Julian Date
A continuous count of days since noon Universal Time on January 1, 4713 BCE.
The Julian Date (JD) is a continuous count of days and fractions of days since noon Universal Time on January 1, 4713 BCE (in the proleptic Julian calendar). It was introduced by astronomers to provide a single system of dates that could be used when working with different calendars and to simplify calculations involving intervals between dates.
The Julian Date system was proposed by Joseph Scaliger in 1583 and named after his father, Julius Caesar Scaliger, not after the Julian calendar. The starting point (Julian Day 0) was chosen because it is before recorded history and because it is the point at which three major cycles used in ancient chronology coincided.
Julian Dates are expressed as a decimal number, where the integer part represents the day number and the fractional part represents the time of day. For example: - JD 2451545.0 corresponds to noon on January 1, 2000 (UTC) - JD 2459000.5 corresponds to midnight on May 31, 2020 (UTC)
A variant called the Modified Julian Date (MJD) is defined as MJD = JD - 2400000.5, which both reduces the number of digits and places the integer boundary at midnight rather than noon.
Julian Dates are primarily used in astronomy, space science, and satellite tracking, where they simplify calculations involving time intervals and eliminate complications from calendar irregularities.