that if Light requires time for its passage the illumination of the
little planet will be perceived later at C than it would have been at
B, and that there must be added to this time of 30 times 42-1/2 hours
that which the Light has required to traverse the space MC, the
difference of the spaces CH, BH. Similarly at the other quadrature
when the earth has come to E from D while approaching toward Jupiter,
the immersions of the Satellite ought to be observed at E earlier than
they would have been seen if the Earth had remained at D.
Now in quantities of observations of these Eclipses, made during ten
consecutive years, these differences have been found to be very
considerable, such as ten minutes and more; and from them it has been
concluded that in order to traverse the whole diameter of the annual
orbit KL, which is double the distance from here to the sun, Light
requires about 22 minutes of time.
The movement of Jupiter in his orbit while the Earth passed from B to
C, or from D to E, is included in this calculation; and this makes it
evident that one cannot attribute the retardation of these
illuminations or the anticipation of the eclipses, either to any
irregularity occurring in the movement of the little planet or to its
eccentricity.
If one considers the vast size of the diameter KL, which according to
me is some 24 thousand diameters of the Earth, one will acknowledge
the extreme velocity of Light. For, supposing that KL is no more than
22 thousand of these diameters, it appears that being traversed in 22
minutes this makes the speed a thousand diameters in one minute, that
is 16-2/3 diameters in one second or in one beat of the pulse, which
makes more than 11 hundred times a hundred thousand toises; since the
diameter of the Earth contains 2,865 leagues, reckoned at 25 to the
degree, and each each league is 2,282 Toises, according to the exact
measurement which Mr. Picard made by order of the King in 1669. But
Sound, as I have said above, only travels 180 toises in the same time
of one second: hence the velocity of Light is more than six hundred
thousand times greater than that of Sound. This, however, is quite
another thing from being instantaneous, since there is all the
difference between a finite thing and an infinite. Now the successive
movement of Light being confirmed in this way, it follows, as I have
said, that it spreads by spherical waves, like the movement of Sound.
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