As shown in the figure, at the position of 1, the longitude difference between the sun and the moon is 0. At this time, the moon is located between the earth and the sun, with its dark side facing the earth and appearing almost at the same time as the sun, so it can't be seen on the ground. This is the new moon, the first day of the lunar calendar.

New Moon: The moon keeps spinning forward. On the eighth day of the seventh lunar month, that is, position 3 in the picture, the longitude difference is 90. The sun went down and the moon was already overhead. It didn't fall until midnight. At this time, just half of the moon illuminated by the sun is displayed in front of you. This is the so-called "first quarter moon".

Full Moon: On the 15th and 16th day of the lunar calendar, the moon turns to the other side of the earth, that is, position 5 in the figure, with longitude difference of 180. At this time, the earth is between the sun and the moon, and the half moon illuminated by the sun is just facing the earth. At this time, we see the full moon, or "looking".

First quarter moon: After the full moon, the moon rises later every day, and the bright part of the moon gets smaller every day. By the 23rd day of the lunar calendar, which is position 7 in the figure, the longitude of the Yellow River is 270. The full moon is half lost, and then half a moon only appears in the eastern sky in the middle of the night, which is the "lower chord".

Extended data

Moon phase change song

The new moon on the first day is invisible only because it is trapped in the sun and the earth.

On the eighth day of the seventh lunar month, the half moon faces west.

When the full moon comes out on the 15th and 16th, the earth shoulders the sun and the moon.

Twenty-two, three and the last quarter of the moon, the moon faces east until midnight.

A formula (convenient for memory): upper west, lower east-meaning: the first quarter moon appears in the first half of the lunar calendar, and the moon faces west, located in the western half of the sky (the concave surface faces east); The last quarter moon appears in the second half of the lunar calendar, and the moon faces east (concave faces west), which is located in the eastern half of the sky.

Baidu encyclopedia-moon phase