So that these orbs may fairly be regarded as bearing the same relation to their primary that Jupiter himself bears to his primary—the sun. It will be seen presently that this consideration is an important one.
But the great interest of the study of Jupiter resides in the fact that, being the nearest of the outer family of planets, the aspect of his globe supplies the best available means of determining the condition of the giant orbs constituting that family.
The first feature which strikes us in the telescopic aspect of the planet is the presence of a series of belts, lying parallel to the planet's equator. Usually the equatorial regions are occupied by a broad, bright belt, of a creamy-white color, and bordered on the north and south by copper-colored belts. Beyond these, again, lie alternate bright and dark belts, the dark belts growing more and more bluish in hue as the pole is approached—while the poles themselves are usually of a somewhat decided blue color in telescopes adapted to display such features to advantage. There are commonly two or three dark belts on each hemisphere.
Now, before inquiring into the peculiarities presented by these belts, and into the remarkable changes which have been noted lately in their general aspect, it may be well for us to consider briefly what such belts seem to imply. That they are due to peculiarities in the planet's atmosphere is admitted on all hands. And it has been usual to compare them with the trade-wind zones and the great equatorial calm zone on our earth. The bright belts, according to this view, are regarded as zones where for the time clouds are prevalent, the dark belts being regions where the comparatively dark hues of the planet's surface are brought into view. And then it has been deemed sufficient to point out that the parallelism of the zones is due to the extreme rapidity of the planet's rotation.
But, setting aside the fact that the trade-wind zones and the great equatorial calm zone on our earth are, in reality, little better than meteorological myths, it must be regarded as a remarkable fact that, in the case of a planet so far away from the sun as Jupiter is, there should be a supply of clouds so abundant as to form belts discernible from the earth. Jupiter is rather more than five times farther from the sun than the earth is, and receives from him about one twenty seventh part of the light and heat which falls upon the earth (equal surface for equal surface). Making every allowance for the possibility pointed out by Prof. Tyndall, that some quality in Jupiter's atmosphere may prevent the solar heat from escaping, and so cause the climate of the planet to be not very different from the earth's, yet the direct heat falling on the planet's oceans cannot be increased in this way nay, it must be rather diminished. It chances, indeed, that the very quality by which the earth's atmosphere retains the solar heat is unquestionably possessed by Jupiter's atmosphere. When our air is full of aqueous vapor (invisible to the eye), the escape of heat is prevented,
