The Moon is about 2000 miles in diameter and it has a mean distance from Earth of about a quarter of a million miles. It orbits the earth every 27.3 days and rotates upon its axis over the same period, 27.3 days. This is why the same face of the Moon is always turned towards us. We never see the back of the Moon from Earth.
The Moon appears to shine, but in fact it is only reflecting sunlight off its surface back to us on earth. The phases of the Moon, (when it appears to change shape from ‘new moon’ to ‘half moon’, ‘full moon’, etc back to new moon) are caused by the change in the amount of the sunlit face of the moon we can see from earth as it orbits us.
The Moon takes 27.3 days to orbit the Earth but the time between ‘new moons’ is 29.5 days. This is because the Earth itself moves as it travels around the Sun. Strictly speaking, we can see 59% of the Moon’s surface from Earth, although it always keeps the same face towards us. Its orbit is not exactly circular and its orbital speed varies according to its distance from us. This causes the Moon to seem to ‘rock’ to and fro over a period of several days, revealing areas just over the usual horizons.
We know from samples returned to Earth by the Apollo astronauts that the Moon and the Earth seem to be made from the same material but ‘Moon’ is only about three fifths as dense as ‘Earth’. This may be the result of material from the Earth’s ‘Mantle’ being thrown out into space and condensing to form the Moon after a gigantic Earth/Impactor event early in the history of the Solar System. (The Mantle is less dense than other parts of the Earth like the Core.) A cataclysmic event of this sort might also explain why our Moon is so big compared to the planet it orbits. No other planet in our Solar System has a moon so big compared to itself.
Earth has 81 times the mass of the Moon. Saturn, for example, has 4150 times the mass of Titan, its biggest moon, though Titan is bigger than our Moon, of course. Our Moon is solid with ‘craters’ and ‘seas’ easily visible. Craters are the result of cosmic bombardment billions of years ago. Some are 100 miles in diameter, some tiny. The more spectacular ones tend to be named after famous people, often scientists, Copernicus for example. They can rise 12000 feet above the surface. Seas, or ‘Maria’are smooth areas of the Moon’s surface, and tend to have romantic names like Sea of Clouds (Mare Nubium)or Bay of Rainbows (Sinus Iridum).
The surface layer of the Moon is about 13-30 feet deep and beneath it lies a half mile thick layer of fractured rocks. Beneath that lies solid rock and below that a central core perhaps 600-900 miles in diameter, which may be molten. However, the Moon has no magnetic field.
No doubt the Moon still has some surprises for us when next we venture back!
