What are the shorthand formulas for moon phase change?

"Up to the west and down to the east" is the shorthand formula of the moon phase. "Up to the west" means that the first quarter moon appears in the first half of the lunar calendar, and the moon faces west and is located in the western half of the sky (the concave surface faces east).

"Last quarter moon" means that the last quarter moon appears in the second half of the lunar calendar, and the moon faces east (concave faces west) and is located in the eastern half of the sky.

Related introduction:

First quarter moon: The moon continues to rotate forward. On the seventh and eighth day of the lunar calendar, the sun sets and the moon is 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".

First quarter moon: After the full moon, the moon rises later every day, and the bright part of the moon gets smaller every day until the 23rd day of the lunar calendar. 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

Correlation principle:

The moon revolves around the earth, so the relative positions of the sun, the earth and the moon change regularly within a month. Because the moon itself is not luminous and opaque, the visible part of the moon is the part that reflects sunlight.

Only the part of the moon that is directly illuminated by the sun can reflect sunlight. We see the part of the moon that is directly illuminated by the sun from different angles, which is the source of the moon phase. The moon phase is not caused by the earth covering the sun (this is an eclipse), but because we can only see the part of the moon illuminated by the sun, and its shadow part is the dark side of the moon itself.