Bodies which orbit planets are called moons. They vary in size. The Earth has only one moon. Some other planets have many moons, and some have none. When people write just ""the" moon", they are usually talking about the moon of the Earth. Earth's moon is written with a capital letter, "Moon". The Latin word for the moon is "luna", which is why the adjective used to talk about the moon is "lunar". For example, lunar eclipse.