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The current standard epoch for star catalogues etc. 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 was.
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.

تمرين:

When the Gregorian calendar was introduced in 1582,
the accumulated error in the Julian calendar was 10 days.
What was the error by the time the Gregorian calendar was adopted in Turkey, in 1927?

الحل:

1600 was a leap-year in both calendars,
but 1700, 1800 and 1900 were not leap-years in the Gregorian calendar.
Thus the Julian calendar was 13 days adrift, by 1927.

To avoid complications in calculating calendar dates,
astronomers number days in a continuous sequence called the Julian Date (JD).
(This system was devised by the French astronomer Joseph B. Scaliger, in 1582;
he named it, not after Julius Caesar,
but after his father - who was called Julius Caesar Scaliger.)
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 2000 January 1st was JD 2451545.0 .

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.
0h on 2000 January 1st was MJD 51544.0 .

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