Page:Astronomy for Everybody.djvu/231

Yet another resemblance between Jupiter and the sun is that they are both brighter in the centre of their disk than toward the circumference. In the case of Jupiter, the shading off is very well marked. The extreme circumference especially is more softened than that of any of the other planets.

The apparent resemblance between the surfaces of these bodies, taken in connection with the brightness of the planet, has led to the question whether Jupiter may not be, in whole or in part, self-luminous. This again is a question which needs investigation. The idea that the planet can emit much light of its own seems to be negatived by the fact that the satellites completely disappear when they pass into its shadow. We may therefore say with entire certainty that Jupiter does not give enough light to enable us to see a satellite by that light alone. We can hardly suppose that this would be the case if the satellite received one per cent as much light from the planet as it does from the sun. It is also found that the light which Jupiter sends out is somewhat less than that which it receives from the sun. That is to say, all the light which it gives out, when estimated in quantity, may be reflected light, without supposing the planet brighter than white bodies on the surface of the earth. But this still leaves open the question whether the white spots, sometimes so much brighter than the rest of the planet, may not give us more light than can fall upon them. This also is a question not yet investigated.

The hypothesis which best lends itself to all the facts seems to be that the planet has a solid nucleus, of which