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

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