http://www.space.com/4314-length-saturn-day-revised.html They measure some presumed solid(ish) center. If the magnetic field axis and the rotation axis are displaced, you can measure when the magnetic field goes by. Or by radio sources. Jupiter day length at the poles is slightly longer than day length at equator, but it's about 1% 9 hr 56 min vs 9 hr 50 min. I figure the Jupiter day length (which has been given as about 10 hours for a very long time, certainly before we sent spacecraft nearby) is probably from visual observation of the Great Red Spot. You can easily see it move in a not very big telescope over the course of a night.

Here's some insight into measurement of jovian rotation back in the good old days. A wee bit more sophisticated than looking at the Great Red Spot. Gardner, FF & Shain, CA (1958). Further Observations of Radio Emission from the Planet Jupiter. Australian Journal of Physics 11(1) 55-69. On Saturday, July 9, 2016, jimlux <jimlux@earthlink.net> wrote: > http://www.space.com/4314-length-saturn-day-revised.html > > They measure some presumed solid(ish) center. > > If the magnetic field axis and the rotation axis are displaced, you can > measure when the magnetic field goes by. Or by radio sources. > > Jupiter day length at the poles is slightly longer than day length at > equator, but it's about 1% 9 hr 56 min vs 9 hr 50 min. > > I figure the Jupiter day length (which has been given as about 10 hours > for a very long time, certainly before we sent spacecraft nearby) is > probably from visual observation of the Great Red Spot. You can easily see > it move in a not very big telescope over the course of a night. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- I am Pulse. Unbreakable.