The first object in space to be visited
by humans in 1969 is the Moon, which is 4.6 billion years old. It is the
second brightest object in the sky, besides the Sun, reflecting light
from the Sun at night; it radiates no light of its own. The Earth's
only natural satellite, the Moon moves around the Earth in an elliptical or oval-shaped
orbit at 36,800 km per hour, once every 29 days, 12 hours and 44 minutes. It also
rotates on its axis in the same amount of time. A day and a night on
th Moon lasts about 15 Earth days each. The unusual slightly egg-shape of
the Moon with the small end of the "egg" pointing toward Earth causes the
Moon to keep the same face toward Earth at all times. This is because its
period of rotation and revolution are the same; 29 days, 12 hours and 44
minutes. This cycle is known as the lunar month as seen from Earth. When the
Moon is closest to the Earth in its orbit, its distance from our Earth is
356,400 km; when the Moon is farthest from the Earth, its distance is
406,700 km from our planet. The Moon's diameter is about 3,476 km, about a
fourth that of the Earth's, and 400 times smaller than the Sun's.
The gravitational pull of the Moon on the Earth affects the tides on Earth. The closer the Moon is to Earth, the greater the attraction. The time between high tides is 12 hours and 25 minutes. Surface temperatures range from -184 degrees Celsius during its night to 214 degrees Celsius in the day, except at the south and north poles where the temperature is a constant -96 degrees Celsius.
The most prominent features of the Moon are its many craters. Since the Moon has no atmosphere and no liquid water, there is no erosion taking place. This has preserved the many thousands of craters on its surface. The surface is coverd with regolith, a fine grain soil which results from the constant bombardment of the Moon's surface by small meteorites. Most rocks on the surface seem to be between 4.6 and 3 billion years old. Information from moon rocks led to the impact theory that millions of years ago, the Earth collided with a large object, perhaps the size of Mercury. The resulting impact flung molten rock far out into space where it cooled and formed the Moon we see today. This theory explains why the Moon is made of mostly lighter material than the Earth, and why it is so large. The impact theory is now the most widely accepted theory of the Moon's origin.
Bibliography and Additional Sources of Information
The Earth, Sun and Moon.
Sea and Sky: The Cosmos.
Sea and Sky: The Cosmos. www.seasky.org/sky7.html