Page:The Solar System - Six Lectures - Lowell.djvu/76



depth of tint that their importance is inferred. But their most amazing attribute is their geometric character. They seem to be generally arcs of great circles drawn from certain salient points on the planet's surface to certain other equally salient ones.

Their number appears to be legion. Schiaparelli discovered 104. But the better the planet is seen the more of them come out. About 350 have now been mapped at Flagstaff, and the number is only limited by our penetration. Like the asteroids, the larger ones have already been detected.

Each opposition now brings out smaller and smaller specimens.

But now comes a most interesting fact connected with them which was discovered by Schiaparelli and found equally true at Flagstaff. They are not always equally visible. Sometimes they are conspicuous, sometimes scarcely discernible even to a practiced eye. And this is not mere matter of distance. The best time for seeing the planet is not the best time for detecting the canals.

At certain oppositions we pass the planet at close quarters, at certain others a good way off. The close approaches are called favorable oppositions, the distant encounters unfavorable ones.