ok, that's not actually true... my .jar file doesn't seem to be working
locally either.

ok then, never mind. I figured out my problem.

Just for the record, in case anybody else hits this issue:

I created the jar file incorrectly. Because the .class files were part of a
package called "ephemeris", I needed to create the jar file from the
directory above using:

jar cvf ephemeris.jar ephemeris

rather than in the directory itself using

jar cvf ephemeris.jar *.class

once I fixed that, everything worked (phew! that was driving me nuts for a
while...)