fifth largest satellite
It is the sole natural satellite of the world and therefore the fifth largest satellite within the system . The diameter of the moon is slightly larger than a quarter of the earth, and its mass is about 1/81 of that of the earth.
It is the largest satellite in volume and mass relative to the planets around it, and the second most dense satellite in the solar system, second only to Io.
It is generally believed that the moon was formed about 4.5 billion years ago, shortly after the earth appeared. There are several hypotheses about its origin, but none of them can be completely and reasonably explained without flaws.
The most widely recognized theory is the Great Collision Theory, which assumes that the moon was formed between the Earth and Mars-sized "Theia". A huge impact between.
rotation is synchronized with its revolution
Its rotation is synchronized with its revolution (tidal lock), so it faces the earth on the same side. The front of the moon is marked by a dark sea of volcanic lava, with bright and ancient crust highlands and obvious craters in the middle.
Observed from the earth with the naked eye, the moon is the brightest celestial body except the sun. Although it looks very bright, its surface is actually very dark and its reflectivity is only slightly higher than that of the old asphalt.
Because the moon is very easy to be observed in the sky, coupled with regular moon phase changes, it has had a major impact on human culture such as calendars, art, and mythology since ancient times.
distance between the moon & earth
The gravitational influence of the moon causes the tides of the earth's oceans and the time of each day is prolonged. The distance between the moon and therefore the earth is about 30 times the diameter of the world . In other words, there is space left between the earth and the moon by squeezing the eight planets of the solar system plus Pluto.
The size ratio of the moon to the sun is similar to the distance ratio, making its visual size almost the same as that of the sun. During a solar eclipse, the moon can completely cover the sun and form a total solar eclipse.
The moon is that the first extraterrestrial planet where humans have ever landed. The former Soviet Union’s Moon Program launched the first unmanned spacecraft to land on the moon in 1959, and NASA’s Apollo program is the only manned mission to the moon so far.
Apollo 8 manned orbited the moon in 1968, and Apollo 11 landed on the moon for the first time in 1969. By 1972, mankind had successfully landed on the moon six times. These missions brought back more than 380 kilograms of lunar rocks, some of which were used to study the geology of the moon to understand the origin of the moon (relevant studies put forward the hypothesis that the moon formed a huge impact 4.5 billion years ago), and the internal structure of the moon was formed And the history of the moon after its formation.
After Apollo 17 in 1972, only unmanned spacecraft continued to visit the moon, the most notable of which was the Soviet lunar walker rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have all launched moon-orbiting satellites. These space probes confirmed the presence of water ice in the soil of the permanently dark crater on the moon's polar regions. In 2019, China's probe Chang'e-4 landed on the back of the moon for the first time and surveyed.
There are plans for manned re-landing and exploration of the moon. For example, the US plan for returning to the moon and the Chinese lunar exploration project have been confirmed and implemented, but the details are still in the research stage and have not been implemented in a short time. Now under the outer space treaty, the moon is still a place where all countries can freely go to explore for peaceful purposes.