Moon Facts

recall the Moon’s diameter (3 500 km) and its approximate distance from Earth (380 000 km)
recall that the Moon’s rotational period and orbital period are both 27.3 days
demonstrate an understanding of why the far side of the Moon is not visible from Earth
describe how astronomers know the appearance of the Moon’s far side and how it differs from the near side
relate the lack of atmosphere to the Moon’s low gravity

The Moon has a diameter of approximately 3,500km and it is approximately 380,000km from Earth.

         
pics - NASA

Although it is not the largest natural satellite of a planet in the solar system it is very large compared to the size of the planet it orbits. Its diameter is only 3.7 times smaller than the Earth.

One of the most remarkable coincidences is that the size of the Moon as it appears in the sky is just about the same as the apparent size of the Sun. More about that later.

The Earth's gravitational field strength is about 10 N/kg ( a mass of 1 kg has a weight of 10N on Earth). On the moon it is much lower, around 1.6 N/kg. Gravity on the Moon is about 1/6th of what it is on Earth. The Moon has never had an atmosphere but if we gave it one (released lots of air on the Moon's surface) it would evaporate off into space after a short time as the Moon's gravity would not be strong enough to hold it.

Days and nights are 14 times longer than on Earth. There is a huge range of temperatures, from -170 to +130 degrees centigrade.

The Moon in Orbit

The Moon orbits the Earth every 27.3 days. Like the Earth the Moon is also spinning but with a period of 27.3 days. This is probably due to tidal forces between the Earth and the Moon.

The result of this is that the same side of the Moon is always facing the Earth. Until man made probes (the first being the Soviet Luna 3 probe in 1959) were sent to orbit the Moon we had never seen the far side.

The far side has a very cratered surface but very few maria. This is possibly because the crust is thicker on the far side so it is harder for molten rock to flow to the surface and form maria. It has been suggested that it would be a perfect place to locate a radio telescope as it would be free from interference from Earth communications.

The diagram below (not to scale) shows that the plane of orbit of the Moon is about 5 degrees to the ecliptical plane (the plane on which the Sun and planets are) which is why solar eclipses do not occur once a month but are very rare. Nevertheless the Moon in the sky stays close to the ecliptic (the path of the Sun in the sky) and passes through the constellations of the zodiac.