after, reckoning from the moment when the Earth was at B, that the
Moon, arriving at C, will be obscured: but this obscuration or
interruption of the light will not reach the Earth till after another
hour. Let us suppose that the Earth in these two hours will have
arrived at E. The Earth then, being at E, will see the Eclipsed Moon
at C, which it left an hour before, and at the same time will see the
sun at A. For it being immovable, as I suppose with Copernicus, and
the light moving always in straight lines, it must always appear where
it is. But one has always observed, we are told, that the eclipsed
Moon appears at the point of the Ecliptic opposite to the Sun; and yet
here it would appear in arrear of that point by an amount equal to the
angle GEC, the supplement of AEC. This, however, is contrary to
experience, since the angle GEC would be very sensible, and about 33
degrees. Now according to our computation, which is given in the
Treatise on the causes of the phenomena of Saturn, the distance BA
between the Earth and the Sun is about twelve thousand diameters of
the Earth, and hence four hundred times greater than BC the distance
of the Moon, which is 30 diameters. Then the angle ECB will be nearly
four hundred times greater than BAE, which is five minutes; namely,
the path which the earth travels in two hours along its orbit; and
thus the angle BCE will be nearly 33 degrees; and likewise the angle
CEG, which is greater by five minutes.
But it must be noted that the speed of light in this argument has been
assumed such that it takes a time of one hour to make the passage from
here to the Moon. If one supposes that for this it requires only one
minute of time, then it is manifest that the angle CEG will only be 33
minutes; and if it requires only ten seconds of time, the angle will
be less than six minutes. And then it will not be easy to perceive
anything of it in observations of the Eclipse; nor, consequently, will
it be permissible to deduce from it that the movement of light is
instantaneous.
It is true that we are here supposing a strange velocity that would be
a hundred thousand times greater than that of Sound. For Sound,
according to what I have observed, travels about 180 Toises in the
time of one Second, or in about one beat of the pulse. But this
supposition ought not to seem to be an impossibility; since it is not
a question of the transport of a body with so great a speed, but of a
successive movement which is passed on from some bodies to others. I
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