a discipline of patience. A better view will require a more powerful telescope whose operation requires a finer mastery of the basics of astronomy. However using a telescope less effective than all those sold nowadays Galileo discovered the moons of Jupiter and confirmed Copernicus's theory: the Earth moves around the Sun, and not the reverse which was commonly believed at the time!

Phases of Venus and evolution of its diameter apparent.

With a refracting telescope, it is also possible to follow the phases of Venus and the change of its visible diameter with the passing of the months. Mars appears like an orange disc, but often without much detail. One can also follow the fluctuation of its visible diameter over the year. When Mars is nearest to the Earth, it is possible to distinguish its polar cap.

The most distant bright planet that one can observe with the refracting telescope is Saturn. If the conditions of observation are good, it unveils the very beautiful spectacle of its rings. One can follow the change of their appearance. In 2002, they were seen at their widest angle and best presentation, and will be seen in profile in 2010. They will then completely invisible for a few days, after which they will again appear as a thin, bright line as the angle gradually increases once more. Meanwhile, their appearance changes from year to year. With experience it is also possible to distinguish the large moon Titan.

The refracting telescope is a suitable instrument for surveying the Sun, but extreme precautions must be taken to avoid burning the retina and permanent blindness. NEVER look directly at the sun through any telescope or binoculars. The only safe method is projection onto a screen, using an appropriate Sun filter to avoid burning or damage to the telescope optics. When these precautions have been taken, the Sun is revealed in all its glory. It can be seen to evolve from day to day and its rotation can be clearly observed.

A refracting telescope also allows us to distinguish binary stars, nebulae (M42) and globular clusters (M13). Finally, let us not forget the Moon, which offers a multitude of details: craters, mountains, etc. As with binoculars, observation of the terminator reveals the best detail, notably the relief of the Moon.

Choice of the refracting telescope

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The main optical problem of refracting telescopes is chromatic aberration (color fringing). When one observes a planet, the Moon, or a bright star at high magnification, it will be surrounded by a diffuse glow of unfocussed color, usually blue or violet. This effect can be minimized by the use of a lens with a long focal length, but this can result in an unwieldy instrument. Refractors can be made essentially free of false color using various panchromatic designs, many of which use three lenses (a triplet) as opposed to the two lenses (a doublet) found in the more common achromatic instruments. This system is costly. Refractors of this type can be less awkward because lenses of a shorter focal length can be used, resulting in a shorter telescope. It is difficult to construct refractors of more than 150 mm aperture because of the expense of the raw glass and the possibility of breakage in manufacturing. Let's add that a refractor is expensive in relation to a telescope of any other design of the same size. 60 mm diameter refractors are cheap, but from 100 mm and up they can be three times as expensive (or more) than a reflecting telescope of the same aperture.
On the other hand a refracting telescope can be transported easily because it doesn't readily go out of alignment. Also in a refracting telescope, the objective is not obstructed in part by the secondary mirror that one finds in reflecting telescopes, which enhances the quality of the picture, the full surface of objective being used to collect light. The best choice (but also the costliest) is an apochromatic telescope that corrects all aberrations (chromatic and spherical).

Observation with a reflecting telescope

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A reflecting telescope is made not of lenses but of mirrors. Being less costly to manufacture, one can, for the same price as a refracting telescope, acquire an instrument larger diameter that gives access to deeper space. Nevertheless, to take advantage of the power of a reflecting telescope, it is necessary to have a good observation site away from the lights of the city, otherwise the use of a good refracting telescope is preferable.

The major advantage of the reflecting telescope over the refracting telescope is its lower manufacturing cost, which allows the purchase, at a reasonable price, of an instrument of bigger diameter, giving a brighter image for observing distant and weakly luminous objects. Also chromatic aberration does not exist with this type of instrument, but the secondary mirror partially obscures the primary mirror, resulting in a loss of brightness of the order of 5 to 10%.

A reflecting telescope, unlike a refractor, needs some maintenance: the primary mirror has a certain degree of freedom in the tube and can in some cases (a physical shock for example) go out of alignment, requiring readjustment which can be done by the owner. This same mirror having a very fine coat of aluminium on its surface, deteriorates in contact with air and has a life of 8 to 10 years. Specialist firms can refurbish the mirror.

The Cassini Division in Saturn's rings

With a 150mm reflecting telescope, the viewer is able to distinguish the spiral arms of some galaxies and details in many star clusters. With such an instrument, most of the Messier objects can be appreciated in good detail. These instruments are also very useful when they are used for planet-gazing which they reveal, thanks to their better resolution, a multitude of details as the Great Red Spot of Jupiter, visible with a telescope of 200 mm or the Cassini Division in the rings of Saturn. It becomes possible to follow the changes in appearance of the main planets of the solar system with the passing of the months, and the craters of the Moon appear with all their details on the terminator.

A sufficiently powerful reflecting telescope (300mm) opens the way to hunt for the comets, the holy grail of the amateur astronomer who dreams of being the first to discover a new object to which they will give their name. Comet hunters constitute a world apart from the average astronomy hobbyist. Besides needing expensive instruments, comet research requires great rigor because it demands systematic observations, but some hobbyists count close to ten of these objects on their scorecard.

Whatever the type of observation, it is while getting involved in astrophotography that one gets the best out of your instrument. Lengthening the time of exposure, the improved brightness and contrast of the picture reveals the finest details. The best results are obtained using a CCD sensor connected to a computer. These sensors are contained in all electronic devices capable of taking images (webcam, digital camera, cell phone, etc.). The sensors in these devices can be used in CCD astrophotography, but the best pictures are taken with specialist monochromatic sensors. In any case, a little tinkering is necessary. The astronomy hobbyist who wants to become an amateur astronomer must start learning the fundamental principles of optics, as instruments of optimal performance cannot be bought in a store.

Types of reflecting telescope

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There are two popular types of reflecting telescopes : the Newtonian and the Catadioptric.

The Newtonian type telescope The Catadioptric type telescope (Schmidt)

Newtonian telescopes

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Newtonian telescopes are characterized by longish tubes, slightly shorter in their focal length and are composed of a parabolic main mirrors fixed to the bottom of the tubes with flat secondary mirrors close to the opening, oriented at 45°, that directs the light outwards through the eyepiece. Observation is therefore from the side of the tube. Unfortunately the open tube allows dust to enter which deposits on the mirror. Another disadvantage is that the temperature inside the tube is slightly higher than the temperature of the surrounding environment (at least at the beginning of the night); the hotter air, while escaping, creates turbulence that reduces the quality of the image.

Catadioptric telescopes

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Catadioptric telescopes combine specifically shaped mirrors and lenses to form an image. This is usually done so that the telescope can have an overall greater degree of error correction than their all-lens or all-mirror counterparts, with a consequently wider aberration-free field of view.

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A Schmidt uses all spherical surfaces, making it an instrument that is easy to manufacture. It employs a cassegrain design with a compound curve corrector plate at the front of the tube that "corrects" all the aberration a spherical system would have (and also gives the telescope the advantage of having a sealed tube). The secondary mirror in a Schmidt is mounted on the corrector plate.

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A Maksutov is similar to the Schmidt except it uses a thick concave corrector plate and the secondary is usually a silvered spot on the backside of the corrector (a "spot-mak"). This design has the added advantage over a Schmidt in that it never needs alignment, all optical elements are fixed in alignment.

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See also

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