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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

   

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