JSkyCalc

JSkyCalc -- A Convenient, Portable Observing Aid John Thorstensen, Dartmouth College; 2007 January

Skycalc expedites the "time-and-the-sky" calculations needed by observational astronomers. The newest version, JSkyCalc, features:

Re-written in pure Java to run on all major platforms, including Mac OS-X, Linux, and Windows

Can be launched from a web browser (requires Java Runtime Environment 1.5+, java enabled browser).

Modern, intuitive graphical user interface

Vivid new planetarium-style sky display

Reads user-supplied object lists (Requires local installation)

You can add your favorite observatories (Requires local installation)

Quantitative estimates of moonlight and twilight interference

Tabulation of hourly airmass through the night, with warning colors that flag potential interference

Sunrise, sunset, twilight, moonrise, and moonset times

Help text included in the program.

New in version 1.1: Airmass-vs.-time graph for multiple objects.

Many more features too numerous to mention.

System requirements are simple:

Java Run-Time Environment (JRE) 1.5 or higher

For the web version, any browswer configured to use Java Web Start.

The Java Runtime Environment is automatically included in Mac OS-X, but if your OS is relatively old you may need to update to 1.5 or higher.

Click here to launch JSkyCalc with Java WebStart.

Alternatively, it is easy to install the program on your local system; this enables the reading of object lists and makes it possible to customize your site menu (with a little more work). Java provides a way of bundling up a bunch of files in an archive, called a "jar" file (much like Unix "tar"). Jar files can be executable:

Click here to download the executable jar file.

Jar files are independent of the CLASSPATH environment variable; if you stick it in a directory like /home/thorstensen/foo/bar, you can run it from wherever with the command

java -jar /home/thorstensen/foo/bar/JSkyCalc.jar

I'd recommend putting the JSkyCalc.jar file in an otherwise empty directory, in case you want to unpack it later.

Customizing the site menu.

If you want to customize your site menu, you'll need to unpack the jar file to get at the small data file that contains the site information. Go into the directory with the jar file -- which should otherwise be empty -- and type

    jar -xvf JSkyCalc.jar

That will pull all the files out of the archive (in place, which is why you want to do this in an otherwise empty directory!). Then edit the file skycalcsites.dat according to the instructions you'll find there. There's a skycalcsites.orig file provided for recovery, in case you mess up the editing.

You may be able to reconstruct the executable jar file by typing

jar -cvmf myManifest.mf JSkyCalc.jar .

-- don't forget the final "."! -- at least on a Unix-ish system. This might require the Java Developer's Kit, but it probably doesn't.

Alternatively, you can set your CLASSPATH environment variable to point at the directory you unpacked into (e.g., /home/thorstensen/java/JSkyCalc). Under the Linux bash shell, you set the CLASSPATH like this:

    
    CLASSPATH=$CLASSPATH:/home/thorstensen/java/JSkyCalc
    export CLASSPATH
    echo $CLASSPATH    (this repeats it back so you can check it).

Environment variables are usually twiddled in your startup files somewhere -- something like .bash_profile, or .cshrc -- so you can make the change permanent by editing the appropriate file.

I don't "do" Windows, but I gather from the Windows documentation that environment variables can be edited using the System tool, under the Advanced tab.

Once the jar file has been unpacked and the CLASSPATH has been set, you can start the program by typing

    java JSkyCalc

Of course, most users will want to put it on a taskbar of some sort, depending on their system.

Thanks to Susan Schwarz of Dartmouth's Peter Kiewit Computing Center for helping prepare JSkyCalc for distribution, and Jan Largent of the Department of Physics and Astronomy for posting it.