Page:The Solar System - Six Lectures - Lowell.djvu/48



were narrow, irregular lines and very dark. They were not in the least like the markings on Mars. There were no large patches of shade on the one hand, nor fine, regular pencilings on the other. Its lines were fairly straight, but broken and of varying width. "Cracks" best explains their appearance, and probably their nature.

Their positions were unmoved, even after as much as five hours' interval. As I continued to map them, I marked that while their relation to the terminator was unchanged by the hours, it was slowly shifting with the days. The lines were gradually passing over its edge, and it dawned on me what I was witnessing: the swaying, or libration, of the planet in longitude due to the eccentricity of the planet's orbit.