This arrangement keeps the same face of the moon turned toward the Earth. The distance to the Moon averages 384,400 km but can be as large as 406,700 km and as small as 356,400 km. The two parts of the shadows of the Earth and the Moon are the penumbra, within which sunlight is partially blocked, and the umbra, within which sunlight is completely blocked.
Total eclipses occur within the umbral shadow.
The Moon's umbral shadow is only about 150 km wide where it reaches the Earth. Eclipses can occur only when the Moon is in its new or full phase and when the Moon is near the ecliptic. Using their knowledge of the saros, ancient astronomers were able to predict eclipses. Tides cause the day to lengthen and the Moon to recede slowly from the Earth. In the remote future, the day and month may become equal in length.