Loading Orbital Elements Broken Link

[Miguel de Val Borro]

I would like to load the orbital elements of minor bodies into PyEphem. However, the link to the list of orbital element catalogs is broken http://rhodesmill.org/pyephem#catalogs and the Xephem database format is not defined in the tutorial.

[Brandon Rhodes (brandon-rhodes)]

Excellent question! The “ephem” and “XEphem” tools were once so popular in my circles that links to the documentation hardly seemed necessary, because everyone probably used those tools before finding PyEphem. But no longer! I have created three (3) separate hyperlinks to the XEphem's documentation of its file format:

From the words "XEphem format" in the Quick Reference;
From the words “ephem database format” in the Tutorial;
From a new entry on the “Catalogs” page.

Oh, yes, and I also corrected the link to the catalogs page that you found in the tutorial that used a "#" when it should have used a "/" so that the Catalogs page can be reached right from the tutorial. (It can also be reached from the main table of contents, in case you ever need to find it from there.) Thanks, and let me know about any other ways that the documentation can be improved!

[Miguel de Val Borro (miguel-deval)]

Thank you very much for the information on the Xephem database format. I have read the catalog entry for 176P from http://www.minorplanetcenter.net/iau/Ephemerides/Comets/Soft03Cmt.txt using the readdb function and found that it gives different RA and DEC compared with JPL Horizons on-line system. When I try to read directly the orbital parameters from JPL into pyephem using the command below I still get a different answer by about 1 degree:

# JPL orbital elements
# EPOCH= 2453784.5 ! 2006-Feb-18.00 (CT) Residual RMS= .5342
# EC= .1924588793996613 QR= 2.581086276205783 TP= 2453661.856807971
# OM= 346.5854859391472 W= 36.15336596682724 IN= .2378390038753938
# A= 3.196228910655303 MA= 21.15391261693265 ADIST= 3.811371545104822
# PER= 5.71433 N= .172483383 ANGMOM= .03017897
# DAN= 2.66388 DDN= 3.64413 L= 22.7386168
# B= .1403125 TP= 2005-Oct-18.3568080

linear = ephem.readdb('176P/LINEAR,e,0.2378390038753938,346.5854859391472,'+
    '36.15336596682724,3.196228910655303,0.172483383,0.1924588793996613,21.15391261693265,'+
    '2/18.00/2006,2000,g 15.0,2.0')

What is the expected relative error in the coordinates calculated in pyephem with respect to the Horizons ephemeris?

[Brandon Rhodes (brandon-rhodes)]

Miguel de Val Borro <email address hidden> writes:

> When I try to read directly the orbital parameters from JPL into
> pyephem using the command below I still get a different answer by
> about 1 degree ... What is the expected relative error in the
> coordinates calculated in pyephem with respect to the Horizons
> ephemeris?

The error should be on the order of tenths-of-an-arcsecond: around
30,000 times smaller than a degree. Could you be comparing different
actual coordinates? Which coordinate does not agree? Try all three
RA/Dec variables on the PyEphem object and see if any of them show a
closer match:

http://rhodesmill.org/pyephem/radec.html

If you cannot find any match, then give me a small PyEphem program that
does the calculation and prints out the expected and actual values, and
I can play with it and try to determine why the values do not match.
(Though, without the JPL Horizons source code, I might never be able to
tell without further clues!)

[Miguel de Val Borro (miguel-deval)]

The attached script has the astrometric RA and DEC from the Herschel data corresponding to the mid-date of the observations compared with the coordinates calculated by Pyephem using the orbital parameters from Horizons. There is a difference of about 1 degree in RA, also for a_ra and g_ra. The coordinates in the data agree with the value from the Horizons on-line ephemeris system when the observer is defined as Herschel observatory (code 500@-486), although I would expect that the astrometric coordinates should not depend on the observer frame. Is it possible to define the location of the observer as Herschel satellite in Pyephem?