March 17, 2008

Watch and explore outer space with Stellarium, Celestia, and Xplanet

Author: David A. Harding

Ubuntu founder Mark Shuttleworth spent about $20 million to go into space, and he never got farther away from Earth than a few hundred miles. Using three free software programs, you can look at and virtually travel to places millions of miles away without leaving your GNU/Linux desktop or paying a dime.

The GNU GPL covers all three programs, and most major distributions include all of them as packages.


Stellarium lets you watch the night sky, learn about the stars, and find constellations without going outside or even waiting for dark. Stellarium won the Project of the Month Award in May 2006, and Digitalis Education Solutions, Inc, uses Stellarium in portable planetarium projectors that sell for as much as $41,100.

Stellarium comes with data to display more than 300,000 planets, stars, and other astronomical light sources, and from Stellarium's download page you get data to display even more stars.

Start Stellarium from a desktop menu icon or type stellarium at a command prompt, and Stellarium will show the current Parisian sky. If you don't live in Paris, click the Settings button, choose the Location tab, and select a nearby city from the map. I chose the nearest city to me, Philadelphia. In the same tab, you can also enter a precise location in longitude/latitude format. You only need to set your location once.

Stellarium displays an empty field and clear sky. I'm glad my view isn't hidden by Philadelphia's tall and bright buildings, but I also feel weird looking at an empty field where downtown Philadelphia should be.

If you start Stellarium during the day, you'll see the Sun and no other stars. Press j to speed up time in Stellarium. For every second of real time, 10 seconds pass in Stellarium's relative time. Press j more times to speed up relative time more, and soon the Sun will set. Press k to return to real-time speed. Stellarium displays the current relative time in the upper left corner of the screen.

Zoom in on the sky by pressing Page Up, and zoom out by pressing Page Down. Zooming out all the way lets you see the entire night sky at once. Use the keyboard arrow keys or click and drag with the mouse to move the sky around. On the ground below the sky, Stellarium prints what direction you face.

Clicking on a star or other light source brings up a description of it in the top left corner of the screen. Stellarium provides different details about planets and stars. If you click on Mars, for instance, Stellarium tells you its name, brightness (magnitude), and distance from Earth. If you click on Polaris, also known as the North Star, Stellarium tells you the color of its light (spectral type) and its star catalogue number.

Press c to draw lines between the stars to indicate constellations, and press v to add labels. Finally, press r to paint a picture of the constellations on top of the stars and lines.

To go backward in time and look at something you saw last night, press l. You can also enter a date in the Date & Time configuration tab. Stellarium accepts dates thousands of years in the past or future.

I run Stellarium every clear weather night before I walk my dog. Stellarium shows me exactly how the stars appear from my house, and where to look for constellations. Stellarium makes my walks more enjoyable for me, but not for my dog; she's tired of being stepped on while I stare at the stars and not at what's in front of me.


Watching the heavens in Stellarium makes me want to visit the stars. Celestia lets me explore almost any of the light sources Stellarium renders. I can hover over Mars, travel to Polaris, or look at our sun from dozens of light-years away, all rendered in stunning graphics.

NASA uses Celestia in its outreach program. The software uses facts when possible, and at other times it guesses how the cosmos look using astronomical theory. Celestia doesn't tell you what's a fact and what's a guess, so you may see something -- for example, the sunspots on Polaris -- and wonder if it's real. I think Celestia's guesses make it more useful, not less.

Celestia needs hardware graphic acceleration on most computers. It has GNOME, KDE, and generic Gimp Toolkit (Gtk) front ends, but you need to choose which one to use when you compile Celestia, or choose the Celestia package for your distribution. I used the KDE front end for this review.

Celestia comes with a lot of detail: it has images and data that help render the surface of all the planets and major moons in our solar system, several planets outside our solar system, and thousands of stars. And if that isn't enough, Celestia's official add-on site, The Celestia Motherlode, has more than 10GB of extensions. Some extensions render high-quality images of specific places or spacecraft. Others add planets and space ships from science fiction and fantasy universes such as Babylon 5 and Star Wars.

Start Celestia from a desktop menu icon or type celestia into a command prompt. You start with the view from an invisible spacecraft orbiting the Sun near Earth.

You can get a feel for Celestia by using its built-in demo. Press d on the keyboard, and Celestia takes you on a tour of the universe and of Celestia's features. Or just start driving your space ship: press a to move forward, use the keyboard arrow keys to change direction, and press z to slow down.

Similar to Stellarium, you can press l to speed up time and press k to slow it down. While watching Earth, speeding up time makes the change from night to day happen every few seconds. Speed up time more to make the moon complete a full orbit every few seconds. Watch carefully and you'll see lunar and solar eclipses. During a solar eclipse, I suggest you slow down time to watch the Moon's shadow travel across Earth. You can compare Celestia to pictures of a real solar eclipse taken by astronauts on the International Space Station.

Click on a light source to display information about it. Double-click on the light source to center it in your display. Press g to go to the selected object or press f to follow it as it moves.

Press p, m, and b to turn on and off the labels of nearby planets, moons, and stars. Press / to draw lines between the stars in constellations. With the constellation lines turned on, you can travel far outside the solar system to see how the constellations change and eventually become unrecognizable.

I don't know of any free software educational program that is more fun or more beautiful than Celestia. I wish it were a little easier to use -- sometimes I think that learning to fly the space shuttle is probably easier than learning to fly Celestia's invisible space ship -- but mastering Celestia puts all the beauty of the cosmos at your fingertips.


After exploring the cosmos, I want to settle down somewhere in the solar system with a grand view and get some work done. Xplanet renders a custom view of the solar system every 10 minutes as your desktop background image, turning your desktop into a window on the universe.

Wikipedia and other Web sites use Xplanet to generate maps, and the GPL ircmarkers program uses Xplanet to plot user locations on Earth. Xplanet is based on Xearth, a proprietary tool that makes a picture of Earth your desktop background. It's also related to the free software SunClock program that displays the current time by showing a picture of sunlight moving across Earth.

The Xplanet source package comes with the image for only one planet: Earth. Debian and Ubuntu split this image of Earth off into a separate package, xplanet-images. To see the texture of any other planet or moon, you need to download images from the flatplanet project.

Unlike Stellarium and Celestia, Xplanet isn't an interactive program. Without any command-line options, Xplanet makes your background a picture of Earth centering over 0 latitude and 0 longitude (about 350 miles south of Ghana, Africa, in the Atlantic Ocean). You'll see sunlight travel across the face of the Earth in real time as Xplanet updates the background every 10 minutes.

The flatplanet project also has an online front end for Xplanet. You can use drop-down boxes and buttons to explore the solar system in your Web browser. After rendering an image, flatplanet gives you an Xplanet command line you can use to make the same image on your own computer. Unfortunately, flatplanet uses an old version of Xplanet, and some options aren't in Xplanet anymore.

Xplanet puts the desktop background on the X root window. The X server displays every other window on top of the X root window, but some desktop environments, including GNOME and KDE, cover the X root window. For example, GNOME places a full-screen Nautilus file browser window on top of the X root window in order to display icons on the desktop. The Xplanet FAQ helps you make Xplanet work with GNOME and KDE, and you can find help for other desktop environments using Google. On any desktop, add the -window option to a command and Xplanet will display the result in a normal window.

Start playing with Xplanet by orbiting Earth over your home. First, find the latitude and longitude of your home using an online tool. Give that to Xplanet with the -latitude and -longitude options. For example, I found the coordinates for Philadelphia and typed the following: xplanet -latitude 39.9496 -longitude -75.1647

You can also use Xplanet to watch past or future events. For example, on August 1, 2008, the Moon will block sunlight traveling to Earth (a solar eclipse). You can watch the Moon's shadow travel across the Earth with the following command: xplanet -latitude 180 -date 20080801.08000000

Although the total eclipse won't last more than two and a half minutes anywhere on Earth, from space you can watch the entire eclipse, from start to finish, for more than four and a half hours. If you watch the eclipse from the Sun's point of view, you'll get to see the Moon move across your screen. To watch the Earth from the Sun, another planet, or moon, use the -origin option and the name of the place to watch from. For example: xplanet -date 20080801.08000000 -origin sun

Xplanet shows Earth unless you tell it to show another place with the -body option. For example, you can see Saturn from its moon Titan with the following command: xplanet -origin titan -body saturn

A friend of mine uses a script, run from cron, to download an image from a beach webcam in Florida and make it his desktop background. He points at his background and says, "That's what I'll see when I retire to Florida." I'm more ambitious: I make my desktop background with Xplanet and -origin moon. I point at my background and say, "That's what I'll see when I retire to the Moon."


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