s nearly if not quite as useful.
We can solve the problem approximately; we can find out what the effect
of one planet on the other is _very nearly_, and by additional labour we
can reduce the limits of uncertainty to as low a point as may be
desired. We thus obtain a practical solution of the problem adequate for
all the purposes of science. It avails us little to know the place of a
planet with absolute mathematical accuracy. If we can determine what we
want with so close an approximation to the true position that no
telescope could possibly disclose the difference, then every practical
end will have been attained. The reason why in this case we are enabled
to get round the difficulties which we cannot surmount lies in the
exceptional character of the problem of three bodies as exhibited in the
solar system. In the first place, the sun is of such pre-eminent mass
that many matters may be overlooked which would be of moment were he
rivalled in mass by any of the planets. Another source of our success
arises from the small inclinations of the planetary orbits to each
other; while the fact that the orbits are nearly circular also greatly
facilitates the work. The mathematicians who may reside in some of the
other parts of the universe are not equally favoured. Among the sidereal
systems we find not a few cases where the problem of three bodies, or
even of more than three, would have to be faced without any of the
alleviating circumstances which our system presents. In such groups as
the marvellous star Th Orionis, we have three or four bodies
comparable in size, which must produce movements of the utmost
complexity. Even if terrestrial mathematicians shall ever have the
hardihood to face such problems, there is no likelihood of their being
able to do so for ages to come; such researches must repose on accurate
observations as their foundation; and the observations of these distant
systems are at present utterly inadequate for the purpose.

The undisturbed revolution of a planet around the sun, in conformity
with Kepler's law, would assure for that planet permanent conditions of
climate. The earth, for instance, if guided solely by Kepler's laws,
would return each day of the year exactly to the same position which it
had on the same day of last year. From age to age the quantity of heat
received by the earth would remain constant if the sun continued
unaltered, and the present climate might thus be preserved indefinitely.