Looking for an add-on

[Guest]

It seems impossible to me that this add-on hasn't been done already, but I find it really hard to figure out where to find it. Have files for Hale-Bopp been created yet and if so, where can I get them? I know this is probably a common question, so sorry for being the 100th+ user to ask it.

[selden]

Yes.

One way to find out if an add-on exists is to use the Forum's Search page (the second icon at the top of the page).

Hale-Bopp is included in a cometary add-on that Chris provides in the files directory on Shatters.net at
http://www.celestiaproject.net/celestia/files/comets.ssc

Also, I looked it up recently on the Minor Planets Web site. See the thread
http://63.224.48.65/forum/viewtopic.php?t=1106 where I provided 7digit orbital elements obtained from the Minor Planets Web site. More precise (16 digit) elements can be obtained from JPL's Horizons ephemeris server, but I don't think Celestia accepts that many.

Note that these are the comet's current Keplerian orbital parameters and do not show the early path of the comet.

[selden]

Both CfA 7digit and Horizons 16 digit SSC files for Hale-Bopp are now available on my Web site. Their current positions differ by about 54,400 kilometers. You'll have to use the "Solar System Browser" to goto the comet, however. There seems to be a bug in the textual Select code which doesn't accept names that have a / in them.

CfA orbital parameters are in
http://www.lns.cornell.edu/~seb/celestia/hale-bopp.ssc
Horizons orbital parameters are in
http://www.lns.cornell.edu/~seb/celestia/hale-bopp-horizons.ssc

[chris]

Both CfA 7digit and Horizons 16 digit SSC files for Hale-Bopp are now available on my Web site. Their current positions differ by about 54,400 kilometers. You'll have to use the "Solar System Browser" to goto the comet, however. There seems to be a bug in the textual Select code which doesn't accept names that have a / in them.

That's not a bug, actually . . . The / character is used for 'paths' For instance, to select the Moon you could enter Sol/Earth/Moon (though just 'Moon' will work within the Solar System) So / is not permitted in object names.

CfA orbital parameters are in
http://www.lns.cornell.edu/~seb/celestia/hale-bopp.ssc
Horizons orbital parameters are in
http://www.lns.cornell.edu/~seb/celestia/hale-bopp-horizons.ssc

Celestia uses double precision numbers for orbital elements, which means it will recognize up to 16 significant digits. I doubt that any orbit has actually been determined to such precision, however.

--Chris

[selden]

[That's not a bug, actually . . . The / character is used for 'paths' For instance, to select the Moon you could enter Sol/Earth/Moon (though just 'Moon' will work within the Solar System) So / is not permitted in object names.

Hmmm. Then I guess I should rephrase that as an enhancement request It'd be nice if somehow a / could be used in an object name, since the prefix C/ is often used when designating comets in the professional databases, and Pascal followed that convention in comets.ssc. Maybe with a quoting character as a prefix?

Celestia uses double precision numbers for orbital elements, which means it will recognize up to 16 significant digits. I doubt that any orbit has actually been determined to such precision, however.

I've been careful to use the term "precision" and not "accuracy" when describing the differences. Highly precise values (with lots of digits) are not necessarily accurate (i.e. not necessarily correct). I was glad to find that Celestia has no problems with the exponential format that Horizons generates. That saves a lot of work! Somehow I had gotten the impression that the input routines didn't like lots of digits. Maybe the limitation is with numbers expressed as nnn.nnn without the exponent at the end? *shrug*