A Text-Book of Astronomy

by George C. Comstock

Fig. 88.—Increase of density in the atmospheres of Jupiter and the earth.

This is essentially the same physical condition that we found for the sun, and we may add, as further points of resemblance between it and Jupiter, that there seems to be a circulation of matter from the hot interior of the planet to its cooler surface that is more pronounced in the southern hemisphere than in the northern, and that has its periods of maximum and minimum activity, which, curiously enough, seem to coincide with periods of maximum and minimum sun-spot development. Of this, however, we can not be entirely sure, since it is only in recent years that it has been studied with sufficient care, and further observations are required to show whether the agreement is something more than an accidental and short-lived coincidence.

Temperature.—The temperature of Jupiter must, of course, be much lower than that of the sun, since the surface which we see is not luminous like the sun's; but below the clouds it is not improbable that Jupiter may be incandescent, white hot, and it is surmised with some show of probability that a little of its light escapes through the clouds from time to time, and helps to produce the striking brilliancy with which this planet shines.

140. The satellites of Jupiter.—The satellites bear much the same relation to Jupiter that the moon bears to the earth, revolving about the planet in accordance with the law of gravitation, and conforming to Kepler's three laws, as do the planets in their courses about the sun. Observe in [Fig. 86] the position of satellite No. 1 on the four dates, and note how it oscillates back and forth from left to right of Jupiter, apparently making a complete revolution in about two days, while No. 4 moves steadily from left to right during the entire period, and has evidently made only a fraction of a revolution in the time covered by the pictures. This quicker motion, of course, means that No. 1 is nearer to Jupiter than No. 4, and the numbers given to the satellites show the order of their distances from the planet. The peculiar way in which the satellites are grouped, always standing nearly in a straight line, shows that their orbits must lie nearly in the same plane, and that this plane, which is also the plane of the planets' equator, is turned edgewise toward the earth.

These satellites enjoy the distinction of being the first objects ever discovered with the telescope, having been found by Galileo almost immediately after its invention, A. D. 1610. It is quite possible that before this time they may have been seen with the naked eye, for in more recent years reports are current that they have been seen under favorable circumstances by sharp-eyed persons, and very little telescopic aid is required to show them. Look for them with an opera or field glass. They bear the names Io, Europa, Ganymede, Callisto, which, however, are rarely used, and, following the custom of astronomers, we shall designate them by the Roman numerals I, II, III, IV.

Fig. 89.—Orbits of Jupiter's satellites.

For nearly three centuries (1610 to 1892) astronomers spoke of the four satellites of Jupiter; but in September, 1892, a fifth one was added to the number by Professor Barnard, who, observing with the largest telescope then extant, found very close to Jupiter a tiny object only 1/600 part as bright as the other satellites, but, like them, revolving around Jupiter, a permanent member of his system. This is called the fifth satellite, and [Fig. 89] shows the orbits of these satellites around Jupiter, which is here represented on the same scale as the orbits themselves. The broken line just inside the orbit of I represents the size of the moon's orbit. The cut shows also the periodic times of the satellites expressed in days, and furnishes in this respect a striking illustration of the great mass of Jupiter. Satellite I is a little farther from Jupiter than is the moon from the earth, but under the influence of a greater attraction it makes the circuit of its orbit in 1.77 days, instead of taking 29.53 days, as does the moon. Determine from the figure by the method employed in [§ 111] how much more massive is Jupiter than the earth.