A Demonstration Concerning the Motion of Light, Communicated from Paris, in the Journal des Scavans, and here Made English

A Demonstration concerning the Motion of Light, communicated from Paris, in the Journal des Scavans, and here made English. Philosophers have been labouring for many years to decide by some Experience, whether the action of Light be conveyed in an instance to distant places, or whether it requireth time. M. Romer of the R. Academy of the Sciences hath devised a way, taken from the Observations of the first Satellit of Jupiter, by which he demonstrates, that for the distance of about 3000 leagues, such as is very near the bigness of the Diameter of the Earth, Light needs not one second of time. Let (in Fig. 11.) A be the Sun, B Jupiter, C the first Satellit of Jupiter, which enters into the shadow of Jupiter, to come out of it at D; and let EFGHKL be the Earth placed at divers distances from Jupiter. Now, suppose the Earth, being in L towards the second Quadrature of Jupiter, hath seen the first Satellit at the time of its emersion or issuing out of the shadow in D; and that about $42\frac{1}{2}$ hours after, (vid. after one revolution of this Satellit,) the Earth being in K, do see it returned in D; it is manifest, that if the Light require time to traverse the interval LK, the Satellit will be seen returned later in D, than it would have been if the Earth had remained in L, so that the revolution of this Satellit being thus observed by the Emersions, will be retarded by so much time, as the Light shall have taken in passing from L to K, and that, on the contrary, in the other Quadrature FG, where the Earth by approaching goes to meet the Light, the revolutions of the Immersions will appear to be shortened by so much, as those of the Emersions had appeared to be lengthened. And because in $42\frac{1}{2}$ hours, which this Satellit very near takes to make one revolution, the distance between the Earth and Jupiter in both the Quadratures varies at least 210 Diameters of the Earth, it follows, that if for the account of every Diameter of the Earth there were required a second of time, the Light would take $3\frac{1}{2}$ minutes for each of the intervals GF, KL; which would cause near half a quarter of an hour between two revolutions of the first Satellit, one observed in FG, and the other in KL, whereas there is not observed any sensible difference. Yet doth it not follow hence, that Light demands no time. For, after M. Romer had examin'd the thing more nearly, he found, that what was not sensible in two revolutions, became very considerable in many being taken together, and that, for example, forty revolutions observed on the side F, might be sensibly shorter, than forty others observed in any place of the Zodiack where Jupiter may be met with; and that in proportion of twenty two for the whole interval of H E, which is the double of the interval that is from hence to the Sun. The necessity of this new Equation of the retardment of Light, is established by all the observations that have been made in the R. Academy, and in the Observatory, for the space of eight years, and it hath been lately confirmed by the Emersion of the first Satellit observed at Paris the 9th of November last at 5 a Clock, 35'. 45''. at Night, 10 minutes later than it was to be expected, by deducing it from those that had been observed in the Month of August, when the Earth was much nearer to Jupiter: Which M. Romer had predicted to the said Academy from the beginning of September. But to remove all doubt, that this inequality is caused by the retardment of the Light, he demonstrates, that it cannot come from any eccentricity, or any other cause of those that are commonly alleged to explicate the irregularities of the Moon and the other Planets; though he be well aware, that the first Satellit of Jupiter was excentrick, and that, besides, his revolutions were advanced or retarded according as Jupiter did approach to or recede from the Sun, as also that the revolutions of the primum mobile were unequal; yet saith he, these three last causes of inequality do not hinder the first from being manifest.