is attainable, but any one who has made such upon a planet's disk swinging like a lantern in the field of view under a variety of causes instrumental and optical, knows how encumbered they inevitably are
Photograph of Jupiter, 1909.
P. L.
with error. To have the disk caught and fixed on a plate where it may be measured at leisure and as often as one likes, is a distinct advance toward fundamental accuracy. Measures thus effected upon the Jupiter images of 1909 proved the bright equatorial belt to lie exactly upon the planet's equator when allowance was made for the tilt of the planet's axis toward the Earth. This showed that the aspect of the planet toward the Sun had no effect upon the position of the belt. Jupiter's cloud formation, therefore, is not dependent, as all ours are, upon the solar heat.
A like indifference to solar action is exhibited in the utter obliviousness of the belts to day or night. To them darkness and light are nugatory alike. They reappear round the sunrise edge of the disk just as they left it when they sank from sight round the sunset one, and they march across its sunlit face without so much as a flicker on their features.
Yet this seeming immobility from moment to moment
