Lunarian Facts

The Moon

The Earth has just one natural satellite, the Moon. At night the Moon is the brightest thing that can be seen in the sky but it does not produce any light of its own: it only reflects sunlight.

The Moon travels once around the Earth in 27.3 days. Over this time, it also spins once on its axis. As a result, we always see the same side of the Moon. Photographs of the Moon's far side were first seen in 1959 when the Soviet space probe Luna 3 orbited the Moon and transmitted pictures back to Earth. These first photographs were of rather poor quality but, between 1966 and 1967, the United States sent a series of five Lunar Orbiters, which took thousands or photographs of the Moon's surface.

Only the half of the Moon facing towards the Sun is lit up: the rest is dark. As the Moon travels around the Earth, the proportion of the bright side visible to us changes. When the Moon is between the Earth and the Sun, we cannot see the lit side at all. This totally dark phase of the Moon is called New Moon. Full Moon is seen when the Earth is between the Sun and the Moon.

At Full Moon, the Sun, Earth, and Moon are sometimes in a straight line so that the Earth casts a shadow on the Moon. This is called a lunar eclipse. The Moon does not normally disappear from sight completely, but looks dark red or orange because some sunlight still reaches it after scattering from the Earth's atmosphere. When a lunar eclipse is taking place, it can be seen from anywhere on Earth, where the Moon has risen.

Maria: flat areas of dark volcanic rock. The large darker areas visible to the naked eye are maria. Before the telescope was invented, it was thought that these areas were seas of water. Maria is the Latin word meaning 'seas'. It is the plural form of 'mare' (pronounced maah-ray). Some dark areas were given other watern names in Latin, such as Oceanus Procellarum (Ocean of Storms) and Palus Nebularum (Marsh of Mists).

Craters: circular hollows with a raised wall around, and sometimes one or more peaks in the middle. Most lunar craters were created by impac of rocks falling from space long ago, soon after the Moon was formed. Some craters were flooded or completely covered when molten rock flowed out from the Moon's interior to make the maria.

One of the most interesting times to observe a particular lunar feature is when it is on the boundary between the dark and bright parts of the Moon, the region called the "terminator". The shadows cast by any raised features are very long at this time, helping to show them up most clearly.

The highest mountains on the Moon, the lunar Apennines, reach over 24,600 ft (7,500 m).

In the 1960s, both the USA and the former USSR developed spacecraft to investigate the Moon. The rivalry between the two great powers was seen by some as a "space race", but it is doubtful whether the USSR ever intended to put a Russian cosmonaut on the Moon. It is difficult to know because the Soviet work was shrouded in secrecy. Major successes were announced, but the world rarely heard anything about plans or failures. It was the Americans who announced in 1961 that they were preparing to land a man on the Moon before 1970.

The first steps in lunar exploration were taken by the former USSR. They launched a series of spacecraft towards the Moon, first called Lunik, the Luna In 1959, Lunik 3 took the first photographs of the far side of the Moon and, in 1966, Luna 9 soft-landed on the Moon, proving that it has a firm surface. Some scientists had feared that the Moon was covered by a thick layer of soft dust, which would have made landing very difficult, if not impossible.

Throughout the 1960s, the Americans prepared for a landing. The final stage was the Apollo project. In 1968, Apollo 8 orbited the Moon with three astronauts on board. On July 21, 1969 the historic moment came. Neil Armstrong stepped out of the lunar module or Apollo 11 and put a foot on the Moon. His words were beamed all over the world, "That's one small step for man, one giant leap for mankind." Later he said that he had made a mistake. He intended to say "a man", which would have made his meaning a little clearer.

In 1972 America sent a last manned flight to the Moon. The Apollo missions had been a thrilling episode in the exploration of space--but an enormously expensive one, too. The Apollo program alone was estimated to have cost $25 billion. The cost of the Soviet space program between 1958 and 1973 has been estimated at $45 billion.

The Apollo crews left several "moonquake" detectors on the surface of the Moon. Scientists have used the measurements of rock tremors, transmitted back to Earth, to work out the structure inside the Moon. THere seems to be a rocky crust about 30 miles thick. Under that there is a rocky mantle about 560 miles thick. The very center of the Moon may be partly molten, perhaps with a small iron-rich, solid core.

Diameter	2160 miles (3476 km)
Mass (kg)	7348 x 10^22 kg
Mean Density	3.34 g/cm^3
Escape Velocity	1.48 mi/s (2.38 km/s)
Length of Day (In Earth days)	29d12h44m
Sidereal Period (One revolution about Earth)	27d7h43m11.6s
Inclination to Ecliptic	5�8'43"

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