On the first day of each month in the lunar calendar, when the moon moves between the sun and the earth, the moon faces the earth with its dark side and rises and falls with the sun, so people can't see it. At this time, the moon phase is called "new moon" or "new moon".

A solar eclipse is another example. The sun, the earth and the moon are also in a straight line. The moon is in the shadow outside the earth, and the moon cannot shine on the sun. At this time, it is an eclipse from the earth.

The change of the moon phase is very regular, and there is a cycle every month. Eclipses change faster, usually only a few hours from the beginning to the end. The new moon is the new moon after the second and third day of the lunar calendar. The solar eclipse must occur on the full moon, the fifteenth day of the lunar calendar.

Eclipse is a special astronomical phenomenon, which refers to the astronomical phenomenon that when the moon moves to the shadow part of the earth, part or all of it that can be irradiated by sunlight cannot be directly irradiated by sunlight, so that observers on the earth can't see the ordinary moon phase.