observer, a
considerable compensation was preparing. The very year that the fire
occurred in Denmark a quiet philosopher in England was speculating and
brooding on a remarkable observation that he had made concerning the
apparent motion of certain stars, and he was led thereby to a discovery
of the first magnitude concerning the speed of light--a discovery which
resuscitated the old theory of Roemer about Jupiter's satellites, and
made both it and him immortal.

James Bradley lived a quiet, uneventful, studious life, mainly at Oxford
but afterwards at the National Observatory at Greenwich, of which he was
third Astronomer-Royal, Flamsteed and Halley having preceded him in that
office. He had taken orders, and lectured at Oxford as Savilian
Professor. It is said that he pondered his great discovery while pacing
the Long Walk at Magdalen College--and a beautiful place it is to
meditate in.

Bradley was engaged in making observations to determine if possible the
parallax of some of the fixed stars. Parallax means the apparent
relative shift of bodies due to a change in the observer's position. It
is parallax which we observe when travelling by rail and looking out of
window at the distant landscape. Things at different distances are left
behind at different apparent rates, and accordingly they seem to move
relatively to each other. The most distant objects are least affected;
and anything enormously distant, like the moon, is not subject to this
effect, but would retain its position however far we travelled, unless
we had some extraordinarily precise means of observation.

So with the fixed stars: they were being observed from a moving
carriage--viz. the earth--and one moving at the rate of nineteen miles a
second. Unless they were infinitely distant, or unless they were all at
the same distance, they must show relative apparent motions among
themselves. Seen from one point of the earth's orbit, and then in six
months from an opposite point, nearly 184 million miles away, surely
they must show some difference of aspect.

Remember that the old Copernican difficulty had never been removed. If
the earth revolved round the sun, how came it that the fixed stars
showed no parallax? The fact still remained a surprise, and the question
a challenge. Picard, like other astronomers, supposed that it was only
because the methods of observation had not been delicate enough; but now
that, since the invention of the telescope and the