world as different from our own as it is possible to imagine. There is no visible surface crust and there are no permanent markings. Different spots on the planet's disk give different periods of rotation showing that it is atmospheric phenomena that we observe. All is constant flux and change on Jupiter. Dense vapors arise from a highly heated interior and spread out into belts parallel to the equator in the direction of the planet's rotation. From its nearest satellite all the interesting changes of color and form that constantly take place in the atmosphere of this great globe could be observed in great detail. The high percentage of light and heat that Jupiter reflects from the sun to its nearer satellites makes it a secondary sun to them of tremendous size though feeble strength. As seen from Satellite I the other three major moons of Jupiter present all the phases of our own moon in rapid succession, due to their constantly changing positions with reference to the sun. The five small moons, discovered in modern times, are so minute that they are simply star-like points of light even when viewed from the other moons of Jupiter. To keep track of the rapidly changing positions and various phases of the moons of Jupiter as seen from any one of them, as well as the rapid _apparent_ motion of the planet through the sky due to the revolutions of the satellite around the planet, would be a troublesome task for an astronomer stationed on one of these far distant worlds. It would be a common sight to see in the sky at one time the huge planet, the far-distant, shrunken sun, and one, two or three moons. Seen from the moons of Jupiter the constellations would appear as they do to us on earth, for such a slight change in position as five hundred million miles, more or less, is trivial when one is looking at the stars. Observations of the stars from the nearest moon of Jupiter would be attended with great difficulties at times, since reflected sunlight from a body nearly twenty degrees in diameter would be extremely troublesome, especially were the phases of the planets near that of the full moon. We know how the presence of our own moon in the heavens at the full dims the brightness of the stars so that only the brightest stars are seen. Even as viewed from the fourth or most distant of the major satellites the planet subtends an angle of nearly five degrees. Occultations of the stars are many and frequent as the huge planet globe glides swiftly through the heavens. Many a moonlight night appears almost as day owing to the presence of the enormous, brilliantly reflecting ball of light and at times two or three moons in addition. Only the brightest stars could possibly be seen under such circumstances. When, however, the small worlds pass into the shadow of the great mother planet and not only the light of the sun but also the reflected light of Jupiter disappears for many minutes, the stars shine forth in all their glory there as here.