A preliminary Celestia User's FAQ

[selden]

Q23:
Celestia's orbit for the ISS is out of date. How can I get a better one?

A23:
The orbit of the ISS changes continuously in ways that are almost impossible to predict due to things like atmospheric drag, light pressure, cargo ship docking, etc. If you want an accurate orbit, you'll have to update it on a daily basis.

ISS TLEs (Two Line Elements) are posted to the AMSAT SAREX mailing list regularly by "Dave Larsen PhD". See http://www.amsat.org/amsat/archive/sare ... reads.html

Here are the ISS TLE orbital parameters for August 16th, 2004:

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    ISS
    1 25544U 98067A   04229.23839543  .00019757  00000-0  15906-3 0  4532
    2 25544  51.6323  19.1941 0005251 117.9988 304.8582 15.70921896327755

Grant Hutchison has provided a spreadsheet to convert TLEs into Celestia SSCs at http://www.lepp.cornell.edu/~seb/celest ... ets.html#2

Also, don't forget that Celestia models the shape of the Earth using a spheroid. The actual shape of our planet is much more complicated. As a result, a view from the Earth's surface in Celestia is not accurate enough to show the correct path across the sky of satellites in low Earth orbit like the ISS. In other words, you can't use Celestia to find out where to look in the sky to see the ISS.

[selden]

Q24:
I have a previous version of Celestia with tons of addons, custom textures, etc. How can I update to the latest version without having to reinstall all of those addons, textures, etc?

A24:
You can have more than one copy of Celestia on your system at the same time.

Rename the directory (folder) where you have Celestia now, maybe to Celestia131.

Verify that things still work:
Double-click on the icon for Celestia131\Celestia.exe and look around in your universe.

Install the new copy of Celestia and tell it to use the directory Celestia. It'll create the folder again and install itself there.

Move your Addons at leisure.

Cel:// URLs run the copy of Celestia that is in the folder named Celestia. If you want to change back to using your old Celestia that way, just rename the directories again: rename Celestia to be Celestia132 and rename Celestia131 to be Celestia.

Also, please take a look at http://www.lepp.cornell.edu/~seb/addon-intro.html for a description of how to organize your Addons so they can be moved easily.

[selden]

Note:

Please don't post unanswered questions here. Post them as a new article.

I reposted Col's question in the Bugs forum. It was answered by Q/A #1: install new graphics drivers.

[jdou]

A complement for Q19:

Harald Schmidt has made a CELX script which display Azimuth and Elevation for Celestia 1.3.2 and newer.

You can download it from : http://www.h-schmidt.net/celestia

[selden]

Q25:
Why are some texture maps upside down and backward?

I compared the Celestia textures for Venus, Ida and Miranda with some I found in a book or on the Web, and Celestia's maps are upside down.

A25:

Many sources for planetary maps (such as the USGS) use a mapping convention called "ecliptic north" - the north pole of any planet or asteroid is defined as being whichever rotational pole points north of the plane of the ecliptic (the plane of the Earth's orbit around the Sun). Each planet or asteroid can then be classified as a "direct" rotator, if it rotates in the same direction as the Earth when observed from above its north pole (ie counterclockwise); or as a "retrograde" rotator, if it rotates in the opposite direction.

In contrast, Celestia uses an equally valid mapping convention called "rotational north" - north is defined as being the pole around which the planet appears to rotate counterclockwise, irrespective of that pole's orientation relative to the ecliptic.

For direct rotators, this difference is irrelevant - "north" turns out to be the same direction in both mapping conventions. But for retrograde rotators, Celestia's north pole corresponds to ecliptic south - so you will find many maps of retrograde rotators like Venus, Ida and the moons of Uranus that appear to be upside down relative to those in Celestia. If you want to convert such maps for use in Celestia, you'll have to turn them through 180 degrees.

(Submitted by Grant Hutchison.)

[selden]

Q26:
Why don't Windows and Mac Addons work under Linux?

A26:
Always use only lower case letters and numbers in file names. Use only the underscore (_) as word separators, not spaces. Avoid using composite characters.

Too often people are careless with filenames when creating Addons. The capitalization of names must be consistant for an Addon to work under all operating systems.

My_Addon.dsc is not the same as my_addon.DSC and not the same as my_addon.dsc. They are names for different files.

Linux and other Unix derived operating systems can tell the difference between upper and lower case. Linux can have three different and separate files with those names.

If you specify

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Texture "texture.jpg"

in the catalog file, then the filename must be texture.jpg in the textures folders. If the file is named Texture.JPG, Celestia won't find it because that's not the file that it's looking for.

[selden]

Q27:
Satellites orbiting the Earth are drawn extremely bright with no shading. What causes this?

A27:

This seems to be an issue most often reported for systems with Nvidia MX series graphics cards. A workaround is to tell Celestia not to draw Clouds. No fix is known for this problem.

[selden]

Q 28:
Stars with HIP or HD designations sometimes show up in the "tab completion" line and sometimes do not. What's going on?

A 28:

Only quoted Star "names" are shown in the tab completion line. Stars that have only a HIP catalog number are not listed in the tab completion line. When you want to select a Star that has only a HIP or HD catalog number, you must type its complete designation, including a space between HIP (or HD) and its number:
[return]
HIP <space> 111111
[return]

Strings which only match entries in the binary stars.dat database or in one of the cross-index databases for HD and SAO designations are not shown by the tab-completion code. There are too many of them. Only names which are in an object's quoted textual name string in a textual catalog (SSC, STC or DSC) are listed by the tab-completion code.

A confusion factor is that some Stars have HIP designations specified as their quoted names, often with no space between HIP and the number. Only these quoted HIP names are shown in the tab completion line. Some examples are in /data/visualbins.stc

[selden]

Q29a. Why does Celestia always use 100% of the CPU?
Q29b. How can I make it use less?

A29a:

Celestia uses 100% of the CPU if it cannot finish recalculating what's shown in its window before your computer decides it's time to refresh the screen.

Celestia has to recalculate the positions and visibility of everything you see in its window every time your system refreshes its screen. If you have set the screen refresh rate to be 75Hz, then Celestia has to recalculate everything in 1/75 of a second. If it finishes recalculating everything before the screen is refreshed, then it will use less than 100% of one CPU. If it takes longer, then it will use 100% of the CPU and Celestia will redraw its window more slowly than the screen's refresh rate.

In a dual processor or dual-core system or in a hyperthreaded system simulating a dual processor system, 100% of one of the CPUs is shown as 50% of total system usage. The calculations in Celestia v1.4.1 are single-threaded and cannot make use of more than one CPU at a time.

Type a ` (=backslanted apostrophe =accent grave) to enable Celestia's fps (frames per second) display in the lower left corner. If you have a keyboard that supports diacritical marks, then you may need to type a [space] after the `

Q29b. How can I make Celestia use less CPU time?
A29b:
On most systems Celestia will use no CPU time if its window has been iconized or minimized. (Under Windows, click on the icon in the upper right corner of Celestia's window which has an underscore in it.)

You also can make Celestia use less of the CPU if you ask it to draw less. Some ways to do this are
- reduce the screen refresh rate
- reduce the size of the window
- reduce the size of objects drawn in the window so fewer stars and galaxies are obscured
- reduce the number of stars and galaxies being obscured by turning down the limiting magnitude (type a [ several times) or by disabling them entirely
- reduce the number of objects being drawn by loading fewer addons

Otherwise, consider upgrading your CPU so more can be calculated between screen refreshes. Upgrading its graphics card will help somewhat, depending on how many OpenGL functions get moved from software into hardware.

[selden]

Q 30. Celestia v1.5 is drawing planets and moons bright red. Why?

A 30

Your computer's graphics drivers are obsolete. Download and install current drivers for free from the Web site of the vendor of your graphics hardware.

Modern drivers require that the program (Celestia) specify the version of vertex shader routines that it uses. Obsolete drivers don't recognize that specification.

When vertex shaders generate an error message, Celestia draws the object bright red.