You must use a very good telescope to see the satellites of Uranus.
They are four in number, bearing the names of Ariel, Umbriel, Titania,
and Oberon. The innermost of these, Ariel, completes a journey round
the planet in two days and a half; Oberon, the most distant, requires
thirteen days and a half. A planet is always tending to pull its
satellite down, and the satellite is kept from falling by the speed
with which it revolves. The heavier the planet, the faster must its
satellites go round. Thus, to take an illustration from our own moon,
we know that, if the earth were to be made four times heavier than it
is, the moon would have to spin round twice as fast as it does, in
order to remain in the same orbit. The speed with which the satellites
of Uranus revolve accordingly affords a measure of the mass of the
planet. Were Uranus heavier than he is, his satellites would revolve
more quickly than they do; were he lighter, the satellites would take a
longer period to go round.
Uranus also seems to be greatly swollen by clouds, in the same manner
as are both Jupiter and Saturn; in fact, if our earth was as big as
Uranus, it would weigh four or five times as much as Uranus does. Hence
we are certain that Uranus must consist of materials less dense on the
whole than are those of which our earth is made.
There is another singular circumstance connected with the moons of
Uranus. I have told you how every body revolving round another by
gravitation will describe an ellipse; but, of course, there are many
different kinds of this curve, and some may be nearly circles. There
is nothing whatever to prevent a satellite from revolving around its
primary in an exact circle if it be started properly; that is, in the
right direction and with the right speed. All the four satellites of
this planet seem to revolve in circles so perfect that we can make an
accurate picture of this system with a pair of compasses. It is further
to be noticed that the four circles seem to lie exactly in the same
plane. The orbits of the other great planets and of their satellites
lie in planes inclined at angles of less than 35° to the ecliptic, the
plane in which the earth moves. Here again the satellites of Uranus are
exceptional. The plane in which they are contained stands up almost
squarely to the plane in which the motion of the planet is performed.
The moons of Uranus seem to have got a twist, from some accidental
circumstance for which we are not able to account.