Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/880

resemblance exists between the condition of Jupiter and that of our earth. This being so, the theory to which we have been already led, that the condition and rapid changes of condition of the deep Jovian atmosphere are due to the planet s intense heat, suggests at onc^ a solution of the difficulties just mentioned. These difficulties arose while ve were dealing with Jupiter as a planet like our earth. But at the outset we noted, in his great volume and mass, the suggestion of a wide difference between his condition and that of our earth. Regarding him as belonging to a different class on account of his enormous size, and being led also to consider him as intensely heated, we may fairly compare him with a body known to be intensely heated, and of size enormously surpassing the earth, viz., with the sun. And though our difficulties are not in this way removed, yet we find that a much more complete analogy can be established between Jupiter and the sun than between the earth and Jupiter. Thus the density of the sun, like that of Jupiter, is small compared with the earth s ; in fact, the mean density of the sun is almost exactly the same as that of Jupiter. The belts of Jupiter may be much more aptly compared with the spot-zones of the sun than with the trade-zones of the earth, which would certainly not present, as viewed from Mercury or Venus, any resemblance to the belts of Jupiter. The spots on Jupiter are not constant, and in this respect resemble the sun spots. They are like these also in having a pro per motion. They change less rapidly, but this is intelli gible when we consider how much less intensely heated

Jupiter is than the sun.

It remains to be mentioned, in support of this theory of the inherent heat of Jupiter, that the light received from fam j g f ar g rea t er than he would reflect if his surface were like that of the moon or of Mars. As the reasoning we have given has not been intended to relate solely to Jupiter, but to Saturn certainly, and probably to Uranus and Neptune, we may suitably present here a table which exhibits the relation between the light received from the various members of the solar system, and appears as markedly to distinguish the giant planets from the inner family, as the relations of size, mass, orbital scale, rapid rotation, and complexity of system, which have been longer known. From the observations of Zollncr, Grundzuge einer all- gemeinen Photometric des Himmels, Berlin, 1861, it appears that the light of the five planets at their mean opposition bears the following proportion to the light of the sun:—

Probable EiTor. Per cent. Sun = 6,994,000,000 times Mars 5 8 Sun= 5,472,000,000 ,, Jupiter 57 Sun= 130,980,000,000 Saturn (without the ring) 5 Sun= 8,486,000,000,000 Suu-79,620,000,000,000 Uranus 6 Neptune 5 5 To which add two estimates of the moon s light, by com paring surfaces, and by comparing point-like images, of the sun and moon:—

Probable Error. Sun = 618,000 times Moon 27 per cent. Sun = 619,600 ,, 1-6 Whence the average reflecting powers of the surfaces of the six orbs compared would be (on the assumption that the outer planets have no inherent light),—

Moon 0-1736 Mars 0-2672 Jupiter. 0-6238 Saturn 4981 Uranus .. , 6400 Neptune... .0 4848

These results seem strongly to suggest that the four members of the outer family of planets shine partly by inherent lustre. The spectroscopic analysis of the light of Tupiter and Saturn, though not altogether satisfactory, indicates clearly the existence ol a very deep vapour-laden atmosphere around each planet.

Soon after the invention of the telescope, Galileo dis- covered that four small orbs or satellites circle around Jupiter. Galileo soon found that their orbits are nearly circular, and their motions nearly uniform. They revolve in planes inclined little to the plane of the ecliptic, so that as viewed from the earth they appear to traverse very nar row ellipses. Even when these ellipses are most open, they all, save the outermost, have minor axes less than Jupiter s apparent diameter, so that any one of the three interior satellites is necessarily occulted when in superior conjunc tion with Jupiter as viewed from the earth, and eclipsed in his shadow when in superior heliocentric conjunction; while when in inferior geocentric conjunction, it is in transit across the planet s disk ; and when in inferior heliocentric conjunction, its shadow is in transit. The same is true of the fourth satellite during about two-thirds of Jupiter s year, viz., for about one-sixth of his year before and after the two equinoxes (four-sixths in all), the plane of the satellite s orbit being nearly coincident with the planet s equator. The elements of the satellites are given in the tables on p. 783, and the orbits are pictured to scale in Plate XXVIII. To the elements tabulated may be added these (the densities usually given in the books being alto gether incorrect):—

Density Density (earth s as 1). (water us 1). Sat. 1 0-198 1-148 II 0-374 2167 ,, III 0-325 1-883 IV 0-258 1-468

Thus all the satellites (except the first) have a density ex ceeding Jupiter s. Probably their real densities are greater, as irradiation increases their apparent size.

The motions of Jupiter s satellites have been very care fully studied. They had not been long observed before a peculiarity was recognised, which Roemer was the first to interpret. Their various phenomena were found to occur earlier when Jupiter was in opposition, and later when he was near conjunction, than the predicted time. (In con junction, of course, he cannot be seen.) Roemer suggested that the time-difference is due to Jupiter s variations of distance from the earth, light, which brings to our earth information of the phenomena, taking a longer time in reaching us when Jupiter is farther away. Ridiculed at first, this theory was before long established by repeated observation, and eventually placed beyond all question by Bradley s discovery of aberration. (See Aberration, vol. i. p. 48.)

A singular relation exists between the motions of Jupiter s three inner satellites. It will be observed from the table of elements that the period of the second is almost exactly double the period of the first, that of the third almost exactly double that of the second; in other words, the sidereal motions of I. II. and III. are almost exactly as the numbers 4, 2, and 1. This will be more clearly shown by the following table:—

Sidereal Revolution Sidereal Revolution in Seconds. per Second. Sat. 1 152853-505 8 478706 ,, II 306822-040 4 223947 III. ... 618153-360 2-096567 IV 1441931-271 0-898795

The coincidence is not exact ; but the following relation holds exactly : The sidereal motion of I. added to twice the sidereal motion of III. is equal to three times the sidereal motion of II. Thus

(8"-478706) + 2(2"-096,567) = 12" 671840 = 3(4"-223947).

A result of this relation, combined with the fact that when