Mercury and Venus do not have any natural satellites. Moon is our planet Earth’s natural satellite. Though it seems to be the largest object in the night sky, it is tiny compared to sun. As it is much closer to earth than the sun, it looks much bigger. Moon is quite close to earth compared to other planets too. It is quite far away from the man made satellites that are launched.
Over the period of time, it appears with different sizes. This is because during most of the month, only part of sunlit half of it is being viewed by us. This means only about 59% of moon’s surface could be seen from the earth. There is always the other side that cannot be seen by us. The moon takes about 27 days to revolve around the earth once. Also it takes almost the same time for one rotation. That’s why we can always see only one side of the moon and the ‘far-side’ of moon is never visible.
Over the course of month, however the moon does change in apparent size. This happens because orbit of moon is actually not a perfect circle. And when the moon is closer, it looks noticeably larger.
Sometimes two full moons appear during a single month. Such a second full moon is called a blue moon and is said to appear may be once in 2.7 years. The word ‘blue’ here does not talk about the color of the moon.
The diameter of moon is only one-fourth of that of earth. There is no atmosphere or ocean of water in moon which could have otherwise made moon habitable. The surface of the moon is also made up of different materials just like the earth. The gravity on the surface of moon is much less than that of earth. That is why we will weigh much less than what we weigh on earth.