Why does the moon change? As you know, the moon itself does not shine, but it reflects some sunlight. Therefore, for observers on the earth, with the change of the relative positions of the sun, the moon and the earth, the moon takes on different shapes on different days, which is the periodic change of the moon phase. Furthermore, although the moon is illuminated by the sun, there is always a hemisphere that is bright. However, because the moon keeps going around the earth and its position is constantly changing, the hemisphere facing the earth sometimes coincides with the hemisphere irradiated by the sun, sometimes it does not coincide at all, sometimes a small part coincides, and sometimes most of it coincides, so the moon shows that the weather has changed.
The moon on the fifteenth and the circle on the sixteenth are astronomical phenomena. According to astronomical experts, the Mid-Autumn Festival in 20 16 (20 16 September 15) started from the 16 full moon during the Tenth Five-Year Plan period, and it will be the same for the next four years. By the Mid-Autumn Festival in 2020, it will be "May 15th".
This is because we can only see the full moon when the longitude difference between the moon and the sun is 180 degrees and the moon and the sun are completely opposite to the earth. However, the moon's orbit around the earth is an ellipse, which is both near and far. The nearest orbit is 360,000 kilometers, and the farthest orbit is 400,000 kilometers. Because of gravity, the moon moves faster in the near direction and slower in the far direction. If the first half of the month is fast, it will arrive at the full moon position on time. If you slow down at 15, it will be "late" and reach the full moon position at 16 or 17. It is not surprising that the tenth five-year plan is not in place. What needs to be explained to the masses is 1. Every 29.5 days, the moon will be full, not just in August 15 or 16. 12 or 13 times a year. 2. The best time to observe the lunar surface with telescope is from 7th to13rd and19th to 23rd. On the roundest day, the shape of the Moon Mountain is not clear.
When the moon is between the sun and the earth, its dark hemisphere is facing us. We can't see any images of the moon at all. This is the "new moon". In astronomy, the new moon refers to the same time of the moon and the ecliptic. At the new moon, the moon and the sun rise from the east at the same time. Even if the earth reflects sunlight to the moon, the part of the light reflected by the moon is completely submerged in the strong sunlight. When the earth is between the moon and the sun, although the three planets are also in a straight line, the moon hemisphere illuminated by the sun is facing the earth at this time, and the soft moonlight shines on the earth all night. This is the full moon, which is "looking". At this time, the distance between the moon and the sun is 180 degrees.
Because the distance from the moon to the earth is much shorter than the distance from the sun to the earth. On the celestial sphere, the moon moves eastward much faster than the sun. The moon moves more than 13 degrees from west to east every day, while the sun only moves 1 degree. Therefore, after the new moon, the moon quickly ran to the east of the sun. A day or two later, as soon as the sun goes down, you can see a new moon in the western sky, with its two corners facing east. From then on, the moon rose later and more round. About seven days after the new moon, the yellow meridian of the moon is 90 degrees higher than that of the sun. The moon we see is a semicircle with an arc to the west, which is the first quarter of the moon. After that, the moon continued to move eastward, becoming more and more round, and then rose again until it was seen. From the new moon to the sun, the moon is getting farther and farther away. Looking back, the moon is getting closer to the sun and losing weight. When the yellow meridian of the moon exceeds the yellow meridian of the sun by 270 degrees, it becomes a semicircle again, but the arc faces the east, which is the last quarter of the moon. At this time, the sun rises in the east and the moon hangs in the southern sky. Naturally, we can't see the moon with our naked eyes at this time. After the last chord, the moon came out of the east in the middle of the night. Its semicircle gradually disappeared and became a narrow sickle at an acute angle with the west. From the new moon to the sun, the distance between the moon and the sun is getting closer and closer, so they disappear into the morning light like the yellow meridian of the sun.
The best days to observe the surface of the moon with a telescope are the 7th to13rd and19th to 23rd of the lunar calendar. On the roundest day, the shape of the Moon Mountain is not clear. Because of gravity, the speed of the moon's revolution around the earth is fast and slow, fast when it is near and slow when it is far away. Because of the speed of the moon's rotation, the time of "seeing" is different from month to month. It may be the 14th, 15th, 16th and 17th lunar calendar. Most of them are 15 years old and 16 years old.