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Alex,

Raper, in his "Practice of Navigation and Nautical Astronomy, from 1840, writes:

"The telescope should have a magnifying power of not less than 40, and the observer should be ready some minutes before the time of observation, estimated by applying the long. by acc. to the time in the Nautical Almanac."

"The sun should not be less than 8° below the horizon, nor Jupiter less than 8° above it, for the phenomenon to be distinctly visible."

"The difference between the M.T. at place, found by observation, and that at Greenwich, is the long."

"This method, though easy and convenient, is not very accurate; the eclipse is not instantaneous; and the clearness of the air, and the power employed, affect considerably the time of the phenomenon, in which observers have been found to differ 40^s or 50^s."

"The observation may be considered complete only when the immersion and emersion of the same satellite are observed on the same evening, and as nearly as possible under the same circumstances. Thus, if the satellite disappear a little sooner than if the air had been clearer, it will emerge a little later from the same cause, and the mean of the two results may be near the truth."

Lars