SATAN

A Hebrew word signifying enemy or adversary; and used as such, without any reference to the Evil Power itself, in one or two pa ages of the Old and New Testament. The equivalent term in Greek for this word is dioiSoXoi, literally one who accuses or calumniates; whence the word devil is derived. SATELLITE (Lat. satelles, an attendant), in the Solar System, is the attendant of a planet; a body which revolves about the planet, and follows it in its orbit round the sun. Hence the satellite is sometimes called a secondary^ planet, or merely a secondary; the planet about which It revolves being the prima 7-1/. The planets which are accompanied by satellites are the Earth, Jupiter, Saturn, and Uranus. The Earth has one satellite, namely, the Moon; Jupiter has four; Saturn seven; and Uranus certainly two, if not six. For the Earth's satellite, see Moon. Satellites of Jupiter. — These bodies were first observed by Galileo, and their discovery followed immpdiately that of the telescope. With a telescope of ordinary power they may be seen (unle when eclipsed by the shadow of the planet, or concealed behind its disc), on any clear night, at different distances from the planet, and arranged nearly in a straight line, in which they appear to oscillate backwards and forwards with different velocities, and performing unequal excursions; so that their arrangement with respect to the planet, or configurations, are constantly changing. Sometimes they are observed to pa before Jupiter, in which case they cast a shadow on his disc like a small round black spot, whence they are inferred to be opake bodies illuminated by the sun; at other times they pa behind the planet and are concealed from our view; and all these phenomena occur in regular order, and, with respect to each satellite, after the same intervals of time. An attentive examination of the apparent motions of the satellites soon renders it evident that they revolve round Jupiter in small but unequal orbits, the planes of which are nearly coincident with that of the equator of the planet, which is inclined in a small angle to the ecliptic. Observation also shows that the motions of the satellites about their primary are regulated by the same laws as are observed by the planets in their revolutions round the sun. The orbits are ellipses of small eccentricity, of which Jupiter occupies one of the foci; the areas described by the radius vector are proportional to the times of description; and the squares of the periodic times are respectively proportional to the cubes of the mean distances. Thus Jupiter and his satellites form a system in miniature entirely analogous to that of the sun and planets. The satellites are distinguished as theirs*, second, third, SiTid fourth, according to their respective distances from Jupiter, the first being that which is nearest the planet. The following table shows their mean distances (in terms of the equatorial radius of Jupiter), times of revolution, ma es as compared with Jupiter, and diameters in English miles: — Satellite. Mean Distance. Periodic Time. Ma . Diameter in Miles. 1 2 3 4 6-04853 9-62.347 15-.'55024 26-99835 Days. 1-76914.3-55118 7.15455 16-68877 0-000017 0-000023 0-000088 0-000043 2508 2068 3377 2890 The mean distances are found by measuring the angular distances from Jupiter at the time of the greatest elongations, and the ma es were determined by Laplace from the theory of gravitation. On account of the minutene of their apparent diameters it is difficult to determine their true diameters with precision. The second, which is the smallest, at the mean distance of the planet subtends an angle of rather le than 1"; and the third, which is the largest, an angle of le than 1-5", (Me. moirs Royal Astronomical Society, vol. iii. p. 301.) Small as the satellites of Jupiter are in comparison of the primary planet, they are in themselves bodies of considerable magnitude. As compared with the Earth their stated as follows: — That diameters may be approximately sta of the first rather le than J, of the second |, of the SATELLITE. third I, and of the fourth rather more than 4. The third is about the size of Mars. These four moons must present to the inhabitants of Jupiter a spectacle of endle variety. Although their orbits are doubtle elliptical, the eccentricities of the first and second are so small as to be insensible to observation. That of the third is sufliciently sensible; and that of the fourth still greater, but subject to considerable variations. The direction of the motions of the satellites in their orbits is from west to east, according to the general analogy of the planetary system; and from observed periodical defalcations of light to which they are subject, it has been inferred that, like our own moon, each of them revolves about its axis in the same time as that in which it completes a sidereal revolution about the planet. From the preceding table it will be seen that the periodic time of the first satellite is nearly half of that of the second, and that of the second nearly half of that of the third. The mean angular motions of these three satellites, therefore, form very nearly the progre ion 1, ^, i; so that the mean motion of the first satellite, added to twice that of the third, is very nearly equal to three times the mean motion of the second. Another equally singular analogy is, that the mean longitude of the first, minus three times that of the second, plus twice that of the third, is always very nearly equal to two right angles. These two results subsist equally in respect both of the sidereal and synodical motions and longitudes; and it follows as a consequence of the last, that for a great number of years at least, the three first satellites cannot be eclipsed at the same time, for in the simultaneous eclipses of the second and third the first will always be in conjunction with Jupiter, and vice vers a. On account of the shortne of the periods of revolution, the eclipses of the satellites (especially of the first) take place very frequently; and they are phenomena of considerable importance in astronomy, from their affording signals by means of which the differences of terrestrial longitudes are determined, in the same manner as in the case of an eclipse of the moon. The method, however, is not capable of the same precision as is afforded by lunar observations. The eclipses of Jupiter's satellites have also an historical interest, from having led Roemer to the important discovery of the succe ive propagation and velocity of light. When Jupiter is in opposition with the sun, and his distance from the earth consequently le than his distance from the sun by the whole radius of the earth's orbit, the eclipses are observed to happen about 16 m. 26 sec. earlier than they happen when the planet Is in conjunction, and its distance from the earth greater than its distance from the sun by the same quantity. This phenomenon can only be explained by supposing that light occupies 16m. 26 sec. in traversing the earth's orbit, and consequently 8 m. 13 sec. in coming from the sun to the earth, which gives a velocity of about 192,000 miles in a second. The theory, with its consequences, has been amply confirmed by Bradley's discovery of the aberration. See Aberration. Satellites of Saturn.— Saturn, as already mentioned, is accompanied by seven satellites. The most distant, which is by far the largest, was discovered by Huygens in 1665. Four others were first seen by Dominic Ca ini about twenty years afterwards; but the two interior ones, which can only be seen under very peculiar circumstances, and with the aid of the most powerful telescopes, were discovered by Sir William Herschel, in 1789. On account of the diflSculty of observing the satellites of this planet, their theory has been little studied. The third law of Kepler, which connects the periods and distances, is found to be preserved, as in the system of Jupiter. The planes of their orbits coincide nearly with that of the ring, with the exception of the seventh, which makes an angle with that plane of about 3 or 4 degrees. The orbit of this last is sensibly elliptical, the eccentricity being 049. Owing to the obliquity of the orbits to Saturn's ecliptic, the satellites are not eclipsed in every revolution, but (with the exception of the two interior ones) only fall into the shadow of the planet at the times when the ring is seen from the earth nearly edgewise. The following table shows their mean distances from Saturn in terms of the equatorial radius of the planet, and their periods of sidereal revolution: — Satellite. Mean Distance. Periodic Time. 1 2 3 4 5 6 7 3-351 4-.'500 5-284 G-819 9-524 22-081 64-359 d. h. m. 0 22 38 1 8 53 1 21 18 2 17 45 4 12 25 15 22 41 79 7 55 The two interior satellites appear to just skirt the exterior edge of the ring. The seventh, like the satellites of Jupiter, exhibits periodical changes in the intensity of [s. 1094]