"A short time after the invention of the telescope and the consequent discovery of Jupiter's satellites, Römer... was engaged in a series of observations... to determine the time which one of these bodies took to revolve round its planet. The method employed by Römer was to observe the successive s of the satellite and to notice the interval that elapsed between each of them. But it at last happened that the interval between the two occultations, which was about forty five hours, became prolonged by periods of 8, 13, and 16 minutes, during that half of the year when the earth was receding from the planet, while it became proportionally cut short during [earth's approach]. Römer was struck by a happy idea he suspected instantly that... an interval of time sufficiently long [was required] to allow the light that had left the satellite immediately after its disappearance to reach the eye of the observer. ...[T]he farther off the earth was from the satellite the longer was the interval of time between its disappearance and that of the arrival of the last portions of its light upon the earth ...It was thus that Römer explained the difference between the calculated and observed time of the occultation and he saw that he was on the threshold of a great discovery. ...he saw that light propagated itself through space with a certain velocity and that the fact... just mentioned furnished the precise means of measuring it. Thus the occultation of the satellite was retarded one second for every 185,000 miles that the earth is distant from Jupiter; the reason being that a ray of light takes a second to travel this distance... because the velocity of light is... 185,000 miles per second."