cture it will be well to emphasise the sharp distinction
between the terms _rotation_ and _revolution_. The term "rotation" is
invariably used by astronomers to signify the motion which a celestial
body has upon an axis; the term "revolution," on the other hand, is used
for the movement of one celestial body around another. Speaking of the
earth, for instance, we say, that it _rotates_ on its axis, and that it
_revolves_ around the sun.

So far, nothing has been said about the sizes of the members of our
system. Is there any stock size, any pattern according to which they may
be judged? None whatever! They vary enormously. Very much the largest of
all is the Sun, which is several hundred times larger than all the
planets and satellites of the system rolled together. Next comes Jupiter
and afterwards the other planets in the following order of
size:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
Very much smaller than any of these are the asteroids, of which Ceres,
the largest, is less than 500 miles in diameter. It is, by the way, a
strange fact that the zone of asteroids should mark the separation of
the small planets from the giant ones. The following table, giving
roughly the various diameters of the sun and the principal planets in
miles, will clearly illustrate the great discrepancy in size which
prevails in the system.

Sun 866,540 miles
Mercury 2,765 "
Venus 7,826 "
Earth 7,918 "
Mars 4,332 "

ZONE OF ASTEROIDS

Jupiter 87,380 "
Saturn 73,125 "
Uranus[3] 34,900 "
Neptune[3] 32,900 "

It does not seem possible to arrive at any generalisation from the above
data, except it be to state that there is a continuous increase in size
from Mercury to the earth, and a similar decrease in size from Jupiter
outwards. Were Mars greater than the earth, the planets could then with
truth be said to increase in size up to Jupiter, and then to decrease.
But the zone of asteroids, and the relative smallness of Mars, negative
any attempt to regard the dimensions of the planets as an orderly
sequence.

Next with respect to relative distance from the sun, Venus circulates
nearly twice as far from it as Mercury, the earth nearly three times as
far, and Mars nearly four times. After this, just as we found a sudden
increase in size, so do we meet with a sudden increase in distance.
Jupiter, for instance, is about thirteen times as far as M