Orbital Mechanics and Astrodynamics by Gerald R. Hintz

Author:Gerald R. Hintz
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

Julian Days

The Julian day system provides a unique number to all days that have elapsed since a selected standard reference day, January 1, 4713 B.C., in the Julian calendar. The days are in mean solar measure. The Julian day (JD) numbers are never repeated and are not partitioned into weeks or months. Therefore, the number of days between two dates can be obtained by subtracting Julian day numbers.

There are 36,525 mean solar days in a Julian century and 86,400 s in a day. The Julian century does not refer to a particular time system; but rather is merely a count of a fixed number of days. Ephemeris calculations are done in Julian days and Julian centuries.

A Julian day starts at noon UT instead of midnight, an astronomical custom that is followed to avoid having the day number change during a night’s observations. Julian days can be calculated from calendar dates between the years 1901 A.D. and 2099 A.D. as follows:

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