SATELLITES

, in Astronomy, are certain secondary planets, moving round the other planets, as the moon does round the earth. They are so called because always found attending them, from rising to setting, and making the tour about the sun together with them.

The words moon and Satellite are sometimes used indifferently: thus we say, either Jupiter's moons, or Jupiter's Satellites; but usually we distinguish, restraining the term moon to the earth's attendant, and applying the term Satellite to the little moons more recently discovered about Jupiter, Saturn, and the Georgian planet, by the assistance of the telescope, which is necessary to render them visible.

The Satellites move round their primary planets, as their centres, by the same laws as those primary ones do round their centre the sun; viz, in such manner that, in the Satellites of the same planet, the squares of the periodic times are proportional to the cubes of their distances from the primary planet. For the physical cause of their motions, see Gravity. See also PLANETS.

We know not of any Satellites beside those above mentioned, what other discoveries may be made by farther improvements in telescopes, time only can bring to light.

Satellites of Jupiter. There are are four little moons, or secondary planets now known performing their evolutions about Jupiter, as that planet does about the Sun.

Simon Marius, mathematician of the elector of Brandenburg, about the end of November 1609, observed three little stars moving round Jupiter's body, and proceeding along with him; and in January 1610, he found a 4th. In January 1610 Galileo also observed the same in Italy, and in the same year published his observations. These Satellites were also observed in the same month of January 1710, by Thomas Harriot, the celebrated author of a work upon algebra, and who made constant observations of these Satellites, from that time till the 26th of February 1612; as appears by his curious astronomical papers, lately discovered by Dr. Zach, at the seat of the earl of Egremont, at Petworth in Sussex.

One Antony Maria Schyrlæus di Reita, a capuchin of Cologne, imagined that, besides the four known Satellites of Jupiter, he had discovered five more, on December 29, 1642. But the observation being communicated to Gassendus, who had observed Jupiter on the same day, he soon perceived that the monk had mistaken five sixed stars, in the effusion of the water of Aquarius, marked in Tycho's catalogue 24, 25, 26, 27, 28, for Satellites of Jupiter.

When Jupiter comes into a line between any of his Satellites and the sun, the Satellite disappears, being then eclipsed, or involved in his shadow.—When the Satellite goes behind the body of Jupiter, with respect to an observer on the earth, it is then said to be occulted, being hid from our sight by his body, whether in his shadow or not.—And when the Satellite comes into a position between Jupiter and the Sun, it casts a shadow upon the face of that planet, which we see as an obscure round spot.—And lastly, when the Satellite comes into a line between Jupiter and us, it is said to transit the disc of the planet, upon which it appears as a round black spot.

The periods or revolutions of Jupiter's Satellites, are found out from their conjunctions with that planet; after the same manner, as those of the primary planets are discovered from their oppositions to the sun. And their distances from the body of Jupiter, are measured by a micrometer, and est imated in semidiameters of that planet, and thence in miles.

By the latest and most exact observations, the periodical times and distances of these Satellites, and the angles under which their orbits are seen from the earth, at its mean distance from Jupiter, are as below: |

Satellites of Jupiter.
					Distances in
Satel-	Periodic Times.	Semidia-	Miles.	Angles of
lites.	meters.	Orbit.
1	1d	18h	27	34″	5 2/3	266,000	3′	55″
2	3	13	13	42	9 1/59	423,000	6	14
3	7	3	42	36	14 5/13	676,000	9	58
4	16	16	32	9	25 3/10	1,189,000	17	30

The eclipses of the Satellites, especially of those of Jupiter, are of very great use in astronomy. First, in determining pretty exactly the distance of Jupiter from the earth. A second advantage still more considerable, which is drawn from these eclipses, is the proof which they give of the progressive motion of light. It is demonstrated by these eclipses, that light does not come to us in an instant, as the Cartesians pretended, although its motion is extremely rapid. For if the motion of light were infinite, or came to us in an instant, it is evident that we should see the commencement of an eclipse of a Satellite at the same moment, at whatever distance we might be from it; but, on the contrary, if light move progressively, then it is as evident, that the farther we are from a planet, the later we shall be in seeing the moment of its eclipse, because the light will take up a longer time in arriving at us; and so it is found in fact to happen, the eclipses of these Satellites appearing always later and later than the true computed times, as the earth removes farther and farther from the planet. When Jupiter and the earth are at their nearest distance, being in conjunction both on the same side of the sun, then the eclipses are seen to happen the soonest; and when the sun is directly between Jupiter and the earth, they are at their greatest distance asunder, the distance being more than before by the whole diameter of the earth's annual orbit, or by double the earth's distance from the sun, then the eclipses are seen to happen the latest of any, and later than before by about a quarter of an hour. Hence therefore it follows, that light takes up a quarter of an hour in travelling across the orbit of the earth, or near 8 minutes in passing from the sun to the earth; which gives us about 12 millions of miles per minute, or 200,000 miles per second, for the velocity of light. A discovery that was first made by M. Roemer.

The third and greatest advantage derived from the eclipses of the Satellites, is the knowledge of the longitudes of places on the earth. Suppose two observers of an eclipse, the one, for example, at London, the other at the Canaries; it is certain that the eclipse will appear at the same moment to both observers; but as they are situated under different meridians, they count different hours, being perhaps 9 o'clock to the one, when it is only 8 to the other; by which observations of the true time of the eclipse, on communication, they find the difference of their longitudes to be one hour in time, which answers to 15 degrees of longitude.

Satellites of Saturn, are 7 little secondary planets revolving about him.

One of them, which till lately was reckoned the 4th in order from Saturn, was discovered by Huygens, the 25th of March 1655, by means of a telescope 12 feet long; and the 1st, 2d, 3d, and 5th, at different times, by Cassini; viz, the 5th in October 1671, by a telescope of 17 feet; the 3d in December 1672, by a telescope of Campani's, 35 feet long; and the first and second in March 1684, by help of Campani's glasses, of 100 and 136 feet. Finally, the 6th and 7th Satellites have lately been discovered by Dr. Herschel, with his 40 feet reflecting telescope, viz, the 6th on the 19th of August 1787, and the 7th on the 17th of September 1788. These two he has called the 6th and 7th Satellites, though they are nearer to the planet Saturn than any of the former five, that the names or numbers of these might not be mistaken or confounded, with regard to former observations of them.

Moreover, the great distance between the 4th and 5th Satellite, gave occasion to Huygens to suspect that there might be some intermediate one, or else that the 5th might have some other Satellite moving round it, as its centre. Dr. Halley, in the Philos. Trans. (numb. 145, or Abr. vol. 1. pa. 371) gives a correction of the theory of the motions of the 4th or Huygenian Satellite. Its true period he makes 11^d 22^h 41′ 6″.

The periodical revolutions, and distances of these Satellites from the body of Saturn, expressed in semidiameters of that planet, and in miles, are as follow.

Satellites of Saturn.
					Distances in
Satel-	Periods.	Semidi-	Miles.	Diam. of
lites.	ameters.	Orbit.
1	1d	21h	18′	27″	4 3/8	170,000	1′	27
2	2	17	41	22	5 1/2	217,000	1	52
3	4	12	25	12	8	303,000	2	36
4	15	22	41	13	18	704,000	6	18
5	79	7	48	0	54	2,050,000	17	4
6	1	8	53	9	3 5/9	135,000	1	14
7	0	22	40	46	2 5/6	107,000	0	57

The four first describe ellipses like to those of the ring, and are in the same plane. Their inclination to the ecliptic is from 30 to 31 degrees. The 5th describes an orbit inclined from 17 to 18 degrees with the orbit of Saturn; his plane lying between the ecliptic and those of the other Satellites, &c. Dr. Herschel observes that the 5th Satellite turns once round its axis exactly in the time in which it revolves about the planet Saturn; in which respect it resembles our moon, which does the same thing. And he makes the angle of its distance from Saturn, at his mean distance, 17′ 2″. Philos. Trans. 1792, pa. 22. See a long account of observations of these Satellites, with tables of their mean motions, by Dr. Herschel, Philos. Trans. 1790, pa. 427 &c.

Satellites of the Georgian Planet, or Herschel, are two little moons that revolve about him, like those of | Jupiter and Saturn. These Satellites were discovered by Dr. Herschel, in the month of January 1787, who gave an account of them in the Philos. Trans. of that year, pa. 125 &c; and a still farther account of them in the vol. for 1788, pa. 364 &c; from which it appears that their synodical periods, and angular distances from their primary, are as follow:

Satellite.	Periods.	Dist.
1	8d	17h	1′	19″	0′	33″
2	13	11	5	1 1/2	0	44 2/9

The orbits of these Satellites are nearly perpendicular to the ecliptic; and in magnitude they are probably not less than those of Jupiter.

Satellite of Venus. Cassini thought he saw one, and Mr. Short and other astronomers have suspected the same thing. (Hist. de l'Acad. 1741, Philos. Trans. numb. 459). But the many fruitless searches that have been since made to discover it, leave room to suspect that it has been only an optical illusion, formed by the glasses of telescopes; as appears to be the opinion of F. Hell, at the end of his Ephemeris for 1766, and Boscovich, in his 5th Optical Dissertation.

Neither has it been discovered that either of the other planets Mars and Mercury have any Satellites revolving about them.