Definition
The Moon is Earth's only natural satellite.
Basics
The Moon (Latin: Luna) is what people generally say when talking about Earth's moon. The moon can be seen from Earth. The moon is actually very large, about a quarter the size of the world, but because it is far away it looks very small. The gravity on the moon is one-sixth of the Earth's gravity. It means that you will be 6 times lighter on the moon than on Earth. The moon is a rocky and dusty place.
The Moon is lit up by the sun as it goes around (Orbits) the Earth. This means sometimes we can see the whole moon and other times only bits of it. This is because the moon gives out no light. We can only see the parts that are reflecting light from the Sun. These different stages are called Lunar Phases. It takes the Moon about 29.53 days (29 days, 12 hours, 44 minutes) to complete the cycle, from big and bright to small and dim and back to big and bright. As the moon passes between the Earth and Sun, this phase is called the New Moon. The next phase of the moon is called the waxing crescent, followed by the first quarter, waxing gibbous, then to a full moon. A full moon occurs when the moon is opposite the Earth, the farthest away from the sun. As the moon continues it's orbit is become a waning gibbous, third quarter, waning crecent, and finally back to a new moon. People used the moon to measure time and present day months are approximately equal in time to a lunar cycle.
Because the moon is tidally locked (always faces the same way) towards Earth there is a half of the moon we can never see. This is called the dark side of the moon even though the sun does shine on it.
The moon has fascinated people since there were people to see it. People often dreamed of standing on the moon and looking at Earth.
Before people stood on the Moon, some of the big countries in the world, the United States and Russia (which used to be part of the USSR) sent robots to the moon. These robots would fly around, crawl, or in the case of Luna 2, crash into the moon. The robots were the first things that humans made that touched the Moon.
Humans finally landed on the Moon on 21 July, 1969. Astronauts Neil Armstrong, Buzz Aldrin, landed their space probe (the Eagle) on the surface of the moon. Then, as half the world watched him on television, Armstrong climbed down the ladder of the Eagle and was the first to touch the Moon as he said, "That's one small step for man, one giant leap for mankind." Later, Buzz Aldrin hops out of the Eagle, and together, they put the American flag near the spot where they landed. Even though their footprints were left on the moon a long time ago, it is likely that they are still there. If you leave footprints on Earth, wind will blow the dirt around, filling in the footprints and making them disappear. Rain could also smooth out the dirt. But on the moon there is no wind or rain. Because of that, the footprints do not get filled in or smoothed out.
Some people say they have never actually been to the moon. They say that the pictures of men on the moon have all been faked, and that the samples of moon dust and moon rocks are not really from the moon. There are many arguments to show that these people are wrong, though.
More people landed on the moon between 1969 and 1972, when the last spaceship, Apollo 17 visited. Eugene Cernan of Apollo 17 was the last person to touch the moon since 2008.
Because it's smaller in size the Moon has less gravity than Earth (only 1/6 of the amount on Earth). So if you weigh 60 pounds on earth, you would only weigh 10 pounds on the moon. But even though the moon's gravity is weaker than the earth's gravity, it is still there. If you threw a ball in the air, it would still fall back down. However, it would fall much more slowly. And if you jumped up as high as you could on the moon, you would still fall back to the ground. You could jump and do lots of tricks in the air and not hurt yourself at all, even if you didn't land on your feet. That's because the moon's gravity wouldn't pull as hard as the earth's gravity does.
As it was said before, the moon has no rain or wind, so footprints would never wear away naturally.
The Moon also has no atmosphere. Without an atmosphere, you wouldn't be protected from heat or cold. Because of thisastronauts have to wear very heavy spacesuits to protect themselves and to carry oxygen to breathe. Of course, since the moon doesn't have a lot of gravity, the spacesuit wouldn't be as heavy as it would be on Earth.
In the Earth, the sky is blue because the blue rays of the sun bounce off the gases in the atmosphere, making it look like blue light is coming from the sky. But on the moon, because there is no atmosphere, the sky looks black, even in the daytime. And because there is no atmosphere to protect the moon from the rocks that fall from outer space, they crash right into the moon and make big dents called craters. The moon has thousands of these craters.
Topics of Interest
The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about 1,700 kilometres (1,100 mi)—a quarter the Earth's radius—beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the phases of the Moon, which repeat every 29.5 days (the synodic period).
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is quite different from that of the Earth. The Moon lacks a significant atmosphere and any bodies of water, which eliminates erosion due to weather; it does not possess any form of plate tectonics, it has a lower gravity, and because of its small size, it cools more rapidly. The complex geomorphology of the lunar surface has been formed by a combination of processes, chief among which are impact cratering and volcanism. The Moon is a differentiated body, possessing a crust, mantle and core.
The lunar maria (singular: mare) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and hence appear dark to the naked eye. The maria cover about 16 percent of the lunar surface, mostly on the near-side visible from Earth. The few maria on the far-side are much smaller, residing mostly in very large craters where only a small amount of flooding occurred. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh) and sinus (bay). The latter three are smaller than maria, but have the same nature and characteristics.
Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapour is quickly decomposed by sunlight and lost to space. Scientists have since the 1960s conjectured that water ice could survive in cold, permanently shadowed craters at the Moon's poles.
The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as the maria Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.
The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the line-of-sight spacecraft acceleration can be measured by small shifts in frequency of the radio signal, and the measurement of the distance from the spacecraft to a station on Earth. Since the gravitational field of the Moon affects the orbit of a spacecraft, it is possible to use these tracking data to invert for gravitational anomalies. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the far-side gravity field is thus only poorly characterized. The gravitational acceleration of the moon is 1.63 m/s2, about 16.7% that of earth's. Because weight is directly dependent upon gravitational acceleration, things on the moon will weigh only 16.7% of which they weigh on the earth.
The external magnetic field of the Moon is very weak in comparison to that of the Earth. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core) and the varying magnetization that is present is almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact-generated plasma cloud around the Moon in the presence of an ambient magnetic field.
The atmosphere of the Moon is very tenuous and insignificant in comparison with that of the Earth--less than one hundred trillionth the density of the Earth's atmosphere at sea level. For most practical purposes, the Moon is considered to be surrounded by a vacuum.
The giant impact hypothesis (sometimes referred to as the big whack, or, less frequently, the big splash) is the currently favored scientific hypothesis for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body that is sometimes called Theia (or Orpheus) for the mythical Greek Titan who ruled the Sun. Evidence for this hypothesis includes Moon samples which indicate the surface of the Moon was once molten, the Moon's apparently relatively small iron core, and evidence of similar collisions in other star systems.
Moon rock describes rock that formed on the Moon (Earth's moon). The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.
The orbit of the Moon around the Earth is completed in approximately 27.3 days. The Earth and Moon orbit about their common center of mass, which lies about 4,700 kilometres from Earth's center (about three quarters of the Earth's radius). On average, the Moon is at a distance of about 385,000 km from the center of the Earth, which corresponds to about 60 Earth radii. With a mean orbital velocity of 1.023 km/s, the Moon moves relative to the stars each hour by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by 5.1°, whereas the Moon's spin axis is inclined by only 1.5°.
Lunar theory attempts to account for the motions of the moon. There are many irregularities (or perturbations) in the moon's motion, and many attempts have been made over a long history to account for them. After centuries of being heavily problematic, the lunar motions are nowadays (see Modern developments) modelled to a very high degree of accuracy. Several aspects of lunar theory have accordingly become largely about history. But newly achieved levels of accuracy have also turned lunar theory into a sensitive instrument for new tests of physical theories.
A lunar eclipse is an eclipse which occurs whenever the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse. The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes. The next total lunar eclipse occurs on December 21, 2010.
A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun. One half of the lunar surface is always illuminated by the Sun (except during lunar eclipses), and hence is bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon) to 0% (new moon). The boundary between the illuminated and unilluminated hemispheres is called the terminator.
A Moon landing is the arrival of a spacecraft on the surface of the Moon. This includes both manned and unmanned (robotic) missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission on September 13, 1959. The United States's Apollo 11 was the first manned mission to land on the Moon on July 20, 1969.
The lunar effect is a pseudoscientific theory which overlaps into sociology, psychology and physiology suggesting that there is correlation between specific stages of the Earth's lunar cycle and deviant behavior in human beings. The claims of a correlation of lunar phases to human behavior do not hold up under scientific scrutiny. Over the past 30 years, even more evidence has emerged to stress that this is pseudoscience.
Although several pennants of the Soviet Union were scattered by Luna 2 in 1959 and by later landing missions, and U.S. flags have been symbolically planted on the Moon, no nation currently claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction (including nuclear weapons).
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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