extras
directory,
or in a folder within extras
.
Celestia scans the directories that are
in extras
and then the SSC files in them in alphabetical
order, adding them to its internal list of things to display as it
finds them. Objects defined in AAA.SSC
can be used by
objects defined in BBB.SSC
, but objects in
BBB.SSC
cannot be referenced in AAA.SSC
.
One could, for example, define planets in AAA.SSC
and then define their moons in BBB.SSC
.
An .SSC file is used to describe one or more objects and their orbits. hyakutake.ssc is a simple example.
Ulrich Dickmann and Bob Hedgewood have written a PDF document describing SSC files. It is available in both English and German at http://www.celestiamotherlode.net/catalog/documentation.html
Thomas Guilpain has provided a description of most of the declarations you can use in a .SSC file on his Web site.
For objects that have been observed by astronomers, making the orbital entries usually is just a matter of cutting and pasting the right numbers. Often you can get orbital parameters for the object you're describing from several different Web sites. Google does a good job of locating them. You'll find that the parameters they provide for a particular object differ to a greater or lesser extent. This is because of limitations in the original measurements or because the object's orbit has changed with time due to gravitational or other effects.
JPL's Horizons Ephemeris System at http://ssd.jpl.nasa.gov/horizons.html is a good centralized resource for orbital parameters of objects in our Solar System, but it probably won't have the most recent discoveries.
You may need to do a little studying to understand what the various orbital parameters mean and why they're necessary. The brief table in " Parameters Describing Elliptical Orbits" may help. "How to transform orbital elements into Celestia's SSC format" describes a cookbook approach to converting comet and asteroid orbital parameters from IAU format into Celestia format. Textbooks have been written about astrodynamics and celestial mechanics, which are available in the usual ways: visit your local bookstores and libraries, as well as Amazon. There also are some Web pages that can help you get started. Some of both are listed on the main page.
extras
directory, or in a folder within extras
.
For you to look at the objects it has defined, you need to find out
what they are.
One way to find them is to inspect the .SSC file itself.
It's just a plain text
file which can be examined with your favorite text display program.
Notepad and Wordpad are included with Windows. Emacs, more, less, and cat
are Unix utilities that can be used.
Every object defined in an .SSC file has a name. The name of the first object is the first quoted string in the file.
For example, the line
"Cassini-xyz" "Sol"
means that the object is named "Cassini-xyz" and that it orbits
around "Sol" (the Sun).
Definitions of new bodies in an SSC file have to
reference bodies that Celestia
already knows about. They have to be bodies declared earlier in that
same .SSC file, or bodies that are defined in .SSC or .STC
catalog files that Celestia has
already read. Celestia reads the files that are in a particular
directory in alphabetic order.
For example, if A_MYSYSTEM.SSC
contains
"planet1" "HIP 999999" {...planet1 stuff...}then
B_MYSYSTEM.SSC
can contain
"moon1" "HIP 999999/planet1" { ... moon1 stuff...}
If there are no errors in the .SSC file, then you also will find that
object in Celestia's "Solar System Browser". This and other browsers
are listed in the Menu "Navigation".
New objects that orbit around the sun will be listed near the bottom
of the browser's popup window.
When an object has an orbit which goes around a planet, then its
entry is associated with that planet's entry. You'll have to select the
[+]
in front of the planet's name to see the list of
objects that orbit around it.
You can use the mouse to select any object that is listed in the broswer. Double-clicking on its name causes that object to be "Selected". The currently "Selected" object is listed at the top left of Celestia's display. Many of Celestia's commands work with respect to the object that is "Selected".
The "Solar System Browser" includes both "Center" and "Goto" buttons. Other commands are available in the "Navagation" menu. They all have keyboard shortcuts, too. See the "Help" menu for a list of them.
Another way to "Select" an object is to type a "carriage return" to Celestia. It'll prompt you for the name of the object. As you type an object's name, Celestia will display a list of objects which have names starting with the letters you've already typed. You can type a "tab" to step through those names. Otherwise you'll have to spell the object's name exactly right.