Jupiter

Dr. Harold Jeffreys has come to the conclusion that the tem perature of the planet is probably low on theoretical grounds. He first found that on any reasonable estimate of the planet's age its original store of heat could not have maintained to the present day an effective temperature as high as that of the earth, and later showed that a low temperature, like that deduced from the radiometric observations, is consistent with conclusions derived from considerations of the planet's density and ellipticity. The problem is accordingly a perplexing one. It will probably be felt by most telescopic students of the planet that whatever the temperature of the visible superficial layers the internal tem perature at any rate must be relatively high. It is tempting to attribute the planet's supply of energy to sub-atomic processes; or, possibly, the enveloping layer in which the observed disturb ances occur, may be composed of substances with a low boiling point.

The Planet's Atmosphere.

Plate I. under PLANET (fig. 6) shows photographs taken by W. H. Wright at the Lick observatory in (a) ultra-violet and (b) extreme red light. The former shows the great extent of the Jovian atmosphere, while the latter gives a representation of the surface detail which is rather more like what is seen telescopically. A comparison of the two photographs is very instructive, and it seems that work on these lines is par ticularly promising from the point of view of determining the relative levels of the features observed and the solution of other problems. The intensity of the red spot in the ultra-violet photo graph is very noticeable, and the white band in the southern hemisphere strongly suggests that the disappearance of surface features mentioned earlier in the article is due to overlying atmos pheric vapours. In the article PLANET reference has been made to the atmospheric absorption bands in the spectrum of Jupiter and the other outer planets, but the nature of their source has not yet been established.

The Satellites.

Jupiter is accompanied by no less than 9 satellites so far as at present known. Four of them were dis covered by Galileo in 161o, and they can be readily seen with a good pair of field glasses. The others are all very small and faint. The fifth was discovered by Barnard with the Lick telescope in Sept. 1892, the sixth and seventh by Perrine, also at the Lick observatory in Dec. and Jan. 1904-1905, the eighth by Melotte at Greenwich in Feb. 1908, and the ninth by S. B. Nicholson at Lick in July 1914, the last four being found photographically. The ninth is probably only about 15 m. in diameter and the eighth may be only slightly larger.

The four principal satellites, commonly known as the Galilean Satellites, are named Io, Europa, Ganymede and Callisto, but they are generally designated by the Roman numerals 1., Iv. in the order of their distance from the planet. They are large enough to show sensible discs even in comparatively small instru ments. Three of them are about equal to or exceed the earth's moon in size and the other (ii.) is very nearly as large. Both III. and iv. have diameters of more than 3,00o miles. If they were not so near the planet they would all be naked-eye objects; indeed it has been reported on various occasions, when the condi tions have been favourable, that one or more, or perhaps two of them when close together seen as one, have actually been so glimpsed. Particulars of the orbits of these satellites are given

in the table on p. 191, but mention must also be made of a re markable relationship between the longitudes of I., 11. and III. discovered by Laplace, viz., that the longitude of 1. plus twice that of III. minus three times that of H. is 18o°, so that they can not all come into conjunction with one another or into opposition or conjunction with the sun at the same time.

The phenomena of these satellites, viz., their transits, occulta tions, eclipses and the transits of their shadows across the planet's disc are easily observed even with quite small telescopes, of, say, 2 in. aperture and upwards. Photometric observations of their eclipses are of special value and the Harvard series of such obser vations formed an important part of the material used some years ago by Professor R. A. Sampson in constructing his Theory of the Satellites and the Tables on which the predictions of their phenomena have since been based. A comparison of the times as computed and the times observed has led to the interesting conclu sion that the diameter of Jupiter is probably subject to small irregularities, or at any rate that the luminous shell which forms the visible surface of the planet is liable to temporary deforma tions from atmospheric causes which in turn affect the boundary of the planet's shadow. When projected on Jupiter's surface dur ing transit a satellite near ingress appears as a small white disc, its brightness being much in excess of that of the background near the planet's margin. Shortly afterwards it disappears as the albedos of the two surfaces become equal ; reappears later as a relatively dark object projected against the lighter background of the central portions of the planet's surface; it again becomes invisible and reappears as a small bright object shortly before egress. The cen tral parts of III. and Iv. are commonly very dark indeed during most of the time these satellites are in transit and iv. especially is often comparable with its shadow in blackness. All four Galilean satellites exhibit definite surface features when observed with suffi ciently powerful telescopes under the best conditions. Satellite being the largest and brightest has naturally received special atten tion and several observers have been led to the conclusion that like the earth's moon it rotates on its axis in the period of its orbital revolution and hence presents always the same face to the planet. This conclusion has been based partly on the fact that certain apparent variations in its form, which are doubtless due to dusky spots on its surface, have been found definitely related to the posi tion of the satellite in its orbit. During the 1927 apparition E. M. Antoniadi, observing with the 33 in. refractor at Meudon under very favourable conditions, studied the details of the surface mark ings with great completeness and his observations will be regarded as definitely establishing the long period above mentioned. More over, photometric observations, and especially those made in 1926 by Stebbins with the photo-electric photometer at the Lick ob servatory, show small variations in the light of all four satellites which are related to their orbital positions and indicate that they all present a constant face to Jupiter.