# Celestia/FAQs
## What is Celestia?

Celestia is a freely-distributed, multi-platform, open source software package that provides photo-realistic, real-time, three-dimensional viewing of the solar system, the galaxy and the universe. It has proved a valuable tool for astronomy education, and is used in homes, schools, museums and planetariums around the world. Versions are available for Windows, Macintosh (MacOS X) and Linux computers.

## Where can I get more information about Celestia?

The Celestia Website has some additional information about Celestia.
The Celestia Forum is a good place to look for or ask for additional information. You'll find lots of friendly people there.
The Celestia Motherlode has many Celestia resources. (Unavailable 2020-11-18)

### Where can I get the most recent version of Celestia?

You can download the latest version of Celestia (v1.6.1) from SourceForge.
Prerelease versions are announced in the Celestia Web Forum in its Users Forum.

### What changes have been made to Celestia since the last version?

The developers maintain a list of all new functionality and bug fixes in the Celestia ChangeLog, which is located at SourceForge (most recent changes listed at bottom).

## Questions about how Celestia works
### Celestia crashes, what it draws is messed up or it's extremely slow. What can I do?

Celestia makes use of the most advanced features of OpenGL that your computer's graphics driver claims to support. Many older OpenGL implementations have serious bugs. Here are some options for improving Celestia's display, with the most likely ones first:
* Make sure full hardware acceleration is enabled in your display properties.
* Upgrade to the most recent drivers for your graphics card. Download them for free from the Web site of the manufacturer of your card, not from Microsoft.
Nvidia drivers can be downloaded from http://www.nvidia.com/content/drivers/drivers.asp
ATI drivers can be downloaded from http://ati.amd.com/support/driver.html
Notes:
* Graphics chipsets integrated into laptop systems usually require drivers provided by the manufacturer of the laptop itself. Too often proprietary "glue chips" prevent the chip vendor's drivers from working properly.
* Reduce or disable hardware acceleration to verify that the problem is hardware related.

#### Square Stars

Intel graphics chipsets and drivers have had this problem for many years. The only fix is to type Ctrl-S several times while running Celestia. That'll select other "Star Styles", one of which might be drawn better.

#### Windows 7, 8, 8.1 or 10

Update your computer's graphics drivers. Follow the instructions on the Web site of the manufacturer of your graphics hardware.

#### Windows XP

* Whenever you upgrade Microsoft's DirectX software, you must upgrade or reinstall the graphics hardware manufacturer's graphics drivers afterward. Installing DirectX installs Microsoft's copies of the drivers, which usually are several generations old.
* To disable hardware acceleration under Windows, open the "Display Properties" window. Select the "Settings/Advanced/Troubleshoot" tab. (not the "Troubleshoot..." button). Move the "Hardware acceleration" slider all the way to the left. Click on the "OK" buttons to change the settings in use. This will cause Windows to use Microsoft's Generic OpenGL v1.1 library, which is limited, but seems to have relatively few bugs. It does everything in software, works on 2D displays, and is quite slow.
* Driver Upgrade Procedure
** download driver installation program
** use the Control Panel / Add or Remove Programs menu to delete the current graphics drivers.
** Reboot
** Cancel out of XP's offer to install new drivers.
** Run the Installation program for the new drivers.
** Reboot
** Configure desktop resolution and other desirable features.
    The two reboots are essential in order to cause the old low-level drivers to be deleted. Without those reboots, the old low-level drivers will not be deleted and the new installation will not work properly, although it may not generate any error messages.

#### MacOS X

For Apple PPC and Intel computers running MacOS X, you must upgrade to the most recent version of the operating system. Updated drivers usually are not available separately.
Apple's OpenGL on MacOS X often has serious bugs which sometimes are not fixed in the most recent release of MacOS. You must report those problems to Apple, otherwise they will not get fixed.

#### Linux

For computers running Linux, you usually can download the drivers for free from the Web site of the manufacturer of your graphics card.
ATI's fglrx drivers for Linux often have serious bugs. Try to use the driver shipped with the most recent version of Xorg's X server software.

### Celestia draws the Moon in shades of purple, blue and red. Why is it doing that and what can I do?

Your graphics chipset and its drivers aren't drawing bumpmaps and normalmaps properly: their OpenGL routine "GL_ARB_vertex_program" is defective. (This is often seen with the newer Intel graphics chips.) Assuming you've already installed the most recent drivers,
* Download and install a different version of Celestia. The program is frequently revised.
* Turn off some of Celestia's advanced display features.
** Type a [Ctrl-V] several times to select "Basic" or "Multitexture" render paths instead of the OpenGL Vertex programs. This disables the use of vertex and shading programs temporarily. If this does improve things after you've followed the previous suggestions, then you need to do the next step:
** Tell Celestia to ignore specific features that your OpenGL library claims (falsely) to support. To do this, edit celestia.cfg. Remove the # that's in front of the line
    Celestia's Help menu lists all of the routines in your system's OpenGL library. Add equivalent Ignore lines for other suspicious routines.
Addendum provided Tech Sgt. Chen:
Shut down all background programs on your system before running Celestia (i.e., antivirus software, multimedia software such as REAL Player, Musicmatch, etc.) Graphics programs are notorious for consuming system resources and even the best of graphics cards are better off without competing for those resources.

### Celestia still crashes, draws funny stuff or is extremely slow. What can I do?

Report the exact circumstances and details of your hardware and software in the "Celestia Bugs" section of the "Celestia forum". Celestia runs on many different hardware and software configurations. It is not appropriate to ask people to guess what you have. For example:
Hopefully you know the System information. If you're running Windows, System details usually are available in the Control Panel's System Properties menu.
Some of the Graphics information can be found in Celestia's Help menu. If you're running Windows, more details can be found in the Control Panel's Display Properties menu.

### Where can I get another version of Celestia that might work better?

Older versions of Celestia are available on SourceForge v1.2.4 is extremely robust, but does not include many recent features.
Visit the "Celestia Forum" and ask for help if you are having problems with Celestia v1.6.1.

### I want to see all possible Celestia eye candy. What kind of graphics card should I get?

As of 2017, you can get whatever graphics card you like. More memory will allow more and higher resolution objects to be viewed, but the most advanced features of Celestia use only the features provided by OpenGL v2.0. All modern graphics hardware, including Intel's embedded graphics support OpenGL v3 or later. The current version of the Mesa software graphics library also provides OpenGL v3.

### Sometimes a screenshot captured in Celestia will have multiple boxes around it, as if Multiview was on. How can I get rid of it?

To make a good clean screenshot image, press [Ctrl + D] before you capture the image. This cancels Multiview.
(provided by ElPelado)

### I can't turn off the Red/Green diamond in the center of the selected planets.

You are seeing a Mark. Disable Marks by pressing Ctrl+k (the "Control" key and the "k" key). Marks also can be turned on and off in the "Render/View Options..." menu of Celestia v1.3.1 pre3 and later.
(provided by ElPelado)

## I want to write some scripts for Celestia. How can I do it?

Briefly:
    Celestia includes a very simple scripting language of its own which understands commands like "go here, look there, set flag, display text". These commands should go into a file with the filetype .CEL
    Celestia also includes Lua, a sophisticated, general-purpose scripting language. Scripts written in that language should be put into a file with the filetype .CELX
For more information see Celestia/Scripting

## Questions about objects (not) seen in Celestia
### Why are my favorite stars not in Celestia?

    Celestia v1.6.1 includes only stars that had their distances measured by the Hipparcos satellite. Hipparcos was not used to measure the distances to many dim, variable or close double stars. Someone may have created an Add-on that includes your stars, though. Or consider creating the necessary STC file yourself and contributing it.
    Celestia v1.6.1 also includes all of the stars within 20 LY of the Sun, as well as about 200 double stars. (There are actually many more double stars than that, but relatively few have had their orbital parameters accurately measured.)

### The Sun and the Moon are much too small in Celestia. What's wrong?

Their diameters are exactly right. As seen from the earth, they both are about a half-degree across. Celestia's default window is about 45 degrees across, so the Sun and Moon are about 1% of that. They are drawn only 10 pixels wide if your screen is 1024x768.
Remember that your computer screen is only about 10-20 degrees wide in your own field of view. Celestia's 45 degree field provides a "wide angle" view of the sky. This makes objects look smaller than you might expect.
The apparent large size of the Sun and Moon as we see them in the sky is a psychological illusion. There are several different explanations for this. If you take a picture of the moon with a camera lens that has the same field of view as Celestia, you may be surprised at the small size of its image.
At least one book has been written about this effect: The Mystery of the Moon Illusion: Exploring Size Perception, by Helen Ross and Cornelis Plug. Review of their book
A simple experiment was suggested by "HarrieS", a guest on the Celestia forum:
    Here is something you might try: a finger at arm's length is about two degrees wide for most people. That means that you can fit four moons side by side. Go outside and check it. Now have a direct look at your finger indoors. Can you still believe that four moons will fit on it?
And Dalle of the Forum wrote
    But if your head is positioned e.g. 70 cm away from your e.g. 17" monitor, which measures 32 cm across (at least mine does), then the effective field of view looking at the Celestia sky "through" your monitor screen is 2*arctan((32/2)/70)) ~ 26 °. Hence, if you decrease the Celestia field-of-view to 26° you may get a more accurate appearence of what you would see looking out the window.

### Celestia's galaxies are ugly, dim, grey blobs. How can I get colorful galaxies that look like the real things?

The real things are dim, grey blobs. Your eyes are not sensitive to color at the very low light levels emitted by distant galaxies.
Many of the colorful pictures you're used to seeing are enhanced by long exposures on sensitive color film. Others are imaginative "false color" combinations of narrow-band CCD images designed to make visible the specific features of interest to the investigators. The colors of those pictures aren't realistic at all.
You can add a colorful object to Celestia by creating a 3DS model with appropriate images as surface textures. Define it as a Nebula in a DSC file. Use the Search command in the Celestia "Development" and "Add-On" forums to find examples.

### Why are there no stars beyond about 16,000 light years? Why are there no stars in distant galaxies?

Hipparcos could only measure parallax to a value of about 1 milli-arc-second. For details, please read the thread Celestia currently has technical limitations which prevent it from drawing stars beyond a distance of about 13 million LY from the sun.
On the second page of the thread mentioned above , Chris wrote about some of the tradeoffs in the current version of Celestia.

### Sometimes the planets and moons are way far away from where their orbits are drawn. Why?

To draw the orbits, Celestia only calculates 100 or so precise locations and then draws straight lines between them. If the orbiting body doesn't happen to be close to one of those 100 points, then it won't be very close to the line, either. The position of the orbiting body is calculated very accurately. The lines aren't.
Celestia only calculates a few points around the orbit in order to minimize the amount of computation needed between frames. The more calculations are required, the slower the frame rate is. You can specify the number of segments in an orbit track. Use your favorite text editor to modify the file celestia.cfg. Change the line
Increase 100 to 500 or larger.

### Why does the illumination level in Celestia not fall off the farther from the Sun I go - surely it should be very dark by the time I get to Pluto?

The human eye can adapt to a very wide range of illuminations, so it wouldn't be as dark out there as you think - about the level of moderate indoor lighting, in which you can see very well. At the other end of the scale, your computer monitor is physically incapable of generating the brightness of illumination that pertains on the inner planets. Fortunately it doesn't need to, since all that would happen would be that your pupils would constrict to reduce the incoming light to a more comfortable level.
So in summary -
* There's no way Celestia can display "realistic" brightnesses on your computer screen; but
* such "realism" is unnecessary because your eyes merely adapt to compensate.
(Provided by Grant Hutchison)

### When I try to capture a picture or movie, the image is stretched out of proportion.

(Thanks to DaveMc for these tips!) Here are three things you can try...
* Check the OpenGL anti-aliasing setting of your graphics card. If it is on, try turning it off, or setting it to another option. This seems unrelated to the anti-aliasing setting inside of Celestia.
* Set your graphics card OpenGL options to "default".
* Get the most recent drivers for your graphics card.
(Provided by Don G.)

### How can I make Celestia work like a planetarium? I want to see how the sky should look from my backyard.

If you're running Celestia v1.3.0 or later
* Select the body from which to observe; e.g. type H then 3 to select Earth,
* Use the GoTo menu to specify your Longitude and Latitude and GoTo there (lat & long are entered in decimal format - there are websites that convert to/from degrees/minute/seconds format, such as this one,)
* Type y = Sync Orbit (this locks you into position above the location you set),
* Type (Windows:Ctrl-G) (Linux:Alt-S) = Goto Surface,
* Type (Windows:Ctrl-F) (Linux:Alt-F) = change Arrow keys to AltAzimuth mode (this keeps the horizon level when panning left & right),
* Use the arrow keys to look toward the sky, you can adjust the field of view (FOV - how much sky you see at once) with the , & . keys.
Celestia provides a Horizontal (alt-az) coordinate system display which can be selected in its Render menu. Also, typing a ; will turn on the equatorial coordinate system display. There are some scripts which add alt-az display, such as Planetarium.

### Why can't I see Mir or Galileo? I know they're defined in Celestia.

Set Celestia's simulation time to be when the spacecraft was in orbit. Celestia does not display spacecraft if they are not in orbit at the time of the simulation.
For example, Mir was launched on February 20, 1986, and reentered the Earth's atmosphere on March 23, 2001, at 05:55 GMT. Celestia will show Mir only if you set the time to be somewhere between those two dates.
This is controlled by Beginning and Ending directives in the definition of Mir in solarsys.ssc. If you remove those statements, Celestia will always draw Mir in orbit.
Addendum provided Tech Sgt. Chen:
Mir, along with certain other spacecraft models, were modeled within an historical time frame and can only be viewed between the mission start and end dates. You can override ending dates by opening the solarsys.ssc or other specific craft related .SSC files and placing a pound sign (#) in front of the ending date string. Then save the new setting. This way you can always view your installed Space Crafts. Consequently, removing the pound sign will return the craft to its natural time frame.

### Positions on Mars are on the opposite side of the planet or bumps seem to be half a world away from the mountains or it's dark where it should be daylight. Why????

You have a misaligned map.
Celestia requires that all maps have 0 degrees of longitude in the center, with 180 degrees of longitude at the edges. All of the surface texture image maps of all of the moons and planets that come with Celestia have this alignment.
In contrast, many maps of Mars have been created with 0 degrees of longitude at the left and right edges, and with 180 degrees of longitude in the center.
Using a map with 0 at the edge would cause the symptoms you describe. You need to cut the map in half and exchange the halves or you need to find a map that's properly aligned.

### My planet's rings are drawn as a featureless oval. It used to work. What's wrong?

You need to use a smaller ring image, one that is no wider than your graphic card's OpenGL texture buffer.
To see how large your OpenGL texture buffer is, use Celestia's menu Help "OpenGL Info". Near the beginning it has a line that starts with "Max texture size:"
Older versions of Celestia scaled down large ring texture images so they'd fit into the smaller texture buffer of your OpenGL graphics driver. Celestia v1.3.1 and later no longer do that.
Some older cards, like 3dfx Voodoos, only have a 256 byte buffer. Modern cards have a 2K or 4K buffer. Microsoft's software version of OpenGL only has a 1K buffer.

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

The orbit of the International Space Station 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".
Current values are also available at http://heavens-above.com/orbit.aspx?satid=25544
Here are the ISS TLE orbital parameters for August 16th, 2004:
and here are the corresponding values for September 30, 2017:
Grant Hutchison has provided a spreadsheet to convert TLEs into Celestia SSCs
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. Another issue is that Celestia v1.5.1 and earlier use a fixed rotation speed for the Earth. Since the Earth's rotation is variable, the surface of Celestia's Earth is displaced from where it should be. Celestia v1.6.1 uses a more accurate variable rotation speed. In other words, you can't use Celestia to find out where to look in the sky to see the ISS.

### 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?

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.
A description of how to organize your Addons so they can be moved easily.

### I've compared the Celestia texture for Venus/Ida/Miranda with one I've found in a book or on the Web, and Celestia's map is upside down. What's going on?

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 (i.e. counterclockwise); or as a "retrograde" rotator, if it rotates in the opposite direction.
But 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.
(Provided by Grant Hutchison)

## What if I have more questions?

If you have a question about Celestia that is not answered above or elsewhere in this WikiBook, please ask your question in the Celestia Web Forum, not here. There are many experts reading the Forum, but very few knowledgeable people read the WikiBook. If a question is asked more than once or twice in the Forum, it'll be added to this FAQ.