host of disturbing and complicated considerations come in--not so
violently disturbing for the first satellite as for the others, because
it moves so quickly, but still complicated enough.

The fact is, the real motion of Jupiter's satellites is a most difficult
problem. The motion even of our own moon (the lunar theory) is difficult
enough: perturbed as its motion is by the sun. You know that Newton said
it cost him more labour than all the rest of the _Principia_. But the
motion of Jupiter's satellites is far worse. No one, in fact, has yet
worked their theory completely out. They are perturbed by the sun, of
course, but they also perturb each other, and Jupiter is far from
spherical. The shape of Jupiter, and their mutual attractions, combine
to make their motions most peculiar and distracting.

Hence an error in the time of revolution of a satellite was not
_certainly_ due to the cause Roemer suggested, unless one could be sure
that the inequality was not a real one, unless it could be shown that
the theory of gravitation was insufficient to account for it. This had
not then been done; so the half-made discovery was shelved, and properly
shelved, as a brilliant but unverified speculation. It remained on the
shelf for half a century, and was no doubt almost forgotten.

[Illustration: FIG. 76.--A Transit-instrument for the British
astronomical expedition, 1874. Shewing in its essential features the
simplest form of such an instrument.]

Now a word or two about the man. He was a Dane, educated at Copenhagen,
and learned in the mathematics. We first hear of him as appointed to
assist Picard, the eminent French geodetic surveyor (whose admirable
work in determining the length of a degree you remember in connection
with Newton), who had come over to Denmark with the object of fixing the
exact site of the old and extinct Tychonic observatory in the island of
Huen. For of course the knowledge of the exact latitude and longitude of
every place whence numerous observations have been taken must be an
essential to the full interpretation of those observations. The
measurements being finished, young Roemer accompanied Picard to Paris,
and here it was, a few years after, that he read his famous paper
concerning "An Inequality in the Motion of Jupiter's First Satellite,"
and its explanation by means of an hypothesis of "the successive
propagation of light."

The later years of his life he spent in Copenhagen as a professor in