Moon
The Moon is the only natural satellite of the Earth, and was sometimes called Luna (Latin for moon). The Moon is distinguished from the satellites of other planets by its initial capital letter; the other moons will be described below.
- Diameter: 3476 km
- Orbital radius: 384,400 kilometers (238,900 miles)
- Mass: 7.34 × 1022 kg
- Rotational period: 27.32 days
- Orbital period: 27.32 days
Since the moon's rotational period is exactly the same as its orbital period, we always see the same face of the Moon pointed towards the Earth. This synchronicity is a result of tidal friction slowing down the moon's rotation in its early history, a process known as tidal locking. As a result of this tidal locking, the Earth's rotation is also gradually being slowed down by the Moon, and the Moon is slowly receding from the Earth as the Earth's rotational momentum is transferred to the Moon's orbital momentum. The gravitional attraction that the Moon exerts on the Earth is the cause of tides in the sea. Tidal flow is synchronised to the Moon's orbit around the Earth.
The other side of the Moon, often incorrectly called "the dark side", was first seen on September 15 1959 when the unmanned Soviet probe Luna 2 was launched into an orbit over it. "The dark side" is a misnomer since it is lit up by the Sun in the same way as the near side, it is just that we never see it from our vantage point on Earth.
The Earth and Moon orbit about their common center of mass, which lies inside the Earth about 4700 km from the Earth's center. When viewed from Earth's North pole, the Earth and Moon rotate counter clockwise about their axes, Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise. The Moon's orbital plane about the Earth is inclined by 5 degrees with respect to the Earth's orbital plane about the Sun (the ecliptic plane). The Moon's orbital plane along with its spin axis rotates clockwise with a period of 18.6 years, always maintaining the 5 degree inclination. The points where the Moon's orbit crosses the ecliptic are called the lunar "nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending ) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.
The inclination of the Moon's orbit makes it rather unlikely that the Moon formed along with the Earth or was captured later; its origin is the subject of strong scientific debate. The most accepted theory states that the Moon originated from the collision between the young Earth and an impactor the size of Mars and was formed from material ejected from Earth as a result of the collision. New simulations published in August 2001 support this theory [1]. This theory is also corroborated by the fact that the Moon has all the same minerals as the Earth, albeit in different proportions.
The Moon exhibits different phases as the relative positions of the Sun, Earth and Moon change, appearing as the full moon when the Sun and Moon are on opposite sides of the Earth, and becoming invisible as the new moon when they are on the same side. The time between two full moons is 29.5 days; it is longer than the time it takes the Moon to orbit the Earth since the Earth-Moon system is orbiting the Sun. The phases are not created by the shadow of the Earth on the moon; instead, they are a result of our seeing only part of the illuminated half of the Moon. In the Northern hemisphere, if the right side of the Moon is dark, the light part is shrinking: the Moon is waning (moving towards a new Moon). If the left side is dark, the Moon is waxing (moving towards a full Moon). The mnemonic "DOC" represents this ("D" is the waxing Moon; "O" the full moon; and "C" the waning moon). In the Southern hemisphere, this is reversed, and the mnemonic is "COD". A french mnemonic is that the waxing moon at its first "premier" quarter phase looks like a 'p', and the waning moon at its last "dernier" quarter looks like a 'd'.
By what can only be a truly extraordinary coincidence, the apparent size of the Moon as seen from the Earth is almost exactly the same as the apparent size of the Sun, so that total solar eclipses are possible where the Moon almost completely covers the Sun and the solar corona becomes visible to the naked eye.
The Moon (and also the Sun) appear larger when close to the horizon. This is a purely psychological effect (atmospheric refraction and its larger distance actually causes the image of the Moon near the horizon to be slightly smaller); it is assumed that size judgments for overhead objects were not important during evolution of the cognitive apparatus and are therefore inaccurate. [2]
Various lighter and darker colored areas create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others. Craters and mountain chains are also prominent lunar features, whereas the lunar plains were believed by ancient astromomers to be water-filled seas, hence their names, such as the Sea of Tranquility. The Moon has figured prominently in various mythologies and folk beliefs. The Greek goddess Artemis and the Roman Diana were associated with the Moon, as were many other female gods (but notice that the Japanese goddess Amateratsu is associated with the Sun and her brother, Susanowo, with the Moon, an unusual inversion that Tolkien's invented Middle Earth mythology repeats, making Isil, the Moon, male, while Anar, the Sun, is female). The term lunacy is derived from Luna because of the folk belief in the Moon as a cause of periodic insanity. Folklore also stated that lycanthropes such as werewolves and weretigers, mythical creatures capable of changing form between human and beast, drew their power from the Moon and would change into their bestial form during the full Moon.
Humans first landed on the Moon on July 20, 1969 as the culmination of a Cold War-inspired space race between Russia and America. The first astronaut on the Moon was Neil Armstrong, captain of Apollo 11. The last man to stand on the Moon was scientist Harrison Schmitt, who as part of Apollo 17 walked on the Moon in December, 1972.
The term moon (never capitalized) is also used by extension to mean any natural satellite of the other planets. There are on the order of 100 moons in our solar system, and presumably many others orbiting the planets of other stars. Typically the larger Jovian planets have extensive systems of moons. Mercury and Venus have no moons at all, Mars has two tiny moons, and Pluto a captured companion called Charon.
The largest moons in the solar system (those bigger than about 3000 km across) are the Moon itself, Jupiter's "Galilean" moons Io, Europa, Ganymede, and Callisto, Saturn's moon Titan, and Neptune's captured moon Triton. For smaller moons see the appropriate planets.