Philip M From philip madsen Sun Apr 29 00:48:03 2007 Robert, I don't think you got my question.. I have to get graphic.. assume the HC system. I wonder if it would help to recall that this basic mechanism was what was used to make the first calculation for the speed of light. A pretty fair description can be found here -- http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html - "Measuring the Speed of Light with Jupiter's Moons" Paul D Send instant messages to your online friends http://au.messenger.yahoo.com