[Spice_discussion] question on coordinates of date

[Bryan Butler]

hello all,

first off - i would have searched the listserv archives for an answer to 
this, but it gives me an error every time i try to access them.  so, 
i'll just see if anybody is still reading these and can help :).

i'm trying to calculate coordinates of date, and running into some problems.

as an example, consider the geocentric position of the moon.  i'll 
calculate it in J2000.0 and also on 2009Jan30 at 0hUTC:

       CALL FURNSH ('naif0009.tls')
       CALL FURNSH ('de421.bsp')
       CALL FURNSH ('tod.tk')

...

       YEAR = 2009
       MONTH = 1
       DAY = 30
       HOUR = 0
       MINUTE = 0
       SECOND = 0
       WRITE (UTCSTR,1) YEAR, MONTH, DAY, HOUR, MINUTE, SECOND
       CALL STR2ET (UTCSTR, ET)

... as a check get the J2000 coordinates

       CALL SPKEZR ("MOON", ET, "J2000", "LT", "EARTH", STATE, LT)
       CALL RECRAD (STATE, RANGEMOON, RAMOON, DECMOON)

... then try the true, of date coordinates

       CALL SPKEZR ("MOON", ET, "ECI_TOD", "LT", "EARTH", STATE, LT)
       CALL RECRAD (STATE, RANGEMOON, RAMOON, DECMOON)


the result is:

J2000:   23:23:44.8371 -00:00:36.156
of-date: 23:24:13.6409 +00:02:28.349

the first comparison i did was with the SOFA routines, calling 
iau_PNM80() on the STATE returned from the J2000.0 call.  this results 
in exactly the same 'of date' coordinates as above, so i'm pretty sure 
i've got the frame file (tod.tk) defined properly.

then i thought i'd compare this with the positions in horizons 
(http://ssd.jpl.nasa.gov/horizons.cgi), which gives:

J2000:   23:23:44.8370 -00:00:36.157
of-date: 23:24:12.6659 +00:02:23.032

which is slightly different in J2000.0 (a few milliseconds of arc in 
right ascension and one millisecond of arc in declination), but quite 
different in of date (15 seconds of arc in RA, 5 seconds in dec).  this 
is also true of other dates i try - the errors are small in J2000.0, but 
quite large in of date coordinates.

so why is it that horizons gives such a different answer than SPICE for 
of date coordinates?

	-bryan butler
	 scientist, EVLA computing division head
	 national radio astronomy observatory