There are three units of time which have a direct basis in astronomy: the day, which is the period of time it takes for the Earth to make one rotation around its axis; the month, which is the period of time it takes for the Moon to revolve around the Earth; and the year, which is the period of time it takes for the Earth to make one revolution around the Sun.

The week has an indirect basis in astronomy—the seven days of the week probably were named for the seven objects which the ancients saw moving on the zodiac, which were the Sun, Moon, and the five planets that can be seen with the naked eye.

A calendar is a system for measuring long units of time, usually in terms of days, weeks, months, and years. The year is the most important time unit in...