J U P I r E R. 



ctiliglitcneJ by the fun ; and when the latter interpofe be- 

 tween the fun and Jupiter, they produce real folar echpfes, 

 precifely fimilar to thofe wliich the moon occafioiis ou the 

 eartli. 



This phenomenon leads to tlie explan;i?ion of another 

 whicli the fatcUites prefcnt. Tlicy arc ofttii obfervcd to 

 difappear, though at fome diftance from tl)f difc of tlic 

 planet : the third and fourth rc-appear fonu'tinies on the 

 fame fide of the difc. 



The fiiadow whicli Jupiter projects behind it, (relatively 

 to the fun,) is the only caufe that can explain thefe difap- 

 pearances, which are perfectly fimilar to eclipfcs of the 

 inoon. 



The circum (lances which accompany them leave no doubt 

 of the reality of this caufe. The fatellites are always ob- 

 ferved to difappear on the fide of the difc oppofite to the 

 fun, and confequently on the fame fid- to whicli the conical 

 fliadow is projefted. They are echpfed nearell the difc, 

 wlien the planet is nearell to its oppofition. 



Finally, the duration of thefe cclipfes anfwers to the 

 time which ftiould elapfe while they traverfe the (hadow of 

 J upiter. 



Thus it appears that th;fe fatellites move from well to enO, 

 in returning orbits roi.nl the planet. Obftrrations of their 

 ec'ipfes are the moil exacl means ofdetermining their motions. 

 Their mean fidereal and fynodical revolutions, as feen from the 

 ccnlrv; of Jupiter, are very accurately determined by com- 

 paring eciiples at long interva's from each other, and ob- 

 ferved near the oppofition of the planet. It is thus dif- 

 coVered that the n.otion of the frtellites of Jupiter is nearly 

 circular and uniform, becaufe this hypothefis correfponds 

 very nearly with thofe cclipfes which happen when we fee 

 this planet in the fame pofition relatively to the fun. There- 

 fore, the pofitions of the fatellites at every inllant, as feen 

 from the centre of Jupiter, may be determined. From 

 hence lefiilts a fimple and futF.ciently exact method of com- 

 paring with each other the diftances of Jupiter and of the 

 fun from the earth: — a method which tiie ancient aftro- 

 nomers did not poffefs. For the parallax of Jupiter is in- 

 fenfible even to the precifion of modern obfervations, when 

 it is neare.l to us. They only judged of its diftance by th.e 

 time of its revolution ; as they etlimated thofe planets to be 

 the fartheil from us, whofe period of revolution was the 

 longed. 



Let u» fuppole that the total duration of an eclipfe of 

 the third fatellite has been obferved. At the middle of the 

 eclipfe, the fatellite, feen from the centre of Jupiter, is 

 nearly in oppofition to the fun. Its fidereal pofition, ob- 

 served from this centre, (which may eafily be deduced from 



its mean motion,"/ u, thercfoi-e, t!:e fame as that of li.e 

 centre of Jupiter ieen from the centre of the fun. 



Direcl obfervation, or the known motion of the fun, gives 

 the pofition of the earth as feen from its centre. Thus, 

 fuppofing a triangle to be formed by the right hnes which 

 join the centres of the fun, the earth, and Jupiter, we have 

 given in this triangle the angle at the fun, obfervation will 

 give that at the earth, and we fliall obtain the rectilinear dif- 

 tance from Jupiter to tiie earth and to the fun, at the inilant 

 of the middle of the eclipfe, in parts of the diilance from the 

 fun to the earth. 



It is found by thefe means that Jupiter is at lead five 

 times farther from us than the Hin, when its apparent dia- 

 meter is ^B'.^. The diameter of the earth, at the fame dif- 

 tance, would not fubtend an angle of 5".j ; the volume of 

 Jupiter is, tlierefore, at leaft a thoufand times greater than 

 that of the earth. 



The apparent diameters of thefe fatellites being infenfible, 

 their magnitude cannot be exactly meafured. An attempt 

 has been made to appreciate it, by the time they take to 

 penetrate the (liadow of the planet. But there is a great 

 difcordance in the obfervations that have been made to afcer- 

 tain this circumllance. This arifes from the various powers 

 of telefcopes, the different degrees of perfection in the light 

 of the obferver, the ilate cf the atmofphere, the altitude of 

 the fatellites above the horizon, their apparent diilance froirr 

 Jupiter, and the change of the hemifphere prefented to u-. 

 The comparative brightnefs is independent of the four firil 

 caufes, which only alter tbeir proportional hght, and ought, 

 therefore, to afford information concTerning the rotatory 

 motion of thefe bodies. Dr. Herfchel, who is occupied in 

 this dclicbte inveftigation, has obferved that they furpafs 

 each other alternately in brilliance : — a circumllance which 

 enables us to judge of the niaxhmim and mimmum of their 

 light. The relation of this mushnum and nun'niwm to their 

 mutual pafitions has perfuaded him, that tluy turn upon 

 their own axis, like the moon in a period equal to the dura- 

 tion of their revolution round Jupiter : a refult which Ma- 

 raldi had already deduced with regard to the fourth fatellite, 

 from the returns of the fame fpot obferved on its difc m its 

 pafTage over the planet. 



The great diilance of the celeftial bodies weakens the 

 phenomena which their furfaccs prefent, till they are re- 

 duced to flight variations of light, which eicape tlie iirit 

 view, and are only rendered fenfible by a long coiirie ot ob- 

 fervations. But information derived from this fource ought 

 to be received with the greateil caution ; conCdering how 

 much we may be milled, on fuch eccaliuiis, by the tfi'ect 

 of imagination. 



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T.\EI.E 



