Modified Julian Day
On this page, you can learn about the
and how they relate to the
Modified Julian Day
numbers used in Teletext.
There is also a
form-based conversion page which
enables you to freely convert between these various systems.
Astronomers and other people who need to deal with events
separated by a large span use the Julian Day to refer to
time, rather than a date in a particular calendar.
The Julian Day is a number which simply increases by 1 every
mean solar day.
Because there are no discontinuities in the count, the elapsed
time between two events expressed as Julian Day numbers can be found
by simple subtraction.
Julian Day numbering was invented in 1583 by
the French scholar, Joseph Justus Scaliger.
He constructed the Julian Period, an interval of
7980 years, based on three cycles of years:
the 28-year solar cycle (the time taken before the
days of the week next align with the Julian year),
the 19-year lunar cycle (when the phase of the moon
aligns with the days),
and the 15-year Roman indiction cycles (used for
taxation, census and other legal purposes which
continued to be used in to the Middle ages).
Assuming all dates are reckoned in the Julian
calendar, for which it was named, the Julian period
began at 12 noon, 1st. January 4713 BC and will end
end at 12 noon, 1st. January 3268 AD, when all three
cycles will once again co-incide.
Days are numbered consecutively from zero within the
Julian Period, without any subdivisions in to
months or years.
Here are some Julian days for some interesting dates:
Gregorian Date Julian Day MJD
Noon 1752-09-14 2361222
Noon 1858-11-16 2400000
Midnight 1858-11-17 2400000.5 0
Noon 1858-11-17 2400001 0.5
Midnight 1900-01-01 2415020.5 15020
Noon 1996-09-03 2450330 50329.5
Midnight 2000-01-01 2451544.5 51544
Midnight 2100-01-01 2488069.5 88069
Midnight 2132-08-31 2499999.5 99999
Midnight 2132-09-01 2500000.5 100000
Although the Julian Day is very useful for astronomical
purposes, it does have some drawbacks:
To remedy these two inconveniences, the
Modified Julian Day
is defined as the
Thus MJD 0 is at midnight between the 16 and 17 November 1858 AD Gregorian.
For any date in the 20th. and 21st. centuries, the MJD will be at most
five decimal digits long.
Before Gaius Julius Caesar (654--710 AUC, 100--44 BC)
and the Julian Calendar named for him,
the Romans counted years of 355 days
ab urbe condita,
from the building of the city of Rome.
1 AUC is conventionally taken to be 753 BC, but as we
shall see, the correspondence between days and years
was sometimes a little confused.
Because the Roman year was 10 days short with respect to the solar year,
the start of the year quickly got out of step with the constellations.
The priesthood arbitrarily added and subtracted days and even months
which helped things drifting too far,
but even so, by 708 AUC (46 BC) the year was well
out of phase with the seasons.
- it begins at noon, rather than at midnight as is
This offset of 0.5 day makes it awkward to
talk about calendar days as single Julian day numbers.
- it is rather long, with all the dates in the current and
next centuries beginning with the decimal digits``24''.
For several years since his triumph over Pompey in 705 AUC (49 BC),
Julius Caesar had been in increasingly complete
command of Rome, regarded as semi-divine, and Emperor in all but name.
Caesar was also a gifted astronomer, and well aware of the
difficulties the existing calendar caused.
He decreed that 708 AUC (46 BC) should be lengthened to
445 days in order that his new calendar be inaugurated
in step with the constellations on 1st. January, 709 AUC (45 BC).
His new calendar had a year of 365 days plus a leap year every fourth
year with 366 days.
This made the average length of the year 365.25 days,
a good approximation to the real length of 365.24219 days,
and the Julian calendar continued almost unchanged for many centuries.
Julius Caesar's nephew and adopted heir,
Gaius Octavian Augustus (691--767 AUC, 63 BC--14 AD)
became first Roman Emperor in 727 AUC (27 BC) and renamed the
month Quinctilis to Augustus and stole a day from February to
give his month 31 days.
The base date for the calendar was moved from the founding of Rome
to various dates of local importance, and it was not until the
6th. century AD when the scholar Dionysius Exiguus made a study of Easter
days that the Anno Domini epoch we now use came in to being.
Although small, the Julian Calendar's error of 0.0078 days per year
mounted up over the centuries until by the late 1500s,
the error was a noticeable 12 days.
So in 1582, Pope Gregory XIII (1572--1585) decreed that the day after the
4th. October 1582 would be 15th. October 1582, and henceforth
century years would not be leap years unless they were
also divisible by 400.
England did not adopt the Gregorian calendar until 1752,
when the 2nd. September (Julian)
was followed by the 14th. September (Gregorian).
To explore these calendar systems further, a
form-based conversion page
which allows dates in the Julian or Gregorian calendars
to be converted to and from Julian Day and Modified Julian Day numbers.
This page was created on
It was last updated
Please send comments to Robin O'Leary
pdc at ro dot nu
Copyright (C)1996--2004 Robin O'Leary.
All rights reserved.