The elements of astronomy; a textbook . han 50 years, until long after his death, whenBradleys discovery of aberration proved the correctness of his views. 353. Eclipses of the satellites recur at intervals which arereally almost exactly equal (the perturbations being very slight),and the interval can easily be determined and the times tab-ulated. But if we thus predict the times of the eclipses during a whole synodic period ofthe planet, then, beginningat the time of opposition, itis found that as the planetrecedes from the earth, theeclipses, as observed, fall con-stantly more and more be-hi
The elements of astronomy; a textbook . han 50 years, until long after his death, whenBradleys discovery of aberration proved the correctness of his views. 353. Eclipses of the satellites recur at intervals which arereally almost exactly equal (the perturbations being very slight),and the interval can easily be determined and the times tab-ulated. But if we thus predict the times of the eclipses during a whole synodic period ofthe planet, then, beginningat the time of opposition, itis found that as the planetrecedes from the earth, theeclipses, as observed, fall con-stantly more and more be-hindhand, and by preciselythe same amount for allfour satellites. The differ-ence between the predictedand observed time continuesto increase until the planetis near conjunction, whenthe eclipses are almost 17minutes later than the pre-diction. After the conjunction they quicken their pace, andmake up the loss, so that when opposition is reached oncemore they are again on time. It is easy to see from Fig. 86 that at opposition the planet. Fig. of the Equation of Light. § 353] THE EQUATION OF LIGHT. 251 is nearer the earth than at conjunction by just two astronom-ical units; , JB — J A = 2SA, and light coming from J tothe earth when it is at A, will, therefore, make the journeyquicker than when it is at B, by twice the time it takes lightto pass from S to A. The whole apparent retardation of eclipses between opposi-tion and conjunction must therefore be exactly twice the time1required for light to come from the sun to the earth. In this waythe light-equation constant is found to be very nearly 499seconds, or 8 minutes, 19 seconds, with a probable error ofperhaps two seconds. 354. Since these eclipses are gradual phenomena, the determinationof the exact moment of a satellites disappearance or reappearance is•very difficult, and this renders the result somewhat uncertain. Prof. Pickering of Cambridge has proposed to utilize photometric observa-tions fo
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