Moon is Earth's only natural satellite. It is the brightest object in the night sky but gives off no light of its own. Instead, it reflects light from the Sun. Like Earth and the rest of the solar system, the Moon is about 4.6 milliard years old.
The Moon is much smaller than Earth. The Moon's average radius (distance from its centre to its surface) is 1,737.4 km), about 27% of the radius of Earth. The Moon is also much less massive than Earth. The Moon has a mass (amount of matter) of 7.35 x 1019 tonnes. Earth is about 81 times that massive. The Moon's density (mass divided by volume) is about 3.34 g/cm3, roughly 60% of Earth's density.
Because the Moon has less mass than Earth, the force due to gravity at the lunar surface is only about 1/6 of that on Earth. Thus, a person standing on the Moon would feel as if his or her weight had decreased by 5/6. And if that person dropped a stone, the stone would fall to the surface much more slowly than the same stone would fall to Earth.
Despite the Moon's relatively weak gravitational force, the Moon is close enough to Earth to produce tides in Earth's waters. The average distance from the centre of Earth to the centre of the Moon is 384,467 km. That distance is growing, but extremely slowly. The Moon is moving away from Earth at a speed of about 3.8 cm per year.
The temperature at the lunar equator ranges from extremely low to extremely high: from about –173°C at night to +127°C in the daytime. In some deep craters near the Moon's poles, the temperature is always near –240°C.
The Moon has no substantial atmosphere, but small amounts of certain gases are present above the lunar surface. People sometimes refer to those gases as the lunar atmosphere. This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density) zone of particles surrounding an airless body. Mercury and some asteroids also have an exosphere.
The Moon has no life of any kind. Compared with Earth, it has changed little over milliards of years. On the Moon, the sky is black (even during the day), and the stars are always visible.
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?
Earth and Moon
Eclipses and occultations
An eclipse is when the light of the Sun or Moon is blocked such as the annular eclipse shown in the image at right on October 3, 2005, observed at Medina del Campo, Valladolid, España.
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.
On April 25, 1838, an occultation of Mercury by the Moon occurred when Mercury was visible to the unaided eye after sunset. An occultation of Venus by the Moon occurred "on the afternoon of October 14", 1874. An earlier such occultation "occurred on May 23, 1587, and is thus recorded by [Tycho Brahe] in his Historia Celestis".
Many of the meteorites that are found on Earth turn out to be from the Moon.
"Lunar origin [of lunar meteors] is established by comparing the mineralogy, the chemical composition, and the isotopic composition between meteorites and samples from the Moon collected by Apollo missions."
"Cosmic ray exposure history established with noble gas measurements have shown that all lunar meteorites were ejected from the Moon in the past 20 million years. Most left the Moon in the past 100,000 years."
Lunokhod-2 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.
At right is a view of the horizon on the Moon's solid surface taken by an Apollo 16 astronaut. The image shows a black sky without stars because the sunlight coming from the left is overwhelming.
At right is the result of an all Moon survey by the Lunar Prospector using an onboard neutron spectrometer (NS). Cosmic rays impacting the lunar surface generate neutrons which in turn loose much of their energy in collisions with hydrogen atoms trapped within the Moon's surface. Some of these thermal neutrons collide with the helium atoms within the NS to yield an energy signature which is detected and counted. The NS aboard the Lunar Prospector has a surface resolution of 150 km.
Gamma-ray spectrometers have been widely used for the elemental and isotopic analysis of airless bodies in the Solar System, especially the Moon ... These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit. Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet. ... They are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon." per the Wikipedia article on the GRS. "[T]he chemical element thorium [is] mapped [by a GRS], with higher concentrations shown in yellow/orange/red in the left-hand side image shown on the right."
"Lunar X-rays are thought to be produced by the scattering or fluorescence of solar X-rays from the Moon's surface (Schmitt et al., 1991)."
X-rays from the dark moon, or shadowed portion of the Moon, can be explained by radiation from Earth's geocorona (extended outer atmosphere) through which orbiting spacecraft such as the Chandra X-ray Observatory move.
Lunar craters "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."
"During its flight, the Galileo spacecraft returned images of the Moon. The Galileo spacecraft took these images on December 7, 1992 on its way to explore the Jupiter system in 1995-97. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. The dark areas are lava rock filled impact basins: Oceanus Procellarum (on the left), Mare Imbrium (center left), Mare Serenitatis and Mare Tranquillitatis (center), and Mare Crisium (near the right edge). This picture contains images through the Violet, 756 nm, 968 nm filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision."
The dark irregular mare lava plains are prominent in the fully illuminated disk. A single bright star of ejecta, with rays stretching a third of the way across the disk, emblazons the lower centre: this is the crater Tycho on the Near side of the Moon. But, on the far side, the full disk is nearly featureless, a uniform grey surface with almost no dark mare. There are many bright overlapping dots of impact craters. And, an almost complete lack of dark maria.
"The topography of the Moon has been measured with laser altimetry [using the sodium D2 line in the yellow] and stereo image analysis. The most visible topographic feature is the giant far side South Pole – Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and the largest known crater in the Solar System. At 13 km deep, its floor is the lowest elevation on the Moon. The highest elevations are found just to its north-east, and it has been suggested that this area might have been thickened by the oblique formation impact of South Pole – Aitken. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. The lunar far side is on average about 1.9 km higher than the near side."
"These images show a very young lunar crater on the side of the moon that faces away from Earth, as viewed by NASA's Moon Mineralogy Mapper [M3] on the Indian Space Research Organization's Chandrayaan-1 spacecraft. On the left is an image showing brightness at shorter infrared wavelengths. On the right, the distribution of water-rich minerals (light blue) is shown around a small crater. Both water- and hydroxyl-rich materials were found to be associated with material ejected from the crater."
"Very precise microwave measurements between two spacecraft, named Ebb and Flow, were used to map gravity with high precision and high spatial resolution. The field shown [at right] resolves blocks on the surface of about 12 miles (20 kilometres) and measurements are three to five orders of magnitude improved over previous data. Red corresponds to mass excesses and blue corresponds to mass deficiencies. The map shows more small-scale detail on the far side of the moon compared to the nearside because the far side has many more small craters."
"Unusual minerals in impact craters on the moon may not have originated on the moon, but may be from asteroids that created the craters"
"Future studies of the moon's composition will have to show that exposed surface rocks really come from the moon and were not delivered by impacts, especially for unusual or exotic minerals ... "We cannot infer the deep composition of the moon from rocks in the centers of large craters without more care than has been used to date".
"[C]omputer models [have been used] to simulate the formation of lunar craters by asteroid impacts and ... [in] some impacts much of the asteroid's material is not vaporized but is instead deposited in the center of the impact craters."
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