Page:The Solar System - Six Lectures - Lowell.djvu/114



true surface that are laid bare. For that the color is due to the selective absorption of the higher regions of the planet's air is negatived by the spectroscope, which shows dark bands in the red.

In spite of its enormous bulk, Jupiter turns on its axis with such speed that its figure is flattened by $$\scriptstyle \frac{1}{15.5}$$. Its mean time of rotation is 9h 55m. We are forced to say its mean time, not because the markings cannot be accurately timed, nor because of any change in the planet's moment of momentum, but because the planet does not rotate as a whole. Different parts of it go round at different rates. Speaking broadly, the nearer the equator the greater the speed. Between the equator and latitude 30° there is a difference of six minutes in the rotation period. But the several belts have each its own period, and this does not always accord with the latitude. In addition, particular spots on the same longitude have particular spins, and pass by each other at speeds from seven miles to four hundred miles an hour. White markings travel faster than dark markings close beside them. Thus the white masses around the great red spot drift by it. The spot itself has changed its rate by six seconds in as many years. It is pretty evident that Jupiter is chaotic.