Xephem -- Small On Requirement, Big On Features Xephem is a free planetarium program for Unix and Linux that can be obtained from The Clear Sky Institute. While the Sky View display isn't up to esthetic appearance of Kstars or especially Stellarium, it is a highly functional and configurable planetarium program. And -- it is much more bug free. This web page gives an overview of some of the features of Xephem. I invite you to check out Tonight's Sky, which uses a number of Xephem displays to present what the sky is showing tonight. Xephem has many types of displays, not just the general Sky View star screen, an example of which is shown below: Xephem Star Screen Shot Segment In the screen shot example, you can see a number of green symbols, some labeled. These are Messier, Caldwell, and Hershel 400 objects. Xephem allows you to determine how many are automatically labeled, and right clicking on any one of them will show the label, location, type, and other pertinent data. It's worth noting that while the Messier objects were already in the Xephem database, I was able to add the Caldwell and Hershel objects by following Xephem documentation. Some of the Main Features Xephem allows the user to enter a location for which to present the night sky. When Xephem comes up, it automatically sets the time for the current time, but the user can select any other time, or simply click a button to advance or retard the display by a selectable time step. On the star map, holding down the center mouse button will allow you to move the star map to a desired section. One can also use a scroll bar at the bottom of the display to move in azimuth, and a vertical scroll bar at the right of the display to move in elevation. A vertical scroll bar at the left of the display zooms the map in and out.

Many features on the display can be set either from an options pull down or from buttons across the top of the display. One can turn on or off the horizon line, constellation lines, object labels, grid lines, solar system objects, and deep space objects. When exiting, the user is given the option to save the current settings of all options.

The star magnitude limit for displaying stars and nebulae can be selected. From the basic control window, object catalogs can be selectively turned off and on.

See the Orientation of Planets and Moon for Any Time

Xephem has views for the Moon, Jupiter, Saturn, and Mars, showing their view from the selected Earth location and time. Each of these displays, when selected, pop up as a separate window. The moon image, shown below, shows the current lunar phase and a long list of lunar features than can be selected, which places their label on the moon map. The moon map can be zoomed in and out for an appropriate display.

An example of the planet views is this Xephem Jupiter view. It shows the planet, the location of the Galilean moons, and the great red spot for any selected time. A great tool for planning your Jupiter observing sessions. The image can be flipped to accommodate any orientation provide by your telescope view, and zoomed in or out to adjust the field of view.

Similar views, helpful for planning planetary viewing sessions, are available for Saturn and its moons, and Mars with it's features.

One of My Favorites -- Favorites!

Xephem has a feature called favorites. It comes pre-loaded with the Sun, Moon, and planets. Clicking on favorites will drop down the favorites menu, and selecting an object will cause the night sky (star) window to place a reticle over the desired object. If the object is not currently in the window view, the window view is moved to place the object in view.

This is really handy for finding the planets, but I've found that it can be much more than that. You can right-click on any object and the pop up info for that object will let you add it to favorites. You can also use the options menu to select different favorites lists.

I enjoy looking at double stars, but often forget exactly where some of them are. I've found some double star listings appropriate for my telescope, found these on the map, and added them to a new favorites I call doubles. Now I can click the favorites button to get the pull down, and click any double in the list to have Xephem show me where it is. Quite handy.

Other Features and Final Thoughts

Xephem has many other features, some probably a bit arcane for the average amateur astronomer. For example under the tools menu, one has some mathematical utilities for solving astronomical equations and plotting functions.

There's a preference menu for setting display colors, precisions, and other features. There's a data menu where you can enter new object lists you prepare, as described in the Xephem help information. On the data/files menu you can select to turn on or off any of the catalogs you've added.

On the Sky View display, the options menu provides an eyepiece function where one can set up different viewing fields of view. Then any of these field of view setups can be selected. Then on the star map, one can right click on a region of the display and select Place Eyepiece. This causes a circle of the selected field of view to appear on the display. Right clicking again gives the option to delete the eyepiece circle. This is a handy option particular for planning use of wide field instruments, such as binoculars.

In the example below, the area of M31 is zoomed in, and a field of view defined for my binoculars is shown superimposed on M31 (the red circle). As you can see by this display, M31 is a pretty spectecular object even in a pair of binoculars, nearly spanning the entire field of view.

Be sure to check out many contributed features available at the The Clear Sky Institute. If you examine their library, you'll find that Xephem is one of the most capable PC planetarium programs available.

I've used Kstars, Stellarium, and Xephem for some time. Each has its handy features. Kstars is nice for it's display and the ability to associate web page links to any object. Stellarium gives the most realistic display, though it requires a 3D card to do so. Xephem gives the most basic display, but is the most responsive. It can run on the smallest computer. It also is the only one of the three that let me add my own object catalogs and display them properly.