The period between one new Moon and the next which naturally averages 29.5 days, but the Western calendar uses, for convenience, a calendar month with a complete number of days, 30 or 31 (February has 28).

For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to February as a 29th day every fourth year (leap year), century years being excepted unless they are divisible by 400; for example, 1896 was a leap year; 1900 was not.

A measure of the year based on twelve lunar months (the time it takes the moon to revolve around the earth), which lasts about 354.37 days.

The passage of the moon into the earth's shadow.