Page:Mars - Lowell.djvu/44

22 first learned that Mars had a day, and that its length was not far from the length of our own.

The importance of these earliest pictures of Mars has not lapsed with the lapse of time. By comparison of this and other early drawings with modern ones, has been deduced a very accurate value of the length of the Martian day (its sidereal day), a determination accurate to the tenth of a second. It amounts to 24 hours, 37 minutes, 22.7 seconds. Our sidereal day, that is, the day reckoned by the stars, not by the Sun, is roughly 23 hours, 56 minutes; so that the Martian day is about 40 minutes longer than our own. The result is not given here closer than the tenth of a second, because the Flagstaff observations have shown that the value of the length of the Martian day hitherto accepted is probably a trifle too small.

From the discovery of the rotation followed the approximate position of the planet's poles. Round about the poles so determined appeared two white patches, the first study of which we owe to Maraldi. They are the planet’s polar caps. They are to be detected with the smallest modern telescope.

The apparent position of the planet poles as presented to the Earth gives the tilt of the planet’s axis to the plane of its orbit. It turns out to be about 25°. This is very nearly the same as the Earth’s axial tilt to the plane of