Astronomers have always been concerned with time and its measurements. If you read any astronomical text on the subject you are sure to be bewildered by the seemingly endless range of times and their definitions. There's universal time and Greenwich mean time, apparent sidereal time and mean sidereal time, ephemeris time, local time and mean solar time, to name but a few. Then there's the sidereal year, the tropical year, the Besselian year and the anomalistic year. And be quite clear about the distinction between the Julian and Gregorian calendars! (See the Glossary for the definitions of these terms.)

All these terms are necessary and have precise definitions. Happily, however, we need concern ourselves with but a few of them as the distinctions between them become apparent only when very high precision is required.

Calendars

A calendar helps us to keep track of time by dividing the year into months, weeks and days. Very roughly speaking, one month is the time taken by the Moon to complete one circuit of its orbit around the Earth, during which time it displays four phases, or quarters, of one week each, and a year is the time taken for the Earth to complete one circuit of its orbit around the Sun. By common consent we adopt the convention that there are seven days in each week, between 28 and 31 days in each month (see Table 1) and 12 months in each year. By knowing the day number and name of the month we are able to refer precisely to any day of the year.