When a thin crescent of the Moon is visible just before sunrise, the Moon is in its waning phase. This means the moon is moving toward the new moon phase, and the illuminated part that is visible is a small crescent shape on the left side (as seen from the northern hemisphere), typical of the waning crescent phase seen in the early morning sky before sunrise.

More specifically:

• The waning crescent moon rises in the early morning hours before sunrise and sets in the afternoon.
• This phase occurs as the Moon completes its orbit approaching the new moon.
• The crescent appears thin because most of the Moon's illuminated side faces away from Earth during this time.