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.
Extended data:
The period of the moon phase is 29.53 days, which is called the new moon. It is the source of calendar months and weeks.
This time is longer than the moon's revolution (sidereal moon), because the moon revolves around the earth at the same time, and the moon will revolve around it for a lunar month (360+360 * 29.53/365.24) = 389.438+01degree (revolution only revolves around 360 degrees). So a sidereal month is about 29.53 * 360/389.11= 27.32 days.
The order of phase change of the moon is: new moon-waning moon-first quarter moon-full moon-last quarter moon-new moon, and so on. The phase change of the moon is periodic, and the period is about one month.
Baidu Encyclopedia-Moon Phase Change
Baidu encyclopedia-moon phase