216. It is found by observation, that when the Earth is between the Sun and Jupiter, as at _g_, his Satellites are eclipsed about 8 minutes sooner than they should be according to the Tables: and when the Earth is at _B_ or _C_, these Eclipses happen about 8 minutes later than the Tables predict them. Hence it is undeniably certain, that the motion of light is not instantaneous, since it takes about 16-1/2 minutes of time to go through a space equal to the diameter of the Earth’s Orbit, which is 162 millions of miles in length: and consequently the particles of light fly about 164 thousand 494 miles every second of time, which is above a million of times swifter than the motion of a cannon bullet. And as light is 16-1/2 minutes in travelling across the Earth’s Orbit, it must be 8-1/4 minutes in coming from the Sun to us: therefore, if the Sun were annihilated we should see him for 8-1/4 minutes after; and if he were again created he would be 8-1/4 minutes old before we could see him. [Sidenote: Fig. V. Illustrated by a Figure.] 217. To illustrate this progressive motion of light, let _A_ and _B_ be the Earth in two different parts of it’s Orbit, whose distance is 81 millions of miles, equal to the Earth’s distance from the Sun _S_. It is plain, that if the motion of light were instantaneous, the Satellite 1 would appear to enter into Jupiter’s shadow _FF_ at the same moment of time to a spectator in _A_ as to another in _B_. But by many years observations it has been found, that the immersion of the Satellite into the shadow is seen 8-1/4 minutes sooner when the Earth is at _B_, than when it is at _A_. And so, as Mr. ROMER first discovered, the motion of light is thereby proved to be progressive, and not instantaneous, as was formerly believed. It is easy to compute in what time the Earth moves from _A_ to _B_; for the chord of 60 degrees of any Circle is equal to the Semidiameter of that Circle; and as the Earth goes through all the 360 degrees of it’s Orbit in a year, it goes through 60 of those degrees in about 61 days. Therefore, if on any given day, suppose the first of _June_, the Earth is at _A_, on the first of _August_ it will be at _B_: the chord, or straight line _AB_, being equal to _DS_ the Radius of the Earth’s Orbit, the same with _AS_ it’s distance from the Sun. 218. As the Earth moves from _D_ to _C_, through the side _AB_ of it’s Orbit, it is constantly meeting the light of Jupiter’s Satellites sooner, which occasions an apparent acceleration of their Eclipses: and as it moves through the other half _H_ of it’s Orbit, from _C_ to _D_, it is receding from their light, which occasions an apparent retardation of their Eclipses, because their light is then longer ere it overtakes the Earth.