Our Moon

This is 3D model of our Moon taken from here: https://solarsystem.nasa.gov/resources/2366/earths-moon-3d-model/ The Moon orbits around the Earth in about 28 days, the same time it takes to spin around its axis, so from Earth it does show always the same face. The surface visible from Earth has large regions called "Maria" or seas from latin and are the product of intense volcanism following the formation of the Moon. This happened when the solar system was still young and populated by many small bodies and planets. One of these, Theia, about the size of Mars, impacted the young Earth, from the collision the debris grew around the Earth and formed the Moon. At that time the Moon was much closer to the Earth and since then it is receeding and its orbital period is slowing down. So it is the rotation of the Earth, millions of years ago one year was composed by 400 days, because the Earth was spinning faster than nowadays.