[Illustration: PLATE X. TYCHO AND ITS SURROUNDINGS. (AFTER NASMYTH.)] CHAPTER IV. THE SOLAR SYSTEM. Exceptional Importance of the Sun and Moon--The Course to be pursued--The Order of Distance--The Neighbouring Orbs--How are they to be discriminated?--The Planets Venus and Jupiter attract Notice by their Brilliancy--Sirius not a Neighbour--The Planets Saturn and Mercury--Telescopic Planets--The Criterion as to whether a Body is to be ranked as a Neighbour--Meaning of the word _Planet_--Uranus and Neptune--Comets--The Planets are illuminated by the Sun--The Stars are not--The Earth is really a Planet--The Four Inner Planets, Mercury, Venus, the Earth, and Mars--Velocity of the Earth--The Outer Planets, Jupiter, Saturn, Uranus, Neptune--Light and Heat received by the Planets from the Sun--Comparative Sizes of the Planets--The Minor Planets--The Planets all revolve in the same Direction--The Solar System--An Island Group in Space. In the two preceding chapters of this work we have endeavoured to describe the heavenly bodies in the order of their relative importance to mankind. Could we doubt for a moment as to which of the many orbs in the universe should be the first to receive our attention? We do not now allude to the intrinsic significance of the sun when compared with other bodies or groups of bodies scattered through space. It may be that numerous globes rival the sun in real splendour, in bulk, and in mass. We shall, in fact, show later on in this volume that this is the case; and we shall then be in a position to indicate the true rank of the sun amid the countless hosts of heaven. But whatever may be the importance of the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation in asserting how immeasurably his influence on the earth surpasses that of all other bodies in the universe together. It was therefore natural--indeed inevitable--that our first examination of the orbs of heaven should be directed to that mighty body which is the source of our life itself.