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JUPITER

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Originally appearing in Volume V15, Page 565 of the 1911 Encyclopedia Britannica.
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JUPITER , in See also:

astronomy, the largest See also:planet of the See also:solar See also:system; his See also:size is so See also:great that it exceeds the collective See also:mass of all the others in the proportion of 5 to 2. He travels in his See also:orbit at a mean distance from the See also:sun exceeding that of the See also:earth 5.2 times, or 483,000,000 See also:miles. The eccentricity of this orbit is consider-able, amounting to 0.048, so that his maximum and minimum distances are 504,000,000 and 462,000,000 miles respectively. When in opposition and at his mean distance, he is situated 390,000,000 miles from the earth. His orbit is inclined about 1° 18' 40" to the See also:ecliptic. His sidereal revolution is completed in 4332.585 days or 11 years 314.9 days, and his synodicalperiod, or the mean See also:interval separating his returns to opposition, amounts to 398.87 days. His real polar and See also:equatorial diameters measure 84,570 and 90,190 miles respectively, so that the mean is 87,38o miles. His apparent See also:diameter (equatorial) as seen from the earth varies from about 32", when in See also:conjunction with the sun, to 50" in opposition to that luminary. The oblateness, or See also:compression, of his globe amounts to about -; his See also:volume exceeds that of the earth 1390 times, while his mass is about 300 times greater. These values are believed to be as accurate as the best See also:modern determinations allow, but there are some See also:differences amongst various observers and See also:absolute exactness cannot be obtained. The See also:discovery of telescopic construction See also:early in the 17th See also:century and the See also:practical use of the See also:telescope by Galileo and others greatly enriched our knowledge of Jupiter and his system. Four of the satellites were detected in 1610, but the dark bands or belts on the globe of the planet do not appear to have been noticed until twenty years later.

Though Galileo first sighted the satellites and perseveringly studied the See also:

Jovian See also:orb, he failed to distinguish the belts, and we have to conclude either that these features were unusually faint at the See also:period of his observations, or that his telescopes were insufficiently powerful to render them visible. The belts were first recognized by See also:Nicolas Zucchi and See also:Daniel See also:Bartoli on the 17th of May 1630. They were seen also by See also:Francesco See also:Fontana in the same and immediately succeeding years, and by other observers of about the same period, including Zuppi, Giovanni Battista Riccioli and Francesco Maria See also:Grimaldi. Improvements in telescopes were quickly introduced, and between 1655 and 1666 C. See also:Huygens, R. See also:Hooke and J. D. See also:Cassini made more effective observations. Hooke discovered a large dark spot in the planet's See also:southern hemisphere on the 19th of May 1664, and from this See also:object Cassini determined the rotation period, in 1665 and later years, as 9 See also:hours 56 minutes. The belts, spots and irregular markings on Jupiter have now been assiduously studied during nearly three centuries. These markings are extremely variable in their tones, tints and relative velocities, and there is little See also:reason to doubt that they are atmospheric formations floating above the See also:surface of the planet in a See also:series of different currents. Certain of the markings appear to be fairly durable, though their rates of See also:motion exhibit consider-able anomalies and prove that they must be quite detached from the actual See also:sphere of Jupiter.

At various times determinations of the rotation period were made as follows: Date. Observer. Period. See also:

Place of Spot. 1672 J. D. Cassini 9 h. 55 M. 50 s. See also:Lat. 16° S. 1692 „ 9 h.

5o m. See also:

Equator. 1708 J. P. Maraldi 9 h. 55 M. 48 s. S. tropical See also:zone 1773 J. Sylvabelle 9 h. 56 m. 1788 J. H.

See also:

Schroter 9 h. 55 M. 33.6 s. Lat. 12° N. 1788 9 h. 55 M. 17.6 s. Lat. 20° S. 1835 J. H.

Madler 9 h. 55 M. 26.5 s. Lat. 5° N. 1835 G. B. See also:

Airy 9 h. 55 m. 21.3 s. N. tropical zone. A great number of Jovian features have been traced in more See also:recent years and their rotation periods ascertained.

According to the researches of See also:

Stanley See also:Williams the rates of motion for different latitudes of the planet are approximately as under: Rotation Period. . +85° to +28° . . 9 h. 55 m. 37.5 s. +28° to +24° • • . 9 h. 541 M. to 9 h. 56; m. +24° to +20° . . . 9 h.

48 m. to 9 h. 491 m• +20° to +10° .. 9 h. 55 m. 33.9 s. +10° to -12° .. 9 h. 50 M. 20 S. -12° to -18° . 9 h. 55 M.

40 s. -18° to -37° . 9h.55m.18•rs. -37° to -55° . 9h.55m.5S. W. F. Denning gives the following relative periods for the years 1898 to 1905: See also:

Latitude. Rotation Period. N.N. temperate . . 9 h. 55 m.

41.5 s. N. temperate . . 9h.55m.53.8s. N. tropical . . 9 h. 55 M. 30 s. Equatorial . . 9 h. 5o m. 27 s. S. temperate .

. 9h. 55 M. 19.5 s. S.S. temperate . . 9h.55m.7s. The above are the mean periods derived from a large number of markings. The See also:

bay or hollow in the great southern equatorial See also:belt See also:north of the red spot has perhaps been observed for a longer period than any other feature on Jupiter except the red spot itself. H. See also:Schwabe saw the hollow in the belt on the 5th of See also:September 183 1 and on many subse-N.Polar quent See also:dates. The rotation period of this object during the seventy years to the 5th of September 1901 was 9 h. S5 M. 36 s. from 61,813 rotations.

Since 1901 the mean period has been 9 h. 55 M. 40 S., but it has fluctuated between 9 h. 55 M. 38 s. and 9 h. 55 M. 42 s. The motion of the various features is not therefore dependent upon their latitude, though at the equator the See also:

rate seems swifter as a See also:rule than in other zones. But exceptions occur, for in 188o some spots appeared in about 23° N. which rotated in 9 h. 48 m. though in the region immediately N. of this the spot motion is ordinarily the slowest of all and averages 9 h. 55 M. 53.8 S.

(from twenty determinations). These differences of See also:

speed remind us of the sun-spots and their proper motions. The solar envelope, how-ever, appears to show a See also:pretty See also:regular retardation towards the poles, for according to Gustav SpSrer's See also:formula, while the equatorial period is 25 d. 2 h. 15 m. the latitudes 46° N. and S. give a period of 28 d. 15 h. o m. The Jovian currents flow in a due See also:east and See also:west direction as though mainly influenced by the See also:swift rotatory See also:movement of the globe, and exhibit little sign of deviation either to N. or S. These currents do not blend and pass gradually into each other, but seem to be definitely bounded and controlled by See also:separate, phenomena well capable of preserving their individuality. Occasionally, it is true, there have been slanting belts on Jupiter (a prominent example occurred in the See also:spring of 1861), as though the materials were evolved with some force in a polar direction, but these oblique formations have usually spread out in See also:longitude and ultimately formed bands parallel with the equator. The See also:longitudinal currents do not individually See also:present us with an equable rate of motion. In fact they display some curious irregularities, the spots carried along in them apparently oscillating to and fro without any reference to fixed periods or cyclical See also:variations. Thus the equatorial current in 188o moved at the rate of 9 h.

50 M. 6 s. whereas in 1905 it was 9 h. 50 M. 33 s. The red spot in the S. tropical zone gave 9 h. 55 M. 34 s. in 1879-1880, whereas during 1900-1908 it has varied a little on either See also:

side of 9 h. 55 M. 40.6 s. Clearly therefore no fixed period of rotation can be applied for any spot since it is subject to drifts E. or W. and these drifts sometimes come into operation suddenly, and may be either temporary or durable. Between 1878 and 1900 the red spot in the planet's S. hemisphere showed a continuous retardation of speed. It must be remembered that in speaking of the rotation of these markings, we are simply alluding to the irregularities in the vaporous envelope of Jupiter.

The rotation of the planet itself is another See also:

matter and its value is not yet exactly known, though it is probably little different from that of the markings, and especially from those of the most durable See also:character, which indicate a period of about 9 h. 56 m. We never discern the actual landscape of Jupiter or any of the individual forms really diversifying it. Possibly the red spot which became so striking an object in 1878, and which still remains faintly visible on the planet, is the same feature as that discovered by R. Hooke in 1664 and watched by Cassini in following years. It was situated in approximately the same latitude of the planet and appears to have been hidden temporarily during several periods up to 1713. But the lack of fairly continuous observations of this particular marking makes its identity with the present spot extremely doubtful. The latter was seen by W. R. See also:Dawes in 1857, by See also:Sir W. See also:Huggins in 1858, by J. Baxendell in 1859, by See also:Lord See also:Rosse and R.

Copelandin 1873, by H. C. See also:

Russell in 1876-1897, and in later years it has formed an object of See also:general observation. In fact it may safely be said that no planetary marking has ever aroused such wide-spread See also:interest and attracted such frequent observation as the great red spot on Jupiter. The slight inclination of the equator of this planet to the See also:plane of his orbit suggests that he experiences few seasonal changes. From the conditions we are, in fact, led to expect a prevailing See also:calm in his See also:atmosphere, the more so from the circumstance that the amount of the sun's See also:heat poured upon each square mile of it is (on the See also:average) less than the 27th See also:part of that received by each square mile of the earth's surface. Moreover, the seasons of Jupiter have nearly twelve times the duration of ours, so that it would be naturally expected that changes in his atmosphere produced by solar See also:action take place with extreme slowness. But this is very far from being the See also:case. Telescopes reveal the indications of rapid changes and extensive disturbances in the aspect and material forming the belts. New spots covering large areas frequently appear and as frequently decay and vanish, implying an agitated See also:condition of the Jovian atmosphere, and leading us to admit the operation of causes much more active than the See also:heating See also:influence of the sun. When we See also:institute a comparison between Jupiter and the earth on the basis that the atmosphere of the former planet bears the 2.5o a.m. 5.50 p.m.

same relation to his mass as the atmosphere of the earth bears to her mass, we find that a See also:

state of things must prevail on Jupiter very dissimilar to that affecting our own globe. The See also:density of the Jovian atmosphere we should expect to be fully six times as great as the density of our See also:air at See also:sea-level, while it would be comparatively shallow. But the telescopic aspect of Jupiter apparently negatives the latter supposition. The belts and spots grow faint as they approach the See also:limb, and disappear as they near the edge of the disk, thus indicating a dense and deep atmosphere. R. A. See also:Proctor considered that the observed features suggested inherent heat, and adopted this conclusion as best explaining the surface phenomena of the planet. He regarded Jupiter as belonging, on See also:account of his immense size, to a different class of bodies from the earth, and was led to believe that there existed greater See also:analogy between Jupiter and the sun than between Jupiter and the earth. 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 identical with that of Jupiter, and the belts of the latter planet may be much more aptly compared with the spot zones of the sun than with the See also:trade zones of the earth. In support of the theory of inherent heat on Jupiter it has been said that his See also:albedo (or See also:light reflected from his surface) is much greater than the amount would be were his surface similar to that of the See also:moon, See also:Mercury or See also:Mars, and the reasoning has been applied to the large See also:outer See also:planets, See also:Saturn, See also:Uranus and See also:Neptune, as well as to Jupiter. The average reflecting capacity of the moon and five outer planets would seem to be (on the See also:assumption that they possess no inherent light) as follows: Moon . . 0.1736 Jupiter .

0.6238 Uranus . 0.6400 Mars . 0.2672 Saturn . 0.4981 Neptune . 0.4848 S S N These values were considered to support the view that the four larger and more distant orbs shine partly by inherent lustre, and the more so as spectroscopic See also:

analysis indicates that they are each involved in a deep vapour-laden atmosphere. But certain observations furnish a See also:contradiction to Proctor's views. The absolute extinction of the satellites, even in the most powerful telescopes, while in the See also:shadow of Jupiter, shows that they cannot receive sufficient light from their See also:primary to render them visible, and the darkness of the shadows of the satellites when projected on the planet's disk proves that the latter cannot be self-luminous except in an insensible degree. It is also to be remarked that, were it only moderately self-luminous, the See also:colour of the light which it sends to us would be red, such light being at first emitted from a heated See also:body when its temperature is raised. Possibly, however, the great red spot, when the colouring was intense in 1878 and several following years, may have represented an opening in the Jovian atmosphere, and the ruddy belts may be extensive rifts in the same envelope. If Jupiter's actual globe emitted a See also:good See also:deal of heat and light we should probably distinguish little of it, owing to the obscuring vapours floating above the surface. See also:Venus reflects relatively more light than Jupiter, and there is little doubt that the albedo of a planet is dependent upon atmospheric characteristics, and is in no case a See also:direct indication of inherent light and heat. The colouring of the belts appears to be due to seasonal variations, for Stanley Williams has shown that their changes have a See also:cycle of twelve years, and correspond as nearly as possible with a sidereal revolution of Jupiter.

The variations are of such character that the two great equatorial belts are alternately affected; when the S. equatorial belt displays maximum redness the N. equatorial is at a minimum and See also:

vice versa. The most plausible See also:hypothesis with regard to the red spot is that it is of the nature of an See also:island floating upon a liquid surface, though its great duration does not favour this See also:idea. But it is an open question whether the belts of Jupiter indicate a liquid or gaseous condition of the visible surface. The difficulty in the way of the liquid hypothesis is the great difference in the times of rotation between the equatorial portions of the planet and the spots in temperate latitudes. The latter usually rotate in periods between 9 h. 55 M. and 9 h. 56 m., while the equatorial markings make a revolution in about five minutes less, 9 h. 50 M. to 9 h. 51 M. The difference amounts to 7.50 in a terrestrial See also:day and proves that an equatorial spot will circulate right See also:round the enormous sphere of Jupiter (circumference 283,000 m.) in 48 days. The motion is See also:equivalent to about 6000 m. per day and 250 M. per See also:hour. (W.

F. D.) Satellites of Jupiter. Jupiter is attended by eight known satellites, resolvable as regards their visibility into two widely different classes. Four satellites were discovered by Galileo and were the only ones known until 1892. In September of that See also:

year E. E. See also:Barnard, at the Lick See also:Observatory, discovered a fifth extremely faint See also:satellite, per-forming a revolution in somewhat less than twleve hours. In 1904 two yet fainter satellites, far outside the other five, were photo-graphically discovered by C. D. Perrine at the Lick Observatory. The eighth satellite was discovered by P. J.

Melotte of See also:

Greenwich on the 28th of See also:February 1908. It is of the 17th magnitude and appears to be very distant from Jupiter; a re-observation on the 16th of See also:January 1909 proved it to be See also:retrograde, and to have a very See also:eccentric orbit. These bodies are usually numbered in the See also:order of their discovery, the nearest to the sun being V. In apparent brightness each of the four Galilean satellites may be roughly classed as of the See also:sixth magnitude; they would therefore be visible to a keen See also:eye if the brilliancy of the planet did not obscure them. Some observers profess to have seen one or more of these bodies with the naked eye notwithstanding this See also:drawback, but the See also:evidence can scarcely be regarded as conclusive. It does not however seem unlikely that the third, which is the brightest, might be visible when in conjunction with one of the others. Under good conditions and sufficient telescopic See also:power the satellites are visible as disks, and not See also:mere points of light. See also:Measures of the apparent diameter of See also:objects so faint are, how-ever, difficult and uncertain. The results for the Galilean satellites range between o"•9 and 1"•5, corresponding to diameters of between 3000 and 5000 kilometres. The smallest is therefore about the size of our moon. Satellite I. has been found to exhibit marked variations in its brightness and aspect, but the See also:law governing them has not been satisfactorily worked out. It seems probable that one hemisphere of this satellite is brighter than the other, or that there is a large dark region upon it.

A revolution on its See also:

axis corresponding with that of the orbital revolution around the planet has also been suspected, but is not yet established. Variations of light somewhat similar, but less in amount, have been noticed in the second and third satellites. The most interesting and easily observed phenomena of these bodies are their eclipses and their transits across the disk of Jupiter. The four inner satellites pass through the shadow of Jupiter at every See also:superior conjunction, and across his disk at every inferior conjunction. The outer Galilean satellite does the same when the conjunctions are not too near the See also:line of nodes of the satellites' orbit. When most distant from the nodes, the satellites pass above or below the shadow and below or above the disk. These phenomena for the four Galilean satellites are predicted in the nautical almanacs. When one of the four Galilean satellites is in transit across the disk of Jupiter it can generally be seen projected on the See also:face of the planet. It is commonly brighter than Jupiter when it first enters upon the limb but sometimes darker near the centre of the disk. This is owing to the fact that the planet is much darker at the limb. During these transits the shadow of the satellites can also be seen projected on the planet as a dark point. The theories of the motion of these bodies See also:form one of the more interesting problems of See also:celestial See also:mechanics.

Owing to the great See also:

ellipticity of Jupiter, growing out of his rapid rotation, the influence of this ellipticity upon the motions of the five inner satellites is much greater than that of the sun, or of the satellites on each other. The inclination of the orbits to the equator of Jupiter is quite small and almost See also:constant, and the motion of each See also:node is nearlyuniform around the plane of the planet's equator. The most marked feature of these bodies is a relation between the mean longitudes of Satellites I., II. and III. The mean longitude of I. plus twice that of III. minus three times that of II. is constantly near to 18o°. It follows that the same relations subsist among the mean motions. The cause of this was pointed out by See also:Laplace. If we put L, L2 and La for the mean longitudes, and define an See also:angle U as follows: U =L,—3L2+2L3. it was shown mathematically by Laplace that if the longitudes and mean motions were such that the angle U differed a little from 18o°, there was a See also:minute residual force arising from the mutual actions of the several bodies tending to bring this angle towards the value 180°. Consequently, if the mean motions were such that this angle increased only with great slowness, it would after a certain period tend back toward the value 180°, and then beyond it, exactly as a pendulum See also:drawn out of the perpendicular oscillates towards and beyond it. Thus an oscillation would be engendered in virtue of which the angle would oscillate very slowly on each side of the central value. Computation of the mean longitude from observations has indicated that the angle does differ from 180°, but it is not certain whether this deviation is greater than the possible result of the errors of observation. How-ever this may be, the existence of the See also:libration, and its period if it does exist, are still unknown.

The following are the See also:

principal elements of the orbits of the five inner satellites, arranged in the order of distance from Jupiter. The mean longitudes are for 1891, loth of See also:October, G.M.T., and are referred to the See also:equinox of the See also:epoch, 1891, 2nd of October: Satellite V. I. II. III. IV. Mean See also:Long. 264°.29 313°.7193 39° .1187 1710.2448 62°.2000 Synodic Period I I h. 58 m. i d. 18 h. •48 3d. 13h.

.30 7d. 3h. •99 16d. 18m. •09 Mean Distance 106,400 M. 260,000 M. 414,000 M. 661,000 m. 1,162,000 m. Mass =Mass of Jup. (?) •00002831 .00002324 •00008125 .00002149 Stellar Mag. 13 6•o 6• i 5.6 6.6 The following See also:

numbers See also:relating to the planet itself have been supplied mostly by See also:Professor See also:Hermann See also:Struve.

Filar Mic. Heliom. Equatorial diameter of Jupiter (Dist. 5.2028) . 38".50 370•50 Polar diameter of Jupiter 36".o2 35"•23 Ellipticity 1+15.5 1+16.5 Theoretical ellipticity from motion of 900" in the pericentre of Sat. V I =15.3 Centrifugal force gravity at equator . . . 0.0900 Mass of Jupiter =Mass of Sun, now used in tables . 1=1047.34 Inclination of planet's equator to ecliptic 2° 9'•07+o•oo6t orbit . . . 3° 4'.80 Long. of Node of equator on ecliptic . . 336° 2I'•47+0'•762t orbit .

. . 135°25'•81+0•729t The longitudes are referred to the mean terrestrial equinox, and t is the See also:

time in years from 1900.0. For the elements of Jupiter's orbit, see SOLAR SYSTEM; and for See also:physical constants, see PLANET. (S.

End of Article: JUPITER

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