From Kurt.Feigl@verizon.net Fri Mar 29 10:35:28 2002 Subject: Julian Day does not equal day of year Dear colleagues, I wish to correct what I believe is an incorrect definition on your web site: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html, >Epoch Date and Julian Date Fraction: The Julian day fraction is just the >number of days passed in the particular year. For example, the date above >shows "86" as the epoch year (1986) and 50.28438588 as the Julian day >fraction meaning a little over 50 days after January 1, 1986. The >resulting time of the vector would be 1986/050:06:49:30.94. Note that NASA >SkyWatch will expect the time tag to be the same format as shown above >when entering a TLE data. This erroneous definition is for a quantity more properly called "day of year". Furthermore, I would argue that the "fraction" part (after the decimal point) should refer to the fraction of a day after UTC noon. The correct definition can be found at www.codeguru.com/date_time/Seasons.html >Julian Day Numbers (JDN) are used by astronomers as a date/time measure >independent of calendars and convenient for computing the elapsed time >between dates. The JDN for any date/time is the number of days (including >fractional days) elapsed since noon, 1 Jan 4713 BC. Julian Day Numbers >were originated by Joseph Scaliger in 1582 and named after his father >Julius, not after Julius Caesar. They are not related to the Julian calendar. I believe that the common confusion between JD and DOY may be a hold-over from Fortran programs where the former is implicitly an integer variable. I hope you will be able to correct your web site to avoid propagating this misconception further. This important because your web site aims at students and educators and because it seems to come appear first in web searches for "define Julian Day". Sincerely yours, Kurt Feigl, Ph.D. 2002/03/28 19:46 hrs = DOY 87.8242 = JD 2452362.3242 Kurt Feigl Kurt.Feigl@cnes.fr Permanent address: CNRS UMR 5562 tel. +33 5 61 33 29 40 14 ave. E. Belin fax. +33 5 61 33 29 00 31400 Toulouse FRANCE (air mail) Sabbatical address 6 August 2001 - 2 August 2002: MIT 54-617 tel. +1 617-253-3589 400 Main Street fax. +1 617-253-1699 Cambridge MA 02142 USA