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RESOLUTION B1

ON THE USE OF JULIAN DATES
The XXIIIrd International Astronomical Union General Assembly,


recognizing

a. the need for a system of continuous dating for the purpose of analyzing time-varying astronomical data, and

b. that both Julian Dates and Modified Julian Dates have been employed for this purpose in astronomy, geodesy, and geophysics.

recommends

a. that Julian Date (as defined in the appendix) be used to record the instants of the occurrences of astronomical phenomena,

b. that for those cases where it is convenient to employ a day beginning at midnight, the Modified Julian Date (equivalent to the Julian Date minus 2 400 000.5) be used, and

c. that where there is any possibility of doubt regarding the usage of Modified Julian Date, care be exercised to state its definition specifically,

d. that, in all languages, Julian Date be abbreviated by "JD" and Modified Julian Date be abbreviated by "MJD".

APPENDIX. PROPOSED DEFINITIONS

The following definitions are recommended

1. Julian day number (JDN)

The Julian day number associated with the solar day is the number assigned to a day in a continuous count of days beginning with the Julian day number 0 assigned to the day starting at Greenwich mean noon on 1 January 4713 BC, Julian proleptic calendar -4712.

2. Julian Date (JD)

The Julian Date (JD) of any instant is the Julian day number for the preceding noon plus the fraction of the day since that instant. A Julian Date begins at 12h 0m 0s and is composed of 86400 seconds. To determine time intervals in a uniform time system it is necessary to express the JD in a uniform time scale. For that purpose it is recommended that JD be specified as SI seconds in Terrestrial Time (TT) where the length of day is 86,400 SI seconds.

In some cases it may be necessary to specify Julian Date using a different time scale. (See Seidelmann, 1992, for an explanation of the various time scales in use). The time scale used should be indicated when required such as JD(UT1). It should be noted that time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as UTC, may need to be corrected for changes in time scales (e.g. leap seconds).

An instant in time known in UTC can be converted to Terrestrial Time if such precision is required. Values of TT-UT are available using tables in McCarthy and Babcock (1986) and Stephenson and Morrison (1984, 1995). Table 1 provides the difference between TAI and UTC from 1961 through 1 January 1996. The difference between TT and UTC can be calculated knowing that TT = TAI + 32.184s. The Annual Reports of the International Earth Rotation Service should be consulted for dates after 1996. The data of Table 1 are also available electronically at

http://hpiers.obspm.fr or
ftp://hpiers.obspm.fr/iers/bul/bulc/UTC-TAI.history
or at
http://maia.usno.navy.mil or
ftp://maia.usno.navy.mil/ser7/tai-utc.dat.

Differences between the TAI and UTC time scales.

TT-UTC can be calculated by adding 32.184s to TAI-UTC.
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1961 Jan 1 JD 2437 300.5 TAI-UTC = 1.4228180s + (MJD - 37300.) x 0.001296s
1961 Aug 1 JD 2437 512.5 TAI-UTC = 1.3728180s + (MJD - 37300.) x 0.001296s
1962 Jan 1 JD 2437 665.5 TAI-UTC = 1.8458580s + (MJD - 37665.) x 0.0011232s
1963 Nov 1 JD 2438 334.5 TAI-UTC = 1.9458580s + (MJD - 37665.) x 0.0011232s
1964 Jan 1 JD 2438 395.5 TAI-UTC = 3.2401300s + (MJD - 38761.) x 0.001296s
1964 Apr 1 JD 2438 486.5 TAI-UTC = 3.3401300s + (MJD - 38761.) x 0.001296s
1964 Sep 1 JD 2438 639.5 TAI-UTC = 3.4401300s + (MJD - 38761.) x 0.001296s
1965 Jan 1 JD 2438 761.5 TAI-UTC = 3.5401300s + (MJD - 38761.) x 0.001296s
1965 Mar 1 JD 2438 820.5 TAI-UTC = 3.6401300s + (MJD - 38761.) x 0.001296s
1965 Jul 1 JD 2438 942.5 TAI-UTC = 3.7401300s + (MJD - 38761.) x 0.001296s
1965 Sep 1 JD 2439 004.5 TAI-UTC = 3.8401300s + (MJD - 38761.) x 0.001296s
1966 Jan 1 JD 2439 126.5 TAI-UTC = 4.3131700s + (MJD - 39126.) x 0.002592s
1968 Feb 1 JD 2439 887.5 TAI-UTC = 4.2131700s + (MJD - 39126.) x 0.002592s
1972 Jan 1 JD 2441 317.5 TAI-UTC = 10.0s
1972 Jul 1 JD 2441 499.5 TAI-UTC = 11.0s
1973 Jan 1 JD 2441 683.5 TAI-UTC = 12.0s
1974 Jan 1 JD 2442 048.5 TAI-UTC = 13.0s
1975 Jan 1 JD 2442 413.5 TAI-UTC = 14.0s
1976 Jan 1 JD 2442 778.5 TAI-UTC = 15.0s
1977 Jan 1 JD 2443 144.5 TAI-UTC = 16.0s
1978 Jan 1 JD 2443 509.5 TAI-UTC = 17.0s
1979 Jan 1 JD 2443 874.5 TAI-UTC = 18.0s
1980 Jan 1 JD 2444 239.5 TAI-UTC = 19.0s
1981 Jul 1 JD 2444 786.5 TAI-UTC = 20.0s
1982 Jul 1 JD 2445 151.5 TAI-UTC = 21.0s
1983 Jul 1 JD 2445 516.5 TAI-UTC = 22.0s
1985 Jul 1 JD 2446 247.5 TAI-UTC = 23.0s
1988 Jan 1 JD 2447 161.5 TAI-UTC = 24.0s
1990 Jan 1 JD 2447 892.5 TAI-UTC = 25.0s
1991 Jan 1 JD 2448 257.5 TAI-UTC = 26.0s
1992 Jul 1 JD 2448 804.5 TAI-UTC = 27.0s
1993 Jul 1 JD 2449 169.5 TAI-UTC = 28.0s
1994 Jul 1 JD 2449 534.5 TAI-UTC = 29.0s
1996 Jan 1 JD 2450 083.5 TAI-UTC = 30.0s


Created: 1 Jan 2001. Text provided by the former Central Bureau.

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