Julian Day
How to read it
Astronomers needed one number for a moment, free of months, calendars and reforms. The Julian Day is that number: the days, with a decimal fraction, since noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar.1
The day starts at noon, not midnight, so an observer’s whole night falls under one day number. The Modified Julian Date beneath it is the same count minus 2,400,000.5, which starts at midnight on 17 November 1858 and gives a smaller number.1
Where it came from
The starting point comes from the Julian period that Joseph Scaliger proposed in 1583: 7,980 years, the product of three old calendar cycles of 28, 19 and 15 years. All three last began together in 4713 BC.1 The Smithsonian Astrophysical Observatory introduced the modified count in 1957.1
Sources
- Julian day (Wikipedia, CC BY-SA 4.0)