Quote:
Originally Posted by Socco
Is this logic correct?
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No it is not correct. The time of an occultation of one of the Jovian moons and Jupiter is not significantly influenced by the difference in longitude of the place of observation and the place assumed for the ephemeris calculation.'
The ephemeris calculation will be a geocentric one (ie for the centre of the earth) because from the distance of Jupiter, the parallax angle subtended by the earth is miniscule, ie it matters not one jot where you are on earth - all observers would see the occultation at the same Universal time.
(This provides a means to determine longitude, by the way, much as James Cook did with the transit of Venus to check the accuracy of Harrison's clock).
The real problem is more likely that the prediction is in ephemeris time and did not correctly allow for the light-time between Jupiter and Earth - which is 40 +/- 8 minutes depending on where the earth is in its orbit around the sun. Almost certainly the ephemeris applied a correction of 40 minutes as this is the light-time between Jupiter and the Sun (as a first estimate) rather than work out the distance Earth-Jupiter and compute it properly. This alone would easily account for an error of 8 minutes.
In the past the correction for the Earth's position was often not applied because this allowed amateur observers to do two things:
(a) it permits an easy experiment to measure the speed of light as a function of the diameter of the earths orbit by measuring the difference between the observed time and the ephemeris time at different times of the year.
This method provided one of the earliest reliable measurements of the speed of light.
http://www.physlink.com/Education/As...8793E43D2B5D82
(b) you could use simple arithmetic to extrapolate the timings of the occultations of the satellites for a month either way, assuming they have circular orbits without bothering to correct for the earth's position each time. Made life simpler for those without a computer.
Cheers...
