A Demonstration Concerning the Motion of Light, Communicated from Paris, in the Journaldes Scavans, and here Made EnglishSource: Philosophical Transactions (1665-1678), Vol. 12 (1677 - 1678), pp. 893-894Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/101779 .

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C 893 )

A4 DemanIlratio wcrnn be Motiox of Light,cmmtiad from Paris, in the jourinal des Scavans, and heye matde Eo~q)

HiloI'ophers have been labouring for m-any years to decide- Pby fome Experience, whether the action of Light be coDn

veyed in an inftance to d'iftant places,~ or whether it requireth tinie. M. Romerrof the R.Academy of t'he Sciences hath devi. fed a way, takien from. the Obfer vatious of the 6irfl Sate lit of 75iupiler- by w'hich hedemonftrates , that forthe diflance of a* bout 3000 leagues, ftich as is V'ery near the bignefs of the D2ia- mneter of the Earth, Light needs not one fecond of t'ime4

Let (in Fixe. r T.) A be the Srn, B 7fupiter, C the firfi Satellit of jupiter, which -enters into the fhadow of 7upiter , to come. out of it at D;- and let EFGHKL be the Eartb placed at -divers didlances from 2upiter.

Now, flp'pofe the Earth , being 'in L towards the reconcl Quiadrature of 7upi"er, hath ~feen the firft Satellir at the time of its emierfi'on or ifTuinig ouit of the fhadow, in D ; and that about 42,y- hours after, (v~id. after on eolution of this Satel- lit, the Earth being in K, do fee it returfned in D; 'it'is miani- fed, that if the Light require timie to traverfe the interval LN, the Sarellit will be (een returned later in Ds than it would have been. if the Earthi had remained in Li, fo that thie rev&'ution of this Satel'it being th'us obferved by the Elmerflons, will be re- rarded by. ro muich t'ime, .as the Light fbaau have taken in pa frlng from Li to K, and that, on the contraryliu the other Quadrature FQ,.where. t he Earth by approaching goes to-m-eet the Light,thc revolutions of .the Imimerfions will appear to be fhortned by -fo much, as thofe of the Emierrions had appeared to be length- ned. And becaufe in 42~ hours , which this Satellit -very .near takes to make one revoluton , the diftance between the Fan't and Tupiter in both the Ovadratures varies, at leaft a. to Diampe.. ters of the Eartb, it follows, that if for the account of every Diamiete-r of the,Earth.there wereirequired a fecond of time, ,the Light would take 3 ! minutes for each of the inter-vals G F, h L;- which would caufe near, half a quarter of an hour be- tween two revollutilons of the firfi. Satelh.'!t one ob,ferved in FO,) and the other in EL, whe'nreas thePre t' o obfi,,rved auyv venfible diffe'rence.

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( 894 ) Yet dotb it not follow hence, -that L'gbt demands no t'ime.

For, after M. Romer had examin'ld the thing, more nearly, he found, that whiat was not fenfible in two revolutions, became very confidefrable i n many be inrg ta ken toget her, and t ha t, for examiple, forty revolutitons obferved on the fide F,. miight be fenfibly fhiorter, than forty others obferved in any place of the Zodijack where Jupiter mnay be miet with;, and that in pro por- -tion of twenty tWO for thbe whole interval of H E, which is the dosUble df the interval that is frotli hence to the Sun.o

The riecdThfiy of this new Equation of the retrardinient of. Light, is eflablitbed by all the obfervat ions that have been miade in the R,Academy, and in the OL'ferv4try, frtepaeoeIgh years, and it bath been lately ccifirmed by cthe Erwerfion of thec firft Satellit obf'erved at JParui the 9tb of Navember laft at e, a Clock, 3 5.4"..4 1 at Night, x o winutts later than it was tc- be expe&id , by deducing i-t from t hofe that had been obferved'i the Month of A0,uguj, when the Ea4rth was mIUCh nearer co itupi. ter: Which M.Rorner had prediaked to the faid Academ)y fromn the beginning of Sep;eml'er.

Burtto r-emove all doubt, that this inequality is caured by the retardmient of the Light , he demoonftrates , that it cannot come from any excentricity, or any other caufe of tho'fe that are commuonl y alIlIedged to)eNplicate the irregularities of the }Mioon and the other Pl-anets ; t hough 'he be well aware, that the firft Satellit,of Jupiter was excentrick, and that, befides, his revo- luitions were advanced or -retarded according as Jupiter did approach to or recede, from the Sun, as -alfo that the revoluti- ons of the'Primum mobile were unequal ; yet faith he,thefe three lat caufes of inequality do not hinder th.e firif fromi being miani- feft.

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