at the distance of a planet from the sun is not constant. The motion
in a circle is one of such beauty and simplicity that we are reluctant
to abandon it, unless the necessity for doing so be made clearly
apparent. Can we not devise any way by which the circular motion might
be preserved, and yet be compatible with the fluctuations in the
distance from the planet to the sun? This is clearly impossible with the
sun at the centre of the circle. But suppose the sun did not occupy the
centre, while the planet, as before, revolved around the sun. The
distance between the two bodies would then necessarily fluctuate. The
more eccentric the position of the sun, the larger would be the
proportionate variation in the distance of the planet when at the
different parts of its orbit. It might further be supposed that by
placing a series of circles around the sun the various planetary orbits
could be accounted for. The centre of the circle belonging to Venus is
to coincide very nearly with the centre of the sun, and the centres of
the orbits of all the other planets are to be placed at such suitable
distances from the sun as will render a satisfactory explanation of the
gradual increase and decrease of the distance between the two bodies.

There can be no doubt that the movements of the moon and of the planets
would be, to a large extent, explained by such a system of circular
orbits; but the spirit of astronomical enquiry is not satisfied with
approximate results. Again and again the planets are observed, and again
and again the observations are compared with the places which the
planets would occupy if they moved in accordance with the system here
indicated. The centres of the circles are moved hither and thither,
their radii are adjusted with greater care; but it is all of no avail.
The observations of the planets are minutely examined to see if they can
be in error; but of errors there are none at all sufficient to account
for the discrepancies. The conclusion is thus inevitable--astronomers
are forced to abandon the circular motion, which was thought to possess
such unrivalled symmetry and beauty, and are compelled to admit that the
orbits of the planets are not circular.

Then if these orbits be not circles, what are they? Such was the great
problem which Kepler proposed to solve, and which, to his immortal
glory, he succeeded in solving and in proving to demonstration. The
great discovery of the true shape of the planetary orb