astronomy A calendrical scheme comprising a simple day count measured from a pre-historic instant, thus providing a reference base independent of perceived eras and years of the various calendars. Unlike the familiar day, the astronomical day begins at noon, its date that of the half-preceding regular day. Applied holistically around Earth, rather than by local time zones, each day begins at 12:00 Universal Time. Termed the Julian day number and identified by a prefixed JD, the count has its zero at 01 January 4713 BCE. The day number is augmented by fractions to represent instants within a day where needed. Values for the beginning and end instants of a selection of Gregorian dates are:
| JD 2 452 275.5 | 1 Jan 2002 | JD 2 452 276.5 |
| JD 2 452 640.5 | 1 Jan 2003 | JD 2 452 641.5 |
| JD 2 453 005.5 | 1 Jan 2004 | JD 2 453 006.5 |
| JD 2 453 371.5 | 1 Jan 2005 | JD 2 453 372.5 |
[The Astronomical Almanac for the Year 2002 (Washington: US Government Printing Office and London: HMSO, 2001), also Explanatory Supplement to the Astronomical Almanac (Mill Valley, CA: University Science Books, 1994).] cite 0-numbered days within calendars, e.g. 0 January 2003, which ends at JD 2 452 640.5.
Since all Julian dates from 1860 CE to 2130 CE begin with the same two digits, a modified scheme omitting those digits has been adopted. Termed the modified Julian date (MJD), it also reverts to having the day begin at midnight, thus MJD = JD - 2 400 000.5, giving, for example, 52 640 for 1 January 2003.
Astronomical time-keeping uses Terrestrial Time.
