Page:The Solar System - Six Lectures - Lowell.djvu/42



the literal meaning of the word "focus,"—we must note how the main bodies and yet smaller particles are severally ranged about it.

Humboldt divided the planets into two groups : the terrestrial planets and the major planets, and this classification one shall still find in many a text-book. But it has long since ceased to contain even a specious distinction. The so-called terrestrial planets differ among themselves quite as much as any of them do from the major planets. From our present knowledge it would be much nearer the mark to divide the eight into pairs, Mercury and Venus, the Earth and Mars, Jupiter and Saturn, Uranus and Neptune ; yet even between the members of each pair are notable differences, to say nothing of the asteroids which throng the space betwixt Jupiter and Mars.

Of the differences, it will be the province of the succeeding chapters to speak ; but before doing so, let us take a bird's-eye view of the whole.

Our own Solar System has one characteristic, a general family trait, which distinguishes it from many that lie round about it in space ; for we may not doubt that the stars are centres to systems of their own. We have not only analogy to guide us to this deduction, but we already have glints of evidence of the fact. Our system differs, how -