21 



JUNO. 



JUPITER. 



place, as well as increased activity of the veins, particularly of the liver' 

 The external use of sabine, or savine, is proper under skilful direction, 

 more particularly when diffused through fats or cerates, mostly to keep 

 up a perpetual blister. But the internal use of savine is one of the 

 most dangerous forms of popular quackery often causing death. Its 

 employment cannot be too much reprobated. 



JUNO. [HERA.] 



JUXO, one of the group of small planets which revolve between 

 Mars and Jupiter. [ASTEROIDS.] 



JUPITER. [ZEUS.] 



JUPITER, the name of one of the old planets, the largest of all the 

 bodies, except the sun, in the solar system. The astronomical history 

 of this planet (or of any other, except the newly-discovered small 

 planets) is so completely entangled in that of the progress of astro- 

 nomy in general and pure mathematics, that it would be useless to 

 attempt any separation. In the article GRAVITATION will be found the 

 most remarkable points of what an astronomer would call the theory 

 of Jupiter, meaning the explanation of his motions by means of the 

 law of gravitation. We shall here confine ourselves to the statement 

 of the elements of the planet's motion, and that of the satellites. 



The figure of Jupiter ia sufficiently oblate to appear of a sensibly 

 spheroidal form in a moderate telescope. According to the recent 

 researches of Mr. Main, the axis of revolution is to the equatorial 

 diameter in the proportion of 15'84 to 16'84. The apparent equa- 

 torial diameter varies between half and three-quarters of a minute, 

 being 37"'91, when the planet is at its mean distance from the earth. 

 The real equatorial diameter is 11'46 times that of the earth, or 

 upwards of 91,000 miles. Its mean density is nearly the same an that 

 of the sun or about one-quarter of that of the earth. 



The mass of Jupiter, from the time of Newton to the present, was 

 supposed to be to that of the sun in the proportion of 2 to 2141 ; but 

 the recent observations of various astronomers, concluding with those 

 of Mr. Airy, contained in the ' Memoirs of the Astronomical Society,' 

 make it somewhat greater. Mr. Airy's result is that the mass of 

 Jupiter is to that of the sun in the proportion of 1 to 1046'77, or 2 to 

 2094 nearly. 



Element* of the Orbit of Jupiter. 



Epoch 1799, December 31, 12 b mean astronomical time at Paris. 



Semiaxis major 5-20115524, that of the earth being assumed as the 

 unit. 



Excentricity -0481621 ; its secular increase (or increase in 100 years) 

 0001594. 



Inclination of the orbit to the ecliptic 1" 18' 62"; its secular 

 diminution 23". 



^itudes from the mean equinox of the epoch (1) of the ascending 

 code 98 26' 44""90 ; its secular increase (combined with the precession) 

 8430"; (2) of the perihelion 11 7' 38"'26 ; its secular increase 

 (combined with the precession) 6710" ; (3) of the planet (mean) 

 81 52' 19"-38. 



Mean sidereal motion in 365^ days 109256'''59; sidereal revolution 

 4332-5848212 mean solar days. 



\\'h,-n observed with a telescope of sufficient power, the surface of 

 this planet is perceived to be diversified by fault belts extending in 

 parallel directions across hirdisc. These belts are observed to vary in 

 magnitude and number ; but in general three belts are conspicuous, 

 forming a zone across the body of the planet, and extending in a 

 direction nearly parallel to the plane of his orbit. Jupiter, like several 

 of the other planets, is observed to revolve on a fixed axis. The 

 earliest surmise of this fact is due to Kepler, who inferred, from the 

 rapid motion of the satellites of the planet compared with the slower 

 motion of the moon around the earth, that Jupiter rotates more 

 rapidly than the earth does. The subsequent observations of astro- 

 nomers with the telescope have confirmed this interesting conjecture. 

 Hooke appears to have first discovered the rotation of the'planet. In 

 the month of May, 16b'4, while engaged in a telescope survey of the 

 planet he discovered a small spot on the largest of the three belts, 

 which, in the course of two hours, travelled from east to west over a 

 space equal to about half the diameter of the planet. It was reserved 

 for Cassini to assign the period of rotation with some degree of pre- 

 cision. Having found that the planet generally presented a very 

 altered appearance after the lapse of five hours, he concluded that 

 the time of rotation must be somewhere about ten hours. From 

 observations of a spot near, the southern belt of the planet, he found 

 that in 12 d 0' 4, it returned to the same position on the disc. 

 During this interval the planet must have effected twenty-nine coin- 

 ptationi. It hence resulted that the time of rotation amounted 

 to 9 k 66. The subsequent observations of Cassini indicated that the 

 spots, besides participating in the rotation of the planet, have a proper 

 motion on bis surface. The elder Herschel established the existence 

 of this fact beyond all doubt. He found that the time of rotation as 

 indirated by the return of the spots to the same position on the disc 



n ft* 65 40' and 9 h 50 48'. 



reliable determination of the time of rotation is due to 



Mr. Airy, whose observations of the planet were made at Cambridge in 



'ars 1834-5. Having watched the motion of a nuot on tin; 



apianiit. lower belt from Dfceml.er 16, 1834, to March 19, 1835, 



during which period the planet had effected 225 complete rotations, 



he hence concluded that the time of rotation ia'9 h 55"'- 21'3'. The 

 inclination of Jupiter's equator to the ecliptic amounts to 3 5' 30", 



The telescopic appearance of Jupiter seems to indicate that the body 

 of the planet is enveloped in a cloudy atmosphere of great density. 

 This opinion is strengthened by the variable appearance of the spots 

 on his surface. Sir William Herschel supposed that the spots repre- 

 sent dense masses of clouds. He ascribes their variable motion to the 

 prevalence of winds in the atmosphere of the planet, which blow 

 periodically in the same direction. He also endeavours to explain the 

 gradual increase observable in the motion of a spot after its first 

 appearance, by the circumstance, that some period of time must 

 necessarily elapse before the spot can acquire the full velocity of the 

 wind by which it is impelled. From his observations of the spots, he 

 inferred that in some instances the winds travel at the rate of some- 

 what more than 200 miles in an hour. This, however, does not exceed 

 the velocity of occasional winds on the earth's surface. Mr. De La 

 Rue has executed an admirable delineation of the physical features of 

 the planet. 



The discovery of the important fact that Jupiter is accompanied by 

 four satellites is due to Galileo. Having examined the planet with 

 a new telescope on the 7th of January, 1610, his attention was drawn 

 to three small but very bright stars which appeared in the vicinity of 

 the planet, two on the east side of his disc, and one on the west side. 

 On the following evening he again perceived the stars, but their 

 arrangement with respect to the planet was not the same as it was 

 previously. On the 13th he perceived four stars in the vicinity of the 

 planet. By continuing his observations he established beyond all 

 doubt the fact that the four stars seen by him revolved regularly 

 around the planet, and he even arrived at a rough approximation of 

 their times of revolution. 



The four satellites of Jupiter appear to revolve on axes, each in the 

 time of its revolution round the planet, in the same manner as our 

 moon. The elements of their orbits are as follows (Baily, ' Astron. 

 Tables and Formula ') : the units of tune, length, and mass being the 

 earth's mean solar day, the planet's equatorial semidiameter, and the 

 planet's mass. 



The first satellite has no sensible excentrioity, and its orbit is very 

 nearly indeed in the plane of Jupiter's equator. The second has no 

 sensible excentricity ; the inclination of its orbit to the planet's 

 equator is under half a minute, and its nodes have a retrograde revo- 

 lution of about thirty years. The third has a small but very variable 

 excentricity, and the line of apsides has a direct but variable motion. 

 The inclination of its orbit is under a quarter of a minute, and the 

 nodes make a retrograde revolution in about 142 years. The fourth 

 has a greater excentricity, and the direct mean motion of its apsides 

 is nearly three-quarters of a degree per annum. The nodes made by 

 its orbit with that of the planet have a direct motion of 4J minutes 

 per annum, and the inclination of the orbit to that of Jupiter is 

 3 nearly. 



The apparent diameters of the satellites were determined by W. Struvo 

 with a micrometric apparatus attached to the great refractor of the 

 Dorpat Observatory. The following are the apparent diameters which 

 they would subtend if viewed at the mean distance of the planet from 

 the earth : 



Apparent diameter of 1st satellite. 

 2nd . 



,, ,. 3 .. 



4th . 



. r-013 



. 0"-911 



. r-488 



. l"-273 



These measures Indicate the absolute diameters of the satellites, 

 counting outwards from the planet, to be respectively, 2429 miles,' 

 2180 miles, 3561 miles, and 3046 miles, 



In consequence of the smallness of the angle by which Jupiter's 

 equator is inclined to the ecliptic, and of the nearness of the orbits of 

 the satellites to the plane of the equator, all the satellites, except the 

 fourth, which sometimes escapes, undergo one eclipse once in every 

 revolution. Since [ECLIPSE] this is caused by the entry of the satel- 

 lite into Jupiter's shadow, the eclipse is independent of the earth's 

 position, and the observation can certainly be made, unless Jupiter be 

 too near to the sun. It is found that an immersion or emersion is not 

 visible, generally speaking, unless Jupiter be as much as 8" above 

 the horizon, and the sun as much as 8" below it. It never happens 

 that both the immersion and emersion can be observed, in the case of 

 the first satellite, and rarely in that of the second ; though it is other- 

 wise with the third and fourth. The reason is, that the planet itself 



