The moon is an astronomical object revolving around the earth and the only permanent natural satellite on the earth. It is the fifth natural satellite in the solar system, and the largest of the planetary satellites in terms of the size of the planets around it (the main planets). After IO, the moon is the second largest satellite with known density.

The average distance between the moon and the earth is 384400 kilometers (238900 miles), or 1.28 milliseconds.

It is believed that the moon was formed shortly after the earth, about 4.51 billion years ago. There are several hypotheses about its origin. The most widely accepted explanation is that the moon was formed from debris left after a huge collision between earth and a Mars sized object, thea.

The moon rotates synchronously with the earth, always showing the same face, its proximal side marked by dark volcanic limestone, filling the space between bright ancient crustal highlands and prominent impact craters. According to the illuminance measurement of the earth's surface, it is the second brightest regular visible object in the earth's sky after the sun. Its surface is actually dark, although it looks very bright compared to the night sky, and its reflectivity is only slightly higher than that of worn asphalt. Since ancient times, the prominent position of the moon in the sky and the periodic cycle of its laws have made the moon an important cultural influence, affecting language, calendar, art and myth.

The gravitational effects of the moon produce ocean tides, human tides and a slight extension of the day. The current orbit distance of the moon is about 30 times the diameter of the earth, and its apparent size in the sky is almost the same as that of the sun, so that the moon almost completely covers the sun. This apparent visual size matching will not continue in the near future. The linear distance from the moon to the earth is currently increasing at a rate of 3.82 ± 0.07 cm (1.504 ± 0.028 in.) per year, but this speed is not constant.