THE PLANETS URANUS AND NEPTUNE. 415
Its surface is not quite 20 times, its volume somewhat morethan 80 times, that of the Earth .
Its density, which is slightly less than that of Uranus , andsomewhat greater than that of water, is about equal to ith ofthat of the Earth . Notwithstanding its much greater volume,its weight is therefore only about 16 times the Earth ’s weight.The attraction of gravity upon its surface is almost exactly thesame as upon that of Uranus , or about T s, (J ths of what it isupon the Earth .
The intensity of the Sun ’s light and heat received by Nep tune , and the appai’ent area which its disc would present to anobserver upon the planet, are only about ^^th of what theyare for an observer upon tlie Earth . The Sun seen from itwould therefore offer no appreciable disc to the naked eye, butwould simply look like a very brilliant star.
Neptune possesses, as we have already mentioned in ourdiscussion of the moons of Uranus , one Satellite , which wasdiscovered by Mr. Lassell in October, 1846. Its distance fromthe planet’s centre is about 220,000 miles ; its period in itsorbit about 5 d 21 h 8 m . As far as we can judge from itsvisibility at its enormous distance from the Earth , this satel-lite is probably much larger than any other with which we are.acquainted.
It may be interesting to notice that, in the case of Neptune (see also our remarks with regard to the planet Mars , p. 262),it is because it possesses a Satellite that we are able to weighit, as we have above stated, with very considerable accuracy. Ifit were not for this we should have to use indirect methodswhich would render the problem vastly more difficult.
We suppose that our readers must by this time have perceivedthat we really know but very little about either of these twofar-distant planets, Uranus and Neptune . Nevertheless it maynot be amiss if we endeavour to extend our gaze somewhatfurther still by asking:—Do any other planets, even moredistant than these, exist beyond them ?
At present, as we have mentioned, the study of the per-turbations of Neptune ’s movement has not afforded anysatisfactory indications in favour of an affirmative reply to this