In contrast to the Earth, which undergoes continuous changes, the Moon is a lifeless object without an atmosphere or water. The lack of water and air means that there is no weather on the Moon to erode the rock. The Moon has thus remained unchanged for thousands of millions of years.
In the early part of the life of our solar system, about 3. 5 to 4. 5 billion years ago, the Moon was struck by numerous meteorites, covering its surface with a layer of rock debris (regolith). This layer measures between a few centimeters and several meters in thickness. The numerous impact craters, mountains and valleys are the most important features of the Moon's surface.
It was thought for a long time that the dark patches on the Moon were masses of water. For this reason they were called maria (seas). Lighter areas were considered to be continents, or "land" referred to as terrae. The invention of the telescope and later space travel brought more detailed observations and examinations. The largest of the craters are surrounded by circular mountain ranges, while the maria are accompanied by longitudinal chains of hills.
The Bailly crater is the largest crater and can be easily observed from the Earth. It is 295 kilometers in diameter and almost 4000 meters deep. The largest crater lies on the dark side of the Moon and cannot be seen from the Earth.
Craters on the side of the Moon facing the Earth have been given the names of famous historical personalities such as Kepler, Copernicus or Plato. It is also the side on which all landings on the Moon have taken place.
Mountain ranges, such as the Haemus mountains, are several thousand meters high. They are the walls of huge old craters.
The dark side of the Moon had never been seen until 1959. In this year the Soviet Luna 3 space probe sent back the first images from there. This also put an end to speculations about the existence of creatures living on the Moon's unobservable side. Many of the craters on this side were named after philosophers and scientists of the former Soviet Union.
The Moon has no atmosphere to regulate its temperature. For this reason it varies between 100┬░ Celsius during the day and minus 170┬░ Celsius at night.
