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Quoting from Muirden "The Amateur Astronomers Handbook":
".....the Spot is so large that it is not too easy to judge just when the centre is on the planet's meridian. The best method is to take three transits, for the p and f ends as well as the middle."
No details on how this data is reduced, but a variation of 12 seconds in rotation period for the GRS is noted, involving significant forces to speed up or retard this massive object that far.
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