JUPITER 719 at a mean distance of 475,692,000 m. from the sun, his greatest distance being 498,639,000 m., and his least 452,745,000 m. When he is in opposition, his distance from the earth is re- duced by the whole amount of the earth's dis- tance from the sun at the time; and as it chances that the perihelion and aphelion of his orbit lie almost directly opposite the parts of the earth's orbit where she is at her mean dis- tance (91,430,000 in.), it follows that when in opposition Jupiter's distance from the earth varies between 407,209,000 m. (498,639,000 91,430,000) and 361,315,000 m. (452,745,000 91,430,000), a very noteworthy difference. It may be mentioned that Jupiter's perihelion lies in about Ion. 12, so that oppositions occur- ring when the earth's heliocentric longitude is about 12 (in other words, during the first week in October) are under ordinary circum- stances the most favorable occasions for the study of this planet. Nor is the advantage so slight that the oversight of the circumstance in our ordinary test books of astronomy can be readily understood. At an opposition of this kind the apparent area of Jupiter's disk ex- ceeds the apparent area at an opposition early in April, roughly in the proportion of (407)" to (361 )', or as 430 to 338 say as 5 to 4 ; and in addition, Jupiter is more fully illuminated by the sun in the proportion (still roughly) of (499)" to (453)", or as 522 to 430 say as 6 to 5 ; and as the comparatively small illumination of Jupiter limits the magnifying power which can be applied with any given telescope under the most favorable conditions, we may fairly combine these two ratios, and regard 3 to 2 as representing the proportion in which an Octo- ber observation of Jupiter surpasses an April observation, the planet being in either case in opposition. Jupiter circles round the sun in a mean period of 4,332-5848 days ; and his mean synodical period (that is, the interval separating his successive returns to opposition) has a mean value of 398-867 days. Various estimates have been obtained of Jupiter's di- mensions ; but we may take 85,000 m. as the most probable extent (in round numbers) of his equatorial diameter. His polar diameter is considerably less, the compression of the planet being variously estimated at from -fa to T ^. We may assume -^ as approximately correct, according to which estimate his polar axis would be about 5,700 m. less than an equatorial diameter. His volume is about 1,235 times as great as the earth's ; but his density being only about one fourth of the earth's, his mass does not exceed that of the earth in so considerable a proportion. Nevertheless, the disproportion still remains very great, since the mass of the planet exceeds the earth's more than 301 times. It must be remarked that this number 301, being deduced from the observed motions of the planet's satellites, may be relied on as ap- proximately exact, whereas the number 1,235, representing Jupiter's volume (the earth's be- ing 1), depends only on the estimated diam- 459 VOL. ix. 46 eter and compression of the planet, and there- fore cannot be regarded as exactly determined. The estimated density is necessarily affected by any inaccuracy which may exist in the de- termination of the volume; but a moment's consideration will show that the probable limits of error in the determination of the density are not wide. Jupiter rotates on his axis in rather less than 10 hours. The period given by Beer and Madler (see their Beitrdge zur physischen Kenntniss der himmlischen Kdrper im Sonnen-systeme, Weimar, 1841) is 9 h. 55 m. 26-5324 s. ; but no reliance can be placed on the last four digits in this result : first, because it is doubtful whether any mark- ings exist on Jupiter which can be recognized after the lapse of long intervals of time ; and secondly, because if such marks exist, none have been observed during periods long enough to insure that even the seconds in the rotation period should he rightly assigned. Jupiter is the centre of a noble scheme of dependent bodies, called his satellites, which circle round him at the distances indicated in the accom- panying table, which presents the chief ele- ments of this interesting system : ELEMENTS OF JUPITER'S SATELLITES. NO. Sidereal revolution. Distance In railUof V. Inclination of orbit to 2f 's equator. THAMETF.R. Mail, that of Jupiter being 1. Appa- rent. In miles. I... II.... III.... IV.... d. h. m. 1 18 20 8 18 4 7 8 48 19 16 82 6-05 9-62 15-85 26-99 V V
6
S 8 24 1'02" 0-91 1-49 1-2T 2,852 2,099 8,486 2,929 0-000017328 0-000023285 0-000088497 0-000042669 The densities of the satellites have usually been stated incorrectly in the text books of astrono- my at respectively 0-114, 0-171, 0-396, and 1-468 (where the density of water is unity). Whence these values were originally derived we do not know ; hut they are unquestionably incorrect. The following values of the densities have been calculated by the present writer from Laplace's estimates of the masses, combined with the values of the diameters above stated : Density (eartn'i as 1). Density (water a> l). Satellite I 0-198 1-148 "II 0-874 2-167 III 0-825 1-883 "IV 0-2J8 1-468 Thus all the satellites have a greater mean density than Jupiter. Probably their real den- sities are greater than those here tabulated, since irradiation would increase their apparent diameters. The motions of the satellites of Ju- piter have been studied with scrupulous care by astronomers, from the time when Galileo in 1610 first discovered these bodies. They had not been long observed in this way before a peculiarity was recognized which Romer was the first to interpret. It was found that pre-
