Page:Astronomy for Everybody.djvu/203

The great Huygens, who flourished between 1650 and 1700, studying Mars with the telescope, was the first one to recognise the variegated character of its surface, and to make a drawing of the appearance which it presented. The features delineated by Huygens can be recognised and identified to this day. By watching them it was easy to see that the planet rotated on its axis in a little more than one of our days (24h. 37m.).

This time of rotation is the only definite and certain one among all the planets besides the earth. For two hundred years Mars has rotated at exactly this rate, and there is no reason to suppose that the time will change appreciably any more than the length of our day will. The close approach to one of our days, the excess being only thirty-seven minutes, leads to the result that, on successive nights, Mars will, at the same hour, present nearly the same face to the earth. But, owing to the excess in question, it will always be a little farther behind on any one night than on the night before, so that, at the end of forty days, we shall have seen every part of the planet that is presented to the earth.

All that was known of Mars up to a quite recent period could be embodied in a map of the planet, showing the bright and dark regions of its surface, and in the fact that a white cap would be generally seen to surround each of its poles. When a pole was inclined toward us, and therefore toward the sun, this cap gradually grew smaller, enlarging again when the pole was turned from