Concept
In astronomy, a Julian year (symbol: a or aj) is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year. The Julian year is not a unit of measurement in the International System of Units (SI), but it is recognized by the International Astronomical Union (IAU) as a non-SI unit for use in astronomy. Before 1984, both the Julian year and the mean tropical year were used by astronomers. In 1898, Simon Newcomb used both in his Tables of the Sun in the form of the Julian century (36 525 days) and the "solar century" (36524.22 days), a rounded form of 100 mean tropical years of 365.24219879days each according to Newcomb. However, the mean tropical year is not suitable as a unit of measurement because it varies from year to year by a small amount, 6.14e-8 days according to Newcomb. In contrast, the Julian year is defined in terms of the SI unit one second, so is as accurate as that unit and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days. The Julian year is the basis of the definition of the light-year as a unit of measurement of distance. Epoch (astronomy) In astronomy, an epoch specifies a precise moment in time. The positions of celestial objects and events, as measured from Earth, change over time, so when measuring or predicting celestial positions, the epoch to which they pertain must be specified. A new standard epoch is chosen about every 50 years. The standard epoch in use today is Julian epoch J2000.0. It is exactly 12:00 TT (close to but not exactly Greenwich mean noon) on January 1, 2000 in the Gregorian (not Julian) calendar.