Page:The Solar System - Six Lectures - Lowell.djvu/66



body shows but the bleached bones of a once living world.

Now color is conspicuously wanting on Mercury. The disk of the planet is a chiaroscuro of black and white, tones devoid of tints.

Mars is an opal. Colors comparable only to that stone variegate its disk. At top and bottom, collars of pearl-white contrast vividly with light areas of rose-saffron and darker ones of robin's-egg blue. Daylight reveals these colors much better than night, because the contrast of the blue-black sky clothes the disk with yellow it does not really possess, diluting the true tints.

The markings enable the rotation of the planet to be found - The markings move under the observer's eye and yet keep their relative configurations the same, day after day and year after year. They thus reveal the fact that the planet rotates, and by the course of their motion disclose the axis about which the rotation takes place. From the observed data, spherical trigonometry enables us to fix this axis in space and determine its tilt to the plane of the planet's orbit. We thus find that it is inclined to the Martian ecliptic by an angle of 25°, and that the solar day there is 24 hours and 40 minutes long. Thus Mars has both days and seasons, and both days and seasons are practically