The laudatory names of the moon in ancient Chinese poems include: round wheel, jade wheel, golden wheel, Yin Lun, ice wheel, frost wheel, solitary wheel, oblique wheel, laurel wheel, moon wheel, round mirror, jade mirror, gold mirror, water mirror, ice mirror, cold mirror, treasure mirror, Qin Jing, flying mirror, Yaotai mirror and so on.
The moon was formed about 4.5 billion years ago. Shortly after the formation of the earth, there are several hypotheses about its origin. The one supported by more factual evidence is the "theory of the origin of the big collision" formed by the debris generated by a huge collision between the earth and the Martian-sized celestial body "Theia". ?
On the front of the moon, there are a large number of huge impact craters filled with basalt lava flows from dark volcanoes, forming a vast plain called "Moon Sea". In fact, there is not a drop of water in the Moon Sea.
There are bright and ancient plagioclase highlands and obvious impact craters between the periphery of the Moon Sea and the Moon Sea. It is the brightest celestial body except the sun in the sky. Although it is very bright white, its surface is actually very dark, and its reflectivity is only slightly higher than that of old asphalt. Because the moon is very conspicuous in the sky, coupled with the regular moon phase changes, it has had a great influence on human culture such as myths and legends, religious beliefs, philosophical thoughts, calendar compilation, literature and art, customs and traditions since ancient times.
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Looking at the moon from the earth, the surface of the moon is not exactly half of it, because the moon swings like a balance. Observers on the earth will feel that the moon swings back and forth in the north-south direction during its orbit around the earth, that is, it swings like a balance in the dimensional direction, which is called "zonal libration" and the swing angle range is about 6 57'.
The phenomenon that the moon swings back and forth in the east-west direction, that is, the longitude direction, is called "libration", and the swing angle reaches 7 54'. In addition to these two main kinds of libration, the moon also has Sunday libration and physical libration. The first three kinds of libration are not the moon swinging, but the phenomenon caused by the change of the relative position between the observer and the moon. Only the physical balance is the moon itself swinging, and the swing is very small.
Because the orbit of the moon is elliptical, when the moon is at perigee, its rotation speed can't catch up with the revolution speed, so we can see that the eastern part of the moon reaches 98 degrees east longitude. On the contrary, when the moon is at apogee, its rotation speed is faster than the revolution speed, so we can see the western part of the moon at 98 degrees Darcy. This phenomenon is called Libra movement.
Because the orbit of the moon is inclined to the equator of the earth, the polar region will shake about 7 degrees when the moon moves in the starry sky, which is called Libra motion.
Furthermore, since the earth in distance to the moon is only 6 radius of the earth away, if the observer observes the moon from sunrise to sunset, the observation point will have a displacement of the earth's diameter, and the area with the longitude of the moon surface of 1 degree can be seen more often.
Baidu Encyclopedia-Moon