The density of Uranus is about
one fifth greater than that of water, and slightly greater than that of
Neptune. Uranus is 32,000 miles in diameter, and Neptune 35,000 miles.
Curiously enough, the force of gravity upon each of these two large
planets is a little less than upon the earth. This arises from the fact
that in reckoning gravity on the surface of a planet not only the mass
of the planet, but its diameter or radius, must be considered. Gravity
varies directly as the mass, but inversely as the square of the radius,
and for this reason a large planet of small density may exercise a less
force of gravity at its surface than does a small planet of great
density.
The mean distance of Uranus from the sun is about 1,780,000,000 miles,
and its period of revolution is eighty-four years; Neptune's mean
distance is about 2,800,000,000 miles, and its period of revolution is
about 164 years.
Uranus has four satellites, and Neptune one. The remarkable thing about
these satellites is that they revolve _backward_, or contrary to the
direction in which all the other satellites belonging to the solar
system revolve, and in which all the other planets rotate on their axis.
In the case of Uranus, the plane in which the satellites revolve is not
far from a position at right angles to the plane of the ecliptic; but in
the case of Neptune, the plane of revolution of the satellites is tipped
much farther backward.
Pages:
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153