Page:Mars - Lowell.djvu/45

Rh her orbit, which is 23° 24′. As the inclination of the axis to the plane of the orbit determines the seasons, we see that not only has Mars its spring, summer, autumn, and winter, but that these are not very unlike our own.

It is not uninteresting to inquire in what the difference consists. The slight difference of tilt in the Martian axis would slightly extend the breadth of the tropical and the polar regions at the expense of the temperate ones, and thus accentuate the seasons, but the chief seasonal contrast between Mars and the Earth would come in in consequence of the much greater eccentricity of Mars’ orbit. For the more eccentric the ellipse, the greater the variation in the planet’s velocity at different parts of it, inasmuch as the Sun pulls the planet toward himself with a force depending on his distance. The less this distance, the greater the angular velocity. But the angular velocity determines the length of the seasons upon a planet whose pole of rotation is tilted to the plane of its orbit, like the Earth or Mars. The greater the eccentricity of the ellipse, therefore, the greater the difference in the length of the seasons. In the case of the Earth the difference is about eight days, winter in the northern hemisphere being eight days shorter than summer. In the case of Mars, owing to the much greater eccentricity of his orbit combined with his longer