Page:Astronomy for Everybody.djvu/201

More public interest has in recent years been concentrated on the planet Mars than on any other. Its resemblance to our earth, its supposed canals, oceans, climate, snowfall, etc., have all tended to interest us in its possible inhabitants. At the risk of disappointing those readers who would like to see certain proof that our neighbouring world is peopled with rational beings, I shall endeavour to set forth what is actually known on the subject, distinguishing it from the great mass of illusion and baseless speculation which has crept into popular journals during the past twenty years.

We begin with some particulars which will be useful in recognising the planet. Its period of revolution is six hundred and eighty-seven days, or forty-three days less than two years. If the period were exactly two years, it would make one revolution while the earth made two, and we should see the planet in opposition at regular intervals of two years. But, as it moves a little faster than this, it takes the earth from one to two months to catch up with it, so that the oppositions occur at intervals of two years and one or two months. This excess of one or two months makes up a whole year after eight oppositions; consequently, at the end of about seventeen years, Mars will again be in opposition at the same time of the