spring it was less, and in autumn it was more than usual.
This was evidently a puzzling fact: what on earth can our year have to
do with the motion of a moon of Jupiter's? It was probably, therefore,
only an apparent change, caused either by our greater or less distance
from Jupiter, or else by our greater or less speed of travelling to or
from him. Considering it thus, he was led to see that, when the time of
revolution seemed longest, we were receding fastest from Jupiter, and
when shortest, approaching fastest.

_If_, then, light took time on its journey, _if_ it travelled
progressively, the whole anomaly would be explained.

In a second the earth goes nineteen miles; therefore in 42-3/4 hours
(the time of revolution of Jupiter's first satellite) it goes 2.9
million (say three million) miles. The eclipse happens punctually, but
we do not see it till the light conveying the information has travelled
the extra three million miles and caught up the earth. Evidently,
therefore, by observing how much the apparent time of revolution is
lengthened in one part of the earth's orbit and shortened in another,
getting all the data accurately, and assuming the truth of our
hypothetical explanation, we can calculate the velocity of light. This
is what Roemer did.

Now the maximum amount of retardation is just about fifteen seconds.
Hence light takes this time to travel three million miles; therefore its
velocity is three million divided by fifteen, say 200,000, or, as we now
know more exactly, 186,000 miles every second. Note that the delay does
not depend on our _distance_, but on our _speed_. One can tell this by
common-sense as soon as we grasp the general idea of the explanation. A
velocity cannot possibly depend on a distance only.

[Illustration: FIG. 75.--Eclipses of one of Jupiter's satellites. A
diagram intended to illustrate the dependence of its apparent time of
revolution (from eclipse to eclipse) on the motion of the earth; but not
illustrating the matter at all well. TT' T'' are successive positions of
the earth, while JJ' J'' are corresponding positions of Jupiter.]

Roemer's explanation of the anomaly was not accepted by astronomers. It
excited some attention, and was discussed, but it was found not
obviously applicable to any of the satellites except the first, and not
very simply and satisfactorily even to that. I have, of course, given
you the theory in its most elementary and simple form. In actual fact a