The following is what I could coax out of WinJUPOS for ephemeris data, but I am not a skilled user of the software: WinJUPOS 8.0.4 (Jupiter), C.M. transit times, 2009/07/20 14:34 Object longitude: L2 = 216.0° + 0.0000°/d * (T - 2009 Jul 19.5) Time interval: 2009 Jul 20.0 ... 2009 Jul 31.0 Only if celestial body is visible Geographic longitude and latitude: -111 00, +42 00 Output format: Date UT (C.M. of System 2, Altitude) ------------------------------------------------------------------------------ 2009 Jul 20 11:56 ( 216°, 24°) 2009 Jul 21 07:47 ( 216°, 30°) 2009 Jul 23 09:25 ( 216°, 34°) 2009 Jul 24 05:16 ( 216°, 13°) 2009 Jul 25 11:03 ( 216°, 27°) 2009 Jul 26 06:54 ( 216°, 27°) 2009 Jul 28 08:32 ( 216°, 33°) 2009 Jul 29 04:23 ( 216°, 8°) 2009 Jul 30 10:10 ( 216°, 30°) ------------------------------------------------------------------------------ An Sky & Telescope "AstroAlert" just popped up on my desktop and states the following: "The spot is located near Jupiter's System II longitude 210°. For the predicted times when it will cross the planet's central meridian, add 2 hours and 6 minutes to each of our predicted transit times for Jupiter's Great Red Spot." Clear Skies - Kurt