Lectures on astronomical theories . Appendix. {1.) Eclipses and Occultations of Jupiters Satellites. HerscheVs Outlines of Astronomy. (537.) These eclipses (of Jupiters satellites) moreover,are not seen, as is the case with those of the moon, fromthe centre of their motion, but from a remote station^, andone whose situation with respect to the line of shadow isvariable. This, of course, makes no difference in the timesof the eclipses, but a very great one in their visibility, andin their apparent situations with respect to the planet atthe moments of their entering and quitting the shadow. (53
Lectures on astronomical theories . Appendix. {1.) Eclipses and Occultations of Jupiters Satellites. HerscheVs Outlines of Astronomy. (537.) These eclipses (of Jupiters satellites) moreover,are not seen, as is the case with those of the moon, fromthe centre of their motion, but from a remote station^, andone whose situation with respect to the line of shadow isvariable. This, of course, makes no difference in the timesof the eclipses, but a very great one in their visibility, andin their apparent situations with respect to the planet atthe moments of their entering and quitting the shadow. (538) Suppose S. to be the sun, E. the earth in itsorbit, E. F. G. K., J. Jupiter, and the orbit of one ofits satellites. The cone of the shadow, then, will have itsvertex at X., a point far beyond the orbits of all the satel-lites ; and the penumbra, owing to the great distance of the. sun, and the consequent smallness of the angle (about 6only) its disc subtends at Jupiter, will hardly extend, with-in the limits of the satellites orbits, to any perceptibledistance beyond the shadow—for which reason it is notrepresented in the figure. A satellite revolving from west toeast (in the direction of the arrows) will be eclipsed when 60 APPENDIX—Jupiters satellites. • it enters the shadow at a., but not suddenly, because, likethe moon, it has a considerable diameter seen from theplanet; so that the time elapsing from the first perceptibleloss of light to its total extinction will be that which itoccupies in describing about Jupiter an angle equal to itsapparent diameter as seen from the centre of the planet, orrather, somewhat more, by reason of the penumbra ; and thesame remark applies to its emergence at b. Now, owingto the difference of telescopes and of eyes, it is not possibleto assign the precise moment of incipient obscuration, or oftotal extinction at a., or
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