the satellite lay to the body of the planet the more brightly would it be illuminated, and under certain circumstances the more easily would it be seen. The very opposite is, however, notoriously the case. It will be instructive to see wherein the difference lies.
It is, of course, quite true that Jupiter is a brilliant source of light, but in this respect the light that emanates from the planet is of a very different character from the light which radiates from the sun. Jupiter is not a sun-like object diffusing radiance around in virtue of his own intrinsic brilliancy. If the satellites revolving around the planet were really in the condition of the planets themselves as they revolve around the sun, then the closer the satellite was to the planet the greater would be the illumination it would receive. But of course this is not at all the case. Although we have excellent reasons for believing that Jupiter is in truth a highly heated globe, yet we are perfectly assured that the temperature falls far short of that which would be required if the great planet were to dispense its beams around in the same manner as a miniature sun. Indeed, there are facts connected with the satellites themselves which render it perfectly clear that Jupiter, so far from possessing a sun-like radiance, is absolutely devoid of any intrinsic light whatever. It shines merely as the earth itself shines, or as the moon shines, or the planet Mars, or Venus, or Saturn, by the sunbeams which fall upon it. As to the light which illumines the satellites of Jupiter, it also can only come from the sun. We cannot, indeed, say that the light-radiating power of a certain area on Jupiter is the same as that from an equal area on one of his moons. There are certainly intrinsic differences between the material
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