Julian date JD

The comparison of observations over many years is not simple because of the varying numbers of days in a year (due to leap years) and the different numbers of days in the several months. Accordingly, a continuously running counting system for days is used in astronomy; these are known as Julian dates (JD). The Julian date 0.0 was set prior to most dates one would encounter in astronomy, namely at noon on Jan. 1 in 4713 BC. It was defined by Justus Scaligerin 1582 at the time of the initiation of the Gregorian calendar and named after his father Julius Caesar Scaliger, not the Roman emperor. We discussed the several calendars in Section 3 above.

Julian days are counted as continuously running numbers which are now approaching 2.5 million. The Julian day beginning at noon on 2000 Jan. 1 is JD 2 451 545, while for Jan. 2 it is JD 2 451 546, etc. The Astronomical Almanac gives equivalences between Gregorian dates and Julian dates (JD). The beginning of each Julian day is now defined to be at noon at Greenwich, either 12 h UTC or 12 h TT. More precise times within a Julian day are indicated with decimal figures, not h m s. If one uses JD with precision of 1 minute or better, it is important to specify the units one is using, such as JD(TT), JD(TDB), or JD(UTC), because, for example, JD 2 451 545.000 00 (TT) and JD 2 451 545.000 00 (TDB) both occur about one minute before JD 2 451 545.000 00 (UTC).

Julian days can be grouped conveniently into centuries of 100 yr. Now the number of days in a Gregorian century will vary depending on the number of leap years in that given century. This motivates reference to the (Roman) calendar wherein each year has exactly 365.25 d (i.e., a leap year every 4 yr without fail) so that a Julian century always has 36 525 d. Thus J2100.0 will be exactly 36 525 d later than J2000.0. If further, the Julian day is defined with TT time wherein every day has exactly 86 400 s (no leap seconds), we have a system wherein every Julian century has 86 400 s/d x 36 525 d/C = 3 155 760 000 s, and where each second is the atomic (or TT) second, also known as the SI second.

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