File:Lectures on astronomical theories (1876) (14596708728).jpg

Identifier: lecturesonastron14harr (find matches)
Title: Lectures on astronomical theories
Year: 1876 (1870s)
Authors: Harris, John
Subjects: Astronomy
Publisher: Montreal, Lovell Printing and Publishing Co.
Contributing Library: University of California Libraries
Digitizing Sponsor: Internet Archive

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s, and noticing the time between them. Now if it were possible to observe accurately the momentat which the satellite would, after each revolution, eitherenter the shadow or emerge from it, the interval of timebetween these events would enable us to calculate exactlythe velocity and motion of the satellite. It was, then, inthis manner that Roemer proposed to ascertain the motionof the satellite. But, in order to obtain this estimate withthe greatest possible precision, he proposed to continue his-observations for several months. Let us, then, suppose that we have observed the timewhich has elapsed between two successive eclipses, and thatthis time is, for example, forty-three hours. We ought toexpect that the eclipse would recur after the lapse of everysuccessive period of forty-three hours. 24 VELOCITY-OF-LIGHT THEORY. Imagine, then, a lable to be compuei in which we shallcalculate and register before Land the moment at whichevery successive eclipse of the satellite for twelve months
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Pig. 756. VELOCITY-OF-LIGHT THEORY. 25 ^ to come shall occur, and let us conceive that the earth is atA^ at the commencement of our observations; we shall then,as Eoemer did, observe the moment at which the eclipsesoccur, and compare them with the moments registered inthe table. Let the earth, at the commencement of these observa-tions, be supposed at E, fig. 756, where it is nearest toJupiter. When the earth has moved to E, it will be foundthat the occurrence of the eclipse is a little later than thetime registered in the table. As the earth moves, from E.towards E., the actual occurrence of the eclipse is moreand more retarded beyond the computed occurrence, until-at E. in conjunction, it is found to occur about sixteenjninutes later than the calculated time. By observations such as these, Eoemer was struck withthe fact that his predictions of the eclipses proved in■every case to be wrong. It would at first occur to himthat this discrepancy might arise from some errors of hisobser

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