The Julian Calendar
The Julian Calendar is named after Gaius Julius Caesar (100 - 44 B.C.), or, as he is more commonly known, Julius Caesar. Julius Caesar was a Roman politician and general, and was made the dictator of the Roman Republic after defeating his rivals until his assassination, which lead to the creation of the Roman Empire.
Credit: By Tataryn77 (Own work) [Public domain], via Wikimedia CommonsJulius Caesar is recognised for the creation of a calendar system that allowed the use of leap years, which add an extra day on every four years. This does assume that the orbital period of the Earth is exactly 365.25 days, although this was not quite accurate, as the actual day is 11.23 minutes or 0.0078 days less than this. The compound inaccuracy of that this led to resulted in the creation of the Gregorian or Reform Calendar in the sixteenth century.
Why Use It?
If the Julian Calendar is inaccurate enough that using it required entire days to be removed from a year - even if it took about 1,600 years before this needed to be done, what use is there for dates based on this calendar?
As well as local phenomenon Credit: NASA(within our own Solar System), astronomers are also interested in events outside our system, which are at the very least light years away, and potentially millions of light years in distance from us. One light-year is exactly 9,460,730,472,580.8 kilometres or approximately 5,878,625,373,183.608 miles, and is defined by the International Astronomical Union as the distance travelled by light in a vacuum in one Julian Year, that year being based on the Julian Calendar.
The Julian Date
To deal with distant events that have a periodicity that is unrelated to our own calendar, astronomers use a simple counting system called the Julian Date, or, in short, the JD. There are a number of related dates, such as the MJD (below) or the Julian Day Number (JDN).
Every calendar date has a different Julian Day which begins at noon UT (originally GMT). The reference date for the system is noon UT on 1st January 4713 BC. Currently, JDs have numbers in excess of 2,400,000 - said figure being passed on the 17th November 1858. This date is also used as a reference date known as the Modified Julian Date (MJD).
By using decimals in a Julian Date, an exact time of day can also be shown as one number, if admittedly, quite a long one. For example, 13:01 and 34 seconds on the 29th December 2011 would be shown as 2455925.0427546296.
It can be seen from the above figure that the Julian Date is much easier to handle in computations than the conventional calendar, even if it does look unwieldy to the eye. The UNIX timestamp is also based on the JD.