Viewing the Night Sky with Linux, Part II: Visit the Planets With XEphem
By: Akkana Peck
Thursday, September 11, 2008 12:11:13 PM EST
URL: http://www.linuxplanet.com/linuxplanet/reviews/6530/1/
Quirky and Powerful
Part I of this series covered a simple Linux planetarium program, KStars.
But there are some questions KStars isn't very good at answering, like the question that began Part I: "What the heck are those really bright 'stars' in the sunset sky?" For viewing closeups of planets, monitoring the motion of the planets, getting precise predictions of events like eclipses, and other such information, you'll do better with a more powerful tool: XEphem.
XEphem is the oldest and arguably the best-known of the Linux planetarium programs. It's not included in most Linux distributions due to its license terms, but its source is available to download and the program is free for personal, educational or research use. You can buy it, too: $69.95 gets you a pre-compiled version of the program plus three CDs worth of extra data files that aren't in the free version. It's worth it if you're a sky junkie like I am.
You'll find XEphem at http://clearskyinstitute.com/xephem. Extract it and cd:
tar xzvf ~/Tarballs/xephem-3.7.3.tar.gz cd xephem-3.7.3
XEphem builds a bit differently from the usual configure, make, make install sequence. See the file named INSTALL for instructions. You'll need Motif installed, like the libmotif3 package on Ubuntu. You'll probably also need some X development headers: if you get compile errors, try adding the packages xorg-dev and x11proto-print-dev, or your distribution's equivalent.
Once it's compiled, the INSTALL file also has instructions on how to install it on your system, depending on whether you want to install it as root for anyone who might use your system, or just for yourself.
Navigating the Skies
When you run your newly installed XEphem, initially you'll see the control window. Set your location (be sure to click Set before you close the dialog), and set Local Time if you're trying to answer a specific question like "What was that bright object last night?" Then click Update.
To see the sky, go to View->Sky View. You can zoom in with the scrollbar on the left; the right and bottom scrollbars let you move around the sky. The History menu gives you a fast way to some useful views. For instance, try History->West horizon.
You can see that at sunset Venus, Mercury and Mars are all visible. Right-click on any of them to get more information like what times they rise and set. If you want to find any other planets, the Favorites menu will take you there.
Along the top of the window are buttons controlling what you can see. You can easily show or hide constellation names and figures, star names, solar system objects, and various other classes of objects. The buttons down the left and right sides control more esoteric values that you probably won't need to change.
XEphem doesn't show the horizon by default. You can turn it on with a button in the top toolbar. But there's a problem: the default horizon seems to put you in a canyon surrounded by huge mountains! To get a flat horizon, bring up the Control->Horizon... dialog and click on Constant. You can also customize your horizon: click on Edit with Mouse to draw it interactively.
Visiting the Planets
Don't stop at the sky view; check out some of XEphem's other views. The Jupiter view shows you the position of the Great Red Spot and the four Galilean moons by number (right-click to find a moon's name), as well as their shadows when a moon's shadow falls on the planet. The Mars view is great for figuring out which part of Mars is pointing our way. You'll really appreciate that if you observe Jupiter or Mars with a telescope.
Tools->Night at a glance... is a handy table of which solar system objects will be visible throughout the night. XEphem also has some advanced features in the Tools menu that you can use to convert between coordinate systems, solve orbit equations and other useful calculations.
XEphem has been around a long time, and its user interface has some quirks. In particular, once you've changed anything important, like your Location, be sure to save it so XEphem will remember it next time: go to the control window's Preferences->Save... and click Save now.
To find objects outside our solar system, go back to the control window and use Data->Index. Type the object's name or designation and click on Sky Point. It's a bit picky about capitalization, though, and it's better at finding objects by catalog number than by name. You can add your own catalogs, of course. You can also download catalogs for comets, asteroids and artificial satellites in Data->Download. Anything you find in the Index window can be saved as a favorite so it will appear in the sky view's Favorites menu -- assuming you remember Preferences->Save.
Despite its quirks, XEphem is one of the most accurate and reliable planetarium programs available on any platform. Astronomers will appreciate its accuracy and its many useful views and tools.
Akkana Peck is a freelance programmer and writer and the author of Beginning GIMP: From Novice to Professional. She's also a long-time amateur astronomer.