Julian Day Number

The Julian day counts elapsed days from noon on 1 January 4713 BC, and it exists because comparing observations separated by centuries and by calendar reforms is thankless work. With a single increasing number, the interval between two observations is a subtraction. The proposal is Joseph Scaliger's, from 1583, and the name honours his father, Julius Caesar Scaliger — not the Julian calendar, a confusion four centuries old.

The rollover at noon rather than midnight is deliberate: astronomers observe at night, and a date change mid-session would force two dates onto a single night's work. That is why the Julian day of a date at midnight always ends in a half — the whole number falls at the following noon.

MJD fixes two practical irritations at once. Subtracting 2400000.5 from the Julian day shortens the number to five digits in the modern era and moves the rollover to midnight, in step with the civil day. The resulting epoch is 17 November 1858, and the form was popularised in the 1950s by satellite tracking, when every digit still cost money on a punched card. Today it is standard in ephemerides, telemetry and observation archives.