Jupiter, the largest member of our planetary system, and the fifth in order of distance from the sun, so far as the primary members of the system (omitting the asteroids) are concerned. It is designated by the signJupiter travels 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 reduced by the whole amount of the earth's distance 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 distance (91,430,000 m.), 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 lon. 12°, so that oppositions occurring when the earth's heliocentric longitude is about 12° (in other words, during the first week in October) are under ordinary circumstances 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 text books of astronomy can be readily understood.

At an opposition of this kind the apparent area of Jupiter's disk exceeds the apparent area at an opposition early in April, roughly in the proportion of (407)2 to (361)2, 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)2 to (453)2, 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 October 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 dimensions; 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 1/12 to 1/17. Wo may assume 1/15 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 approximately exact, whereas the number 1,235, representing Jupiter's volume (the earth's being 1), depends only on the estimated diameter and compression of the planet, and therefore cannot bo regarded as exactly determined. The estimated density is necessarily affected by any inaccuracy which may exist in the determination 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 Beitrage zur physischen Kenntniss der himmlischen Korper 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 markings 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 be 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 accompanying table, which presents the chief elements of this interesting system:

Elements Of Jupiter's Satellites

The densities of the satellites have usually been stated incorrectly in the text books of astronomy 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; but 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:

Thus all the satellites have a greater mean density than Jupiter. Probably their real densities are greater than those here tabulated, since irradiation would increase their apparent diameters. The motions'of the satellites of Jupiter 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 predieted phenomena of the satellites occurred earlier when Jupiter was in opposition than when he was in quadrature, and that in fact the further Jupiter was from opposition up to the time when he was so near conjunction that his satellites could no longer be observed, the later these phenomena occurred. It was at length suggested by Romer that the discrepancy was due to the increase of distance, the light which brings to our earth information of the phenomena taking longer in reaching the earth when the planet is further away. Repeated observations confirmed this theory, which at first astronomers of repute ridiculed as too fanciful for serious consideration.

Bradley's discovery of the aberration of light placed the theory beyond the possibility of question. - The appearance of Jupiter's disk is such as to suggest the idea that the planet is enveloped in a deep vaporous atmosphere, heavily laden with cloud masses. A series of broad bands or belts, alternately dark and light, and differing in color, lie across the disk, agreeing generally in position with the latitude parallels of the planet. On a close study with telescopic power, these belts are found to present peculiarities of structure exceedingly interesting. Rounded clouds appear to float separately within the deep atmosphere, and from time to time changes of shape and of color are noticed which seem to imply the action of forces of great intensity. Theoretical investigations applied to the subject of an atmosphere of great depth, attracted by the strong gravity of Jupiter, suggest that conditions of pressure would exist incompatible with the gaseity of the envelope. And the known small density of the planet, combined with the result just mentioned, suggests that in the case of Jupiter, as in that of the sun, the increase of pressure and therefore of density, which we should expect from the mere mass of the planet, is counteracted by the expansive effects of intense heat.

This view of the planet's condition has been adopted recently by Prof. Benjamin Peirce on independent mathematical grounds, and may be regarded as altogether more probable than the old-fashioned but quite unsupported opinion that the planet's condition resembles generally that of our own earth.