S AT
SATELL1T Guard , a Perfon attending on ano- ther, eirhcr for his Safety, or to be ready to execute his flcalure. Among the Eaftem Emperors, Satellite ex- preJs'd the Dignity or Office of Captain of the Life- Guard. The Term was afterwards ufed for the Vatfals of Lords, and afterwards for iuch as held Fees, called Serge an ties: Which fee.
SATELLITES, in Aftronomy, certain fecundary Pla- nets, moving round the other Planets, as the Moon does round the Earth ; .thus called, becaule always found at- tending them from Rifing to Setting, and making the Tour of the Sun together with them. See Planet.
The S-tellites move round their primary Planets, as their Genres, by rhe fame Laws as rhofe primary ones do round their Centre, the Sun. Fur the Physical Caule of their Motions, fee Gravity; fee alio SystejA.
The Wurds A/ecu and Sateliir, are fometimes u'ed indifferently ; and thus we ^ay, eirher Jupiter's Akoni, or Jupitfs Satellites: But ordinarily we diltmguifn, re- ft raining rhe Term Mt.cn to the Earth's Satellite and Satellit to rhe little 'Mtoiis lately dilcovered about Jupi- ter and Saturn. See Mccn.
'] he Satellites were unknown till our Time, as need- ing ihe Affftance of the Ttleicope to render them viiible. We don't know of any Satellites, befides thole juft men- tioned ; nor is there any great Foundation to hope ttiat more fhajl be dilcovered hereafter, as the longeif, and the moft exquifne Teleftopes have already been applied.
Satellites of Ju/iter are Four little fecundary Planets, performing their Revolutions about Jupiter, as that Planet does round about the Sun. See Jupiter. Simon AUrivs, Mathematician of the Elector of firau- den ' nigh, about rhe End of November in 1609, difi.en.ed Three little Srars moving round Jupiter's Bo- dy, and pio:eed ; ng along with him ; and' in January 1610, found a Fourth. En January 1610, Gallil<eo ob- served the fame in Italy; and the lame Year puhlifhed his Observation-- ; from which ti e commenced the Ob- iervatinn of the QrcUmhvial Satellites. GalliUo, in Ho- nour of his Patron, firlr. called them AJlra Mediccca, Medicaan Srars. Menus, the firft Difcoverer, called that next Jpiter, Mercurius Jcvialis, Jupiter's Mercury. The iecoiul, (enus Jcvialis, Jupiter s Pmw> the third Jupiter Jovialis, and the fourth. Sal urn us Jcvialis , Jupi- ter's Saturn.
Indeed, Anthony Maria SchyrUus de Rheita, a Capu- chin uf Cokgne, iraag ned, Thar, befides rhe Four known Satellites of Jupiter, he had dilcovered Five more, the 29th of IDiCtmher, Anno 1643, and in Honour of Urban Vlll, the Pope then reigning, denominated them Sidcra Urban'Javiana. But upon Nando 1 s communicating the Observation to Grjfeudus, who had obferved Jupiter on the lame Day, he loon perceived that the Monk had miitaktn Five fixed Stars, in the pouring out of the Water of A'juarius, mark'd in Tyco's Catalogue 24, 25-, 26, 27 and ;8, for Satellites of Jupiter: Whence 'tis no Wonder they fhould appear to the Difcovcrer to move a contrary Way to that uf the reft, viz. from Weft to Eaft. See Epifi. Gajfmd. ad Gab. Naitd. de Novem Stellis. circa Jcvem vifn.
Thduotnena, Nature, Properties, &c. cf Jupiter'j
Satellites-
i°. The Satellites of Jupiter all di [appear in a clear Sky, when Jupiter interpofes between them and the Sun; that is, are ecb'pfed by him.
Hence it follows, That they are deftitute of Light, when the Sun's Rays, which are propagated in right Lines, are intercepted by Jupiter: And hence it follows, That they are opaque Bodies like our Moon, and are illu- mined by the Sun. And hence, fince Jupiter does not illumine his Satellites when placed behind him ; he him- felf, in that Side oppofnc to the Sun, is deftitute of all Light.
2 . When the Satellites are interpuftd between Jupiter and the Sun, a round Macula or Spot is obferved' in Ju- piter's Disk ; which is fometimes found bi"»cr even than the Satellit it fclf. bo
Hence, fince the Satellites are opaque Bodies, and are illumined by the Sun, and mull therefore project a Shadow, oppofite to the Sun; the round Spots feen in Jupiter, are the Shadows of the Satellites. Hence alio, fince the Interfection of the Shadow is a Circle the Shadow it felf is Conical: And hence it follows* That the Figute of the Satellites, at leaft, as to Senfe is Spherical.
3 . If, when the Earth is between Jupiter and the Sun any of the Satellites happen to be between the fame, its Light difappears, and is loft in Jupiter's Light. Thus M. Maralii tells us. That on the ?6th of March \-\q-i, thro' a Telefcope of Thirty Four Foot, he obferved the
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SAT
Fourth of Jupiter s Moons paffing over his Body, in Form of a dark Spot ; but it had no iooner got off of the Disk, than it relumed its ufual Brightnefs. A like Spot he obferved on the 4 th of April, from an Immeriion of the third Satellit; but, on the nth of April, upon watching an Immeriion of the fame Satellit-, he found it appeared wholly, without leaving any Spot at all. The fame Phamomenon was alfb obferv'd at other Times by M. Caffiui : And both CaJJini and Maraldi, have frequent- ly oblerved very iurprizing Cnanges in the apparent Mag- nitudes of the Satellites, when there was notiiing in their Diftance, either from Earth, the Sun, or Jupiter, to oc- cafion fuch Variations. j& gr. The fourth Satellite which is frequently the leaft of all, fometimes appears the biggeft : And the third, which is ordinarily the biggeft, fometimes only appears equal, and fometimes lefs, than any of the reft.
Hence, fince Jupiter's Satellites ate illumined by the Sun, even when immerged in the Light of Jupiter, and yet, notwithstanding this, fometimes appear dark, and fometimes difappear; there muft be ibme Changes in their Atmofphere-s to prevent the equable Reflection of the Sun's Rays, from the feveral Parts of the Atmofpbere. To the fame Caufe 'tis owing that their Shadows ate fbmetimes feen bigger than themfelves.
Periodical Times of f#e Satellites o/'Jupitcr.
The Periods, or Revolutions of the Satellites, are found from their Conjunctions with Jupiter, after the fame Manner as thole of the primary Planets are found from their Oppofitions to the Sun. See Period, &c.
Note, By this Method Cajfmi determined the Periods of
The Firft Satellit Second Satellit Third Satellit Fourth Satellit
iDay 18 Hours 28 Min. 36 Sec.
3 13 18 52
7 3 59 4°
6 18 05 q6
1)ifances of Jupiter's Satellites from Jupiter.
As in the primary Planets, with regard to the Sun, fo in the Satellites, with regard to their Primaries, the Squares of the Periodical Times, are in zTriplicatt Ratio of their Diftances therefrom. To determine the Diftance by Obfervation, they meafure them with a Mi- crometer, in Semi-Diameters of Jupiter. Thefe Di- ftances, according to Cajfmi, are as follow.
The Firft Satellit diftant from Jupiter's Center,
5 and two $ds. Semi-diam.. The Second Satellit 9 Semi-diam. The Third Satellit 14 The Fourth Satellit 25 and one 3d.
Hence, as the Semi- diameter of Jupiter is equal to 27 8 nths Semi-diameters of the Earth, the Diftance of the firft Satellit from the Centre of Saturn is 166 Semi-diameter;: of the Earth: That of the fecond 249 and a Half. That of the third 388 j and that of the fourth 884.
Fxlipfes cf Jupiter's Satellites. See Eclipse.
Satellites of Saturn, are Five little Stars revolving about Saturn. See Saturn. The firft was dilcovered by M. IIuyge?ts, Anno 1655, March the 25th, by means of a Telefcope Twelve Foot long. The other Four at different Times, by M. Cajfmi, viz. Thofe Two next Saturn, in March 1684, by Help of Campani\ GlafTes, of One Hundred, and One Hundred and Thirty-fix Foot long. The third in December 1672, by a Te- lefcope of Campani's, of Thirty-five Foot; and the fifth (that of Uuygens being the fourth) in October \6ll % by a Telefcope of Seventeen Foot.
Moft, perhaps all, of the Phenomena obferved of Jupi- ter's, Satellites, are alfo found exhibited by thofe of Sa- turn: Thus, they are found fometimes bigger, and fome- times lefs : The fifth is fometimes, alfo, found eclipfed, $$c. And hence, there is no doubt, but they are of the fame Nature, tfc. See Satellites of Jupiter.
T'he Periodical Timet of the Satellites of Saturn, ac- cording to M. Caflini, are as follow.
That of the
Firft Satellit 1 Day 21 Hours t% Min. 31 Seconds
Second ^atellit 2 17 41 27
Third Satellit 4 13 47 15
Fourth Satellit 15 22 41 11
Fifth Satellit 74 7 53 57
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