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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eclipse of Io
From: Herbert Prinz
Date: 2002 Oct 9, 11:48 +0000
From: Herbert Prinz
Date: 2002 Oct 9, 11:48 +0000
Ken, Any ephemeris with a practical purpose such as navigation, surveying or star-gazing will always tabulate apparent places or conversely, for phenomena, the times at which they can actually be observed on Earth. Indeed, when an ephemeris is generated from mechanical principles, one first comes up with the "geometric" places of the celestial objects. But these have to be converted into "apparent" places by accounting for light speed (and other effects). Where the prediction of eclipses of Io is concerned, a computation of light time is almost all there is to it. (I am exaggerating here just a tiny bit.) These eclipses occur more or less with the synodic period of revolution of 1d 18h 28m. Correct this for light time and you are pretty much within 3 to 5 minutes of the eclipse. I am sure you know the story of Ole Roemer, who noticed that Io eclipses around conjunction of Jupiter with the Sun were on average a little under 10 minutes early compared with the mean expected value, while they were late by the same amount near opposition . He offered the neat explanation that light might take exactly that time to traverse the diameter of the Earth orbit.Flamsteed was convinced and made a table as I described above. Right now the difference in RA between Sun and Jupiter is 4h. If light time were the problem, I would be some 4 min early with my observation, not the 40 sec that I encountered. But more important, I would be early all the same at immersions and emersions. However, when timing an emersion, I see it too late, which is consistent with assuming telescope error. To cancel this error, it is often suggested to time both ends of an eclipse. This is easier said than done. There was a total of 5 such possibilities this year in my place and 7 in Greenwich. Regards Herbert Prinz Ken Gebhart wrote: > Your story made me wonder about the following: > Doesn't celestial mechanics compute the actual position of the various > bodies in the solar system instead of just their appearance to us on > Earth? If so, the time for light to reach us would give us a different > visual witness of an event. Could this account for the disparity in time > that you experienced? > Ken Gebhart