THE SATELLITES OF JUPITER.

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as a,— the eclipses of the Satellites will precede their occulta-tions ; the arrows showing the direction of the motions in-volved, and the order of the phenomena that will occur beingindicated by the letters I, E, n, e, which respectively stand forIngress, Egress, Disappearance, Reappearance. After theEarth has passed o, and has attained such a position as z, theorder will evidently be reversed, and be d, r, i, e.

It may he noticed in our diagram that Jupiter is supposedto he in the same position j, during the whole time thatthe Earth is moving from a to z ; in fact, the onward motionof Jupiter is not shown. But the figure is only so drawnto save complication, and the explanation given will applyjust as well as if successive positions of Jupiter were shown.This is the case because the Earth moves round the Sunmuch more quickly than Jupiter , so that we may neglect themotion of the latter, by supposing the Earth to he movingwith the difference of its own angular velocity and that ofJupiter . Or even without so doing, we might first suppose zto be omitted in the diagram, and then, secondly, suppose a tobe omitted and z to be inserted: when it would perfectlyapply to the two different cases, and correctly represent foreach of them the relative positions of the Earth and ofJupiter , and the phenomena that would result.

It may also he perceived, that the nearer a Satellite is toJupiter , the less likely is it that all the four phenomena (indi-cated by the letters I, e, d, r) will he visible when the Earth isat a given distance from o, without their interfering with oneanother. For instance, if a Satellite he supposed to move inan orbit represented by the dotted circle, which is drawn atabout frds of the distance of the circle iedr from Jupiter inFig . LXXVII., it is evident that an observer upon the Earth at a would see it pass into the shadow, hut he would not see itemerge, since just as it would otherwise do so it would heocculted. Presently, however, he would perceive it reappearfrom behind the planet, its emersion from eclipse and itsdisappearance by occultation having both been invisible.Similarly if the Earth were at z, the disappearance of sucha Satellite by occultation would be seen, but not its re-

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