The moon is the only eternal celestial body in the sky and its shape can be seen every night. The sun always shines brightly in the sky, and other planets and stars always twinkle in the night sky. Some comets also have their own unique shape changes, but we don't always see them in the night sky.
The shape of the moon always undergoes a series of specific, gradual and repeated changes. On some special nights, the moon will appear in the western sky in the shape of a new moon after sunset. After the night passed, the moon gradually moved eastward, and its thin crescent moon gradually became "fat". After about a week, it gradually turned into a semicircle, and continued to change in a more "fat" direction until another week passed, and finally a "full moon" was formed. After that, it began to get thinner and smaller, and returned to a semi-circular state a week later (however, the other half of the moon brightened this time). Then, another week passed, and finally, before dawn, when it appeared in the eastern sky, it became a crescent moon again. It will disappear in the next few nights, and then reappear, repeating the above change process again.
People naturally think that the moon is like a living body. It grows from birth until it reaches the peak of its life, then it will slowly age and eventually die. Go through this series of stages in one month. Even today, we still call the thin crescent moon that appears in the western sky "new moon", while the thin crescent moon that appears in the eastern sky for one month is called "waning moon" and the full moon in the middle of this process. As I explained earlier, we define the complete period of the moon as "one month". The first calendar was produced on this basis. But why are these changes happening? Is there really a new moon "born" every month?
Thales, the ancient Greek philosopher, didn't think so, and Babylonian astronomers before him probably didn't think so either. The reason for this doubt comes from the research on the change of the position of the sun during the mid-month appearance. At the beginning, it is natural to imagine that the natural laws followed by everything on earth are different from those followed by celestial bodies in the sky. On the earth, everything is falling; In the sky, all the celestial bodies are circling. On the earth, everything is constantly changing until it fails; In the sky, everything seems eternal. Matter on the earth does not emit light; In the sky, all the celestial bodies are constantly flashing. If the materials that make up the moon keep shining like the sun, planets or stars, then the moon will also become an eternal light ball in the sky. Because the moon is not an eternal ball of light, it will grow and age in a month, and it is not permanently luminous. If the moon is as dark as the earth and can only shine by reflecting sunlight, then the sunlight reflected by different parts of it depends on the position of the moon in the sky.
For example, if the moon happens to be between the earth and the sun, then the sun will shine on the side of the moon facing away from us. Therefore, we can't see anything from the moon. When the moon moves from west to east at the speed of 12 times that of the sun, it will appear a little east of the sun the next night, so we can just see a thin crescent on the illuminated western edge of the moon, which appears in the form of a "new moon". As the moon continues to turn eastward, the part we can see will become bigger and bigger, so the "new moon" will gradually become "fat". When the moon turns to 1/4 in the sky, compared with the sun, its west side is illuminated, so we see a bright half moon. Then continue to change until the moon turns to the opposite side of the sun in the sky. At this time, sunlight passes through both sides of the earth and illuminates the whole side of the moon facing us, so we see a "full moon".
Later, the moon gradually caught up with the sun, and the illuminated part we saw began to shrink gradually. A week later, only the eastern half was still on, and then it gradually turned into a crescent moon. At this time, the moon overtook the sun again, and then the above-mentioned change process was repeated. Anyone who carefully considers the above changes will finally come to a conclusion: the moon, like the earth, can't shine by itself, but only by reflecting sunlight.