Page:The Solar System - Six Lectures - Lowell.djvu/77



But the latter are not so unfavorable as they are thought. For another factor beside nearness affects the reckoning. The planet's axis is tilted to the plane of its orbit at an angle of 25°, and is so faced that the southern hemisphere is presented to us at the time of closest approach. Now the canals lie chiefly in the northern hemisphere. In the next place, it is then the northern winter, and careful comparison reveals the fact that the conspicuousness of a canal is a function of the Martian time of year, becoming pronounced in summer and fading out in winter.

This is one reason why the canals so long eluded astronomers. They were not looked for at the proper time.

The first important post-Schiaparellian advance was made in the dark regions of the planet.

For two centuries the dark regions were held to be seas. It became evident, however, from Pickering's observations in 1892 that the great part of them could not be such. In 1894, at Flagstaff, it further became evident that no part of them could be water. From the way in which the clarification of the dark regions progressed with the planet's seasons, it had become patent that the bodily transference of substance, such, for instance, as water, from one place to another,