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.
Exercise:
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?
Click here for the answer.
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 .
Previous section:
Precession
Next section: Final exercise
Return
to index