382. Relative scale of the orbits of Jupiter and the earth. -The relative magnitudes of the distances of Jupiter and the earth from the sun, and the apparent magnitude of the orbit of the earth as seen from Jupiter, are represented in fig. 65, where the planet is at J, the sun at s, and the orbit of the earth E E' E'" E". The direction of the orbital motions being represented by the arrows, it will be evident that when the earth is at E the planet is in opposition, at E"" in conjunction, at E' in quadrature west, and at E" in quadrature east of the sun. 383. Its prodigious orbital velocity. The velocities with which the planets move through space in their circumsolar courses are on the same prodigious scale as their distances and magnitudes. It is impossible, by the mere numerical expression of these enormous magnitudes and motions, to acquire any tolerably clear or distinct notion of them. A cannon ball moving at the rate of 500 miles an hour would take nearly a century to come from Jupiter to the earth, even when the planet is nearest to us, and a steam-engine moving on a railway at 50 miles an hour would take nine centuries to perform the same trip. Taking the diameter of Jupiter's orbit at 1000 millions of miles, its circumference is above 3000 millions of miles, which it moves over in 4333 days. The distance it travels is, therefore, about 700,000 miles per day, 30,000 per hour, 500 per minute, and 8 per second, a speed sixty times greater than that of a cannon ball. 384. Jupiter has no sensible phases. - The mere inspection of the diagram, fig. 65, will show that this planet cannot be sensibly gibbous in any position. The position in which the enlightened hemisphere is in view most obliquely is when the earth is at E' or E", and the planet consequently in quadrature, and even then the centre of the visible hemisphere is only 11° distant from the centre of the enlightened hemisphere. 385. Appearance in the firmament at night. Since between quadrature and opposition the planet is above the horizon during the greater part of the night, and appears with a full phase, it is thus favourably placed for observation during 6 months in 13 months. 386. Stations and retrogression. From a comparison of the orbital motions and distance of Jupiter and the earth, it appears that the planet is stationary at about two months before and two months after opposition; and since the earth gains upon the planet at the daily rate of o°902, the angle it gains in two months or sixty days must be 54°12. The angular distance of the points of station from opposition, as seen from the sun, is therefore about 54°, which corresponds to an elongation of 114°. The planet is therefore stationary at about 66° on each side of its opposition. R Its arc of retrogression is a little less than 10°, and the time of describing it varies from 117 to 123 days. 387. Apparent and real diameters. The apparent diameter of Jupiter when in opposition varies from 42" to 48′′, according to the relative positions of the planet and the earth in their elliptic orbits. At its mean opposition distance from the earth its apparent magnitude is 45". In conjunction the mean apparent diameter is 30", its value at the mean distance from the earth being 37". According to the most accurate methods, the mean diameter is ascertained to be 84,846 miles. The diameter of Jupiter is therefore 10.70 times that of the earth. 388. Jupiter a conspicuous object in the firmament relative splendour of Jupiter and Mars. Although the apparent magnitude of Jupiter is less than that of Venus, the former is a more conspicuous and more easily observable object, inasmuch as when in opposition it is in the meridian at midnight, and when its opposition takes place in winter, it passes the meridian at an altitude nearly equal to that which the sun has at the summer solstice. By reason, therefore, of this circumstance, and the complete absence of all solar light, the splendour of the planet is very great, whereas Venus, even at the greatest elongation, descends generally near the horizon before the entire cessation of twilight. The apparent splendour of a planet depends conjointly on the apparent area of its disk, and the intensity of the illumination of its surface. The area of the disk is proportional to the square of its apparent diameter, and the illumination of the surface depends conjointly on the intensity of the sun's light at the planet, and the reflecting power of the surface. On comparing Mars with Jupiter, we find the apparent splendour of the latter planet much greater than it ought to be, as compared with the former, if the reflecting power of these surfaces were the same, and are consequently compelled to conclude that the surface of Mars is endowed with some physical quality, in virtue of which it absorbs much more of the solar light incident upon it than that of Jupiter does. When the apparent diameter of the latter is twice that of the former, its apparent area is fourfold that of the former. But the intensity of the solar light at Jupiter is at the same time about thirteen times less than at Mars; and if the reflective power of the surfaces were equal, the apparent splendour of Mars would be more than three times that of Jupiter. The reflective power must, therefore, be less in a sufficient proportion to explain the inferior splendour of Mars, unless, indeed, the very improbable supposition be admitted that there may be a source of light in Jupiter independent of solar illumination. 389. Surface and volume.-The surface of Jupiter is above 115 times, and its volume about 1233 times, those of the earth. To produce a globe such as that of Jupiter, it would be necessary to mould into a single globe 1233 globes like that of the earth. The relative magnitudes of the globes of Jupiter and the earth are represented in fig. 66 by J and E. 390. Solar light and heat. -The mean distance of Jupiter from the sun being 52 times that of the earth, the apparent diameter of the sun to the inhabitants of that planet will be less than its apparent diameter at the earth in the proportion of 52 to 1. The relative apparent magnitudes of the disk of the sun at Jupiter and at the earth are represented in fig. 67 at E aud J. The density of solar radiation being in the exact proportion of the apparent superficial magnitudes of the disks, the illuminating and heating powers of the sun will, ceteris paribus, be less in the same proportion at Jupiter than at the earth. As has been already observed, however, this diminished power as well of illumination as of warmth, may be compensated by other physical provisions. 391. Rotation and direction of the axis. - Although the lineaments of light and shade on Jupiter's disk are generally subject to variations, which prove them to be, for the most part, atmospheric, nevertheless permanent marks have been occasionally seen, by means of which the diurnal rotation and the direction of the axis have been ascertained within very minute limits of error. The earlier observers, whose instruments were imperfect, and observations consequently inaccurate comparatively with those of more recent date, ascertained nevertheless the period of rotation with a degree of approximation to the results of the most elaborate observations of the present day which is truly surprising, as may appear by the following statement of the estimates of various astronomers: Cassini (1665) Silvabelle Schröter (1786) Mädler (1835) h m 9 55 33 The estimate of Professor Airy is based upon a set of observations made at the Cambridge Observatory. That of Mädler is founded upon a series of observations, commencing on the 3rd of November, 1834, and continued upon every clear night until April, 1835, during which interval the planet made 400 revolutions. These observations were favoured by the presence of two remarkable spots near the equator of the planet, which retained their position unaltered for several months. The period was determined by observing the moments at which the centres of the spots arrived at the middle of the disk. The direction of the apparent motion of the spots gave the position of the equator, and consequently of the axis, which is inclined to the plane of the planet's orbit at an angle of 3° 6'. The length of the Jovian day is therefore less than that of the terrestrial day in the ratio of 596 to 1440, or 1 to 2:42. 392. Jovian years. - Since the period of Jupiter is 4332.6 terrestrial days, it will consist of 104849 Jovian days.* The day here computed is the sidereal day, which, in the case of the superior planets, differs from the mean solar day by a quantity so insignificant that it may be neglected in such illustrations as these. |