return'd later in D , than it would have been if the Earth had remained in L . So that the Revolution of the Satellit being thus observ'd 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 Emersions will appear to be shortned, by so much as those of the Emersions had appear'd to be lengthned. And because 42½ 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 3½ 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 observ'd 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 observ'd in any place of the Zodiack where Jupiter may be met with; and that in proportion of Twenty two for the whole Interval of HE, 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 establish'd by all the Observations that have been made in the Royal Academy , and in the Observatory , for the space of eight Years; and it hath