, that the rotation periods of the other planets
of our system are usually about twenty-four hours, or under. The fact
that the rotation period of the sun should run into _days_ need not seem
extraordinary when one considers its enormous size.

The periods taken by the various planets to revolve around the sun is
the next point which has to be considered. Here, too, it is well to
start with the earth's period of revolution as the standard, and to see
how the periods taken by the other planets compare with it.

The earth takes about 365-1/4 days to revolve around the sun. This
period of time is known to us as a "year." The following table shows in
days and years the periods taken by each of the other planets to make a
complete revolution round the sun:--

Mercury about 88 days.
Venus " 226 "
Mars " 1 year and 321 days.
Jupiter " 11 years and 313 days.
Saturn " 29 years and 167 days.
Uranus " 84 years and 7 days.
Neptune " 164 years and 284 days.

From these periods we gather an important fact, namely, that the nearer
a planet is to the sun the faster it revolves.

Compared with one of our years what a long time does an Uranian, or
Neptunian, "year" seem? For instance, if a "year" had commenced in
Neptune about the middle of the reign of George II., that "year" would
be only just coming to a close; for the planet is but now arriving back
to the position, with regard to the sun, which it then occupied. Uranus,
too, has only completed a little more than 1-1/2 of its "years" since
Herschel discovered it.

Having accepted the fact that the planets are revolving around the sun,
the next point to be inquired into is:--What are the positions of their
orbits, or paths, relatively to each other?

Suppose, for instance, the various planetary orbits to be represented by
a set of hoops of different sizes, placed one within the other, and the
sun by a small ball in the middle of the whole; in what positions will
these hoops have to be arranged so as to imitate exactly the true
condition of things?

First of all let us suppose the entire arrangement, ball and hoops, to
be on one level, so to speak. This may be easily compassed by imagining
the hoops as floating, one surrounding the other, with the ball in the
middle of all, upon the surface of still water. Such a set of objects
would be described in astronomical parlance as being _in the same
plane_. Suppose, on the other hand,