The current standard epoch is J2000.0; the
previous
one was B1950.0.
In this context, “B” signifies a Besselian year, which begins when
the mean longitude of the Sun is exactly 280°; this always occurs very
close to the start of the calendar year, but not always at the same instant.
For example, “B1950.0” represents the instant 1950 January 0.9235.
“J” signifies a Julian year, which is exactly 365.25 days long.
“J2000.0” represents midday on 2000 January 1, and every other Julian year
begins at an exact multiple of 365.25 days from then. In 1984 the International
Astronomical Union recommended that star positions should be calculated
on the basis of Julian years rather than Besselian ones.
Julian years are named for Julius Caesar, who is credited with the first reform of the calendar. The year (more accurately, the “tropical year”), is measured from one spring equinox to the next, an interval of 365.2421988 mean solar days. In the Julian calendar, most years have 365 days, with an extra day every fourth year (called a leap-year), thus averaging 365.25 days to a year, with an error of 1 day every 128 years.
By the 16th century the accumulated error was 10 days, and Pope Gregory XIII introduced the Gregorian calendar, where century years are only leap-years if they are divisible by 400; thus 1900 was not a leap-year, but 2000 is. The Gregorian year thus averages 365.2425 days to a year, with an error of 1 day every 3320 years. The extra 10 days were arbitrarily omitted, 1582 October 4th being followed by 1582 October 15th. The Gregorian calendar was adopted in different countries at various different dates over the next 350 years.
To avoid complications in calculating calendar dates, astronomers number days in a continuous sequence called the Julian Date (JD). The sequence starts on January 1st, 4713 BC, so that all known astronomical records have positive values of JD. The Julian date changes at midday, so that (in European countries) all observations on a particular night have the same Julian date. Midday on 1998 February 16th is JD 2450861.0 .
JD must be given to 5 decimal places for an accuracy of
1 second of time. A number in this format can cause problems in computing,
so many modern applications use the Modified Julian Date (MJD),
where
MJD = JD - 2400000.5,
meaning that MJD, like the calendar date,
changes
at midnight. Midnight on 1998 February 16th is MJD 50860.0 .
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