Injlrumeiit for exhibiting Jupitef and his Moons. T2Q 



' In obfervlng the eclipfes of Jupiter's fatellites by his (hadow, it has been found that t!ic 

 duration of an eclipfe of each is longer at fome times than at others ; that in fome in- 

 ftances a fatelllte paffcs through the centre of the fliadow, which is fuppofed to be a hi ; c 

 conical, but at others through only a chord of its circular fcdion ; hence an inclination of 

 each of their orbi s to that of Jupiter has been proved, and calculated to be as follows : 

 Of the firft, about 2°S^^ the afcendmg node being at relt near tiie middle of Aquarius; 

 of the fecond, variable from i° 50\in 1608,, to 5" , i' (in i - 15), tiie afcending node being 

 at reft about 5° of Aquarius ; of the third,, variable from 3° (in 1695 , to 5° 24' ,,in 1765), 

 its afcending node being about 25° 58' of Aquarius at this time (1/98), and moving for- 

 wards eight minutes in a year j and of the fourth, about 2° 40', which is very iitic vari- 

 able, the afcending node being about the middle of Aquarius. 



The apojove of the fourth is at about 24^ " of Aries {iT-j^), and moves forward about 

 30 in five years. 



The duration of an eclipfe is the greateft at the nodes and fmalleft at the limits: that of 

 the firfl; varies from 2 h. i6min. to 2 h. 7 min. 40 fee ; of the fecond, from 2 h. 51 min. 

 20 fee. to 2 h. 13 min. 4 fee. ; of the third, from < h. 35 min. 40 fee. to i h. 2 min. 32 fee. ; 

 and of the fourth, from 4 h. 46 min. to o h. o min. o fee. 



The two oppoGte points of the ecliptic, cut by the plane of each orbit extended, are 

 called the geocentric nodes ; and a fatellite appears to move in an exa£l ftraight line 

 only when the earth is in one of thofe ; for at other fiiuacions of the earth the track of 

 each appears, though in a fmall degree, elliptical, aI^d the more fo the farther the earth is 

 removed from their nodes : this is moft apparent with the fourth, notwithftanding the 

 ncides are all in the fame figns, by reafon of the greatnefs of its orbit ; for, when near 

 either limit, it entirely efcapes an occultation, nor is eclipfed if removed above 52° from 

 either node. This happens to be the cafe this year, and will continue fo nearly, if not 

 quite, throughout the next ; and recurs for nearly two years and a half in every fix. 



The line in which the fatellites appear is nearly horizontal, as on the fcreen, when Ju- 

 puer is on the meridian, but becomes the more oblique the farther he is removed there- 

 from. ■ ■ ■ * 



The greateil part of thefe inequalities and peculiarities of motion, as well as their rota- 

 tions, will nor be attempted to be illuftrated by the fatellitian, but fuch phenomena only 

 as are demonftrable'to the eye of an obferver independently of calculations, and which 

 therefore may be confidered as the raoft proper fubje£ls of illuftration by machinery. 



After having given a defcription of the fatellitian and its appendages ; of the method of 

 redifying it for ufe ; and of the principal minutiae relating to the motions of the fatellites j 

 1 come now, in the laft place, to particularize thofe phenomena to be illuftrated, which a 

 telefcope of a moderate magnifying power for celeftial obje£ls will prefent to the obferva.. 

 tion of a fpeflator, and which afford a perpetual fource of amufement to any perfon who 

 is in pofTeffion of a good inftrument. It is neceflary however to make this previous re- 

 mark, that if the telefcope invert the o'^jeil, the pofterior furface of the fcreen mud be 

 viewed ; but if it fhew it direft, the anterior will be proper. 



"When the fatellitian is properly reflilied, the fcreen fixed and marked, and the candle- 

 ftick adjufted with only one, candle, the central one being removed, each. turn of the 



A'^ot.lL— June 1798. S handle 



