VENUS. 



it to Kepler. The letter is extant in the preface to Kepler's 

 Dioptrics, and a tranflation of it may be feeii in Smith's 

 Optics, p. 416. 



Having recited the obfervations which he had made, he 

 adds, " we have hence the mod certain, fenfible decifion 

 and demonitration of two grand quellions, which to this day 

 have been doubtful and dilputed among the greateft matters 

 of reafon in the world. One is, that the planets in their 

 own nature are opaque bodies, attributing to Mercury what 

 we have feen in Venus : and the other is, tliat Venus necef- 

 farily moves round the fun, as alfo Mercury, and the other 

 planets ; a thing well believed indeed by Pythagoras, Co- 

 pernicus, Kepler, and myfelf, but never yet proved, as now, 

 by ocular inipeAion upon Venus." He clofes with ex- 

 plaining the cypher that had been fent, in the following 

 words : " Hzc immatura a me fruftra leguntur, 0, y ; 

 i. e. Cynthias figuras smulatur mater amorum, or, Venus 

 imitates the phafes of the moon." 



M. Maraldi made feveral obfervations on Venus in 1729, 

 but could perceive no fpot ; and, therefore, thofe obferved 

 by Bianchini mud either liave difappeared, or tlie air at 

 Paris was not fo clear as at Rome. 



Martin Folkes, efq. formerly prefident of the Royal 

 Society, fpolce of Bianchini with refpeft, as too accurate 

 to make any miftakes in aftronomical obfervations, and too 

 honeft to publifh any thing that was not exaSly agreeable 

 to truth. See Nature of the Planet.s. 



Sometimes Venus is feen in the difc of tlie fun, in form 

 of a dark round fpot. This happens when the earth is 

 about her nodes at the time of her inferior conjunftion. 

 Thefe appearances, called tranfits, happen but feldom. 

 We have had two in the laft century, tiiz. one in June 1761, 

 and another in June 1769. The next will not occur before 

 the year 1874. See Parallax. 



The effeft of the parallax being determined, for com- 

 puting which Dr. Mafkelyne pi-opofed a new method in 

 relation to the tranfit of Venus in 1769, the tranfit affords 

 a very ready method of finding the difference of the longi- 

 tudes of two places where the fame obfervations are made. 

 For compute the effeft of parallax in time, and reduce the 

 obfervations at each place to the time, if feen from the 

 centre of the earth, and the difference of the times is the 

 difference of the longitudes. From the mean of fixty-three 

 refults from the tranfits of Mercury, Mr. Short found the 

 difference of the meridians of Greenwich and Paris to be 

 9' 15" ; and from the tranfit of Venus in 1761, to be 9' 10" 

 in time. 



Except fuch tranfits as thefe, Venus exhibits the fame 

 appearances to us regularly every eight years ; her conjunc- 

 tions, elongations, and times of rifing and fetting, being 

 very nearly the fame, on the fame days, as before. 



In 1672 and 1686, Caffini, with a telefcope of 34 feet, 

 thought he faw a fatellite moving round this planet, and 

 dillant from it about three-fifths of Venus's diameter. It 

 had the fame phafes as Venus, but without any well-defined 

 form ; and its diameter fcarcely exceeded one-fourth of that 

 of Venus. Dr. Gregory (All. lib. vi. prop. 3.) thinks it 

 more than probable that this was a fatellite ; and fuppofes 

 the reafon why it is not ufually feen, to be the unfitnefs of 

 its furface to refleift the rays of the fun's light ; as is the 

 cafe of the fpots in the moon : of which, if the whole difc 

 of the moon were compofed, he thinks, that planet could 

 not be feen as far as to Venus. 



Mr. Short, in 1740, with a r;f)efting telefcope of 16^ 

 inches focus, perceived a fmall flar near Venus ; with 

 another telefcope of the fame focus, magnifying fifty or 

 fixty times, and fitted with a micrometer, he found its dif- 

 tance from Venus about 10°: with a magnifying power of 



240, he obferved the ftar affume tlie fame phafes with Venus ; 

 its diameter feemed to be about one-third, or fomewhat lefs, 

 of the diameter of Venus ; its light not fo bright and vivid, 

 but exceeding (harp and well defined. He viewed it for the 

 fpace of an hour, but never liad the good fortune to fee it 

 after the firil morning. Phil. Tranf. N° 459. p. 646, or 

 Martyn's Abr. vol. viii. p. 208. 



M. Montaigne, of Limoges in France, we are told in 

 the Encyclopedic, art. Venus, preparing for obferving the 

 tranfit of 1 761, difcovered in the preceding May a fmall 

 ftar about the diftance of 20' from Venus, and its diameter 

 was about one-fourth of the planet. He made other ob- 

 fervations for feveral days, which were communicated to 

 M. Baudouin, who read two memoirs on the fubjeft to the 

 Royal Academy of Sciences, in which he endeavoured to 

 ftate the elements of the orbit of this fatellite ; but it is to 

 be confidered, that Montaigne's telefcope had no micro- 

 meter, and that his diftances muft be very vague and uncer- 

 tain. If Venus has a fatellite, it muft, according to Dr. 

 Herfchel, be lefs in appearance than a ftar of the eighth or 

 ninth magnitude. Phil. Tranf. for 1795. 



After all, it muft be acknowledged, that Venus may 

 have a fatellite, though it is difficult for us to fee it. Its 

 enlightened fide can never be fully turned towards us, but 

 when Venus is beyond the fun ; in which cafe, Venus ap- 

 pears little bigger than an ordinary ilar, and, therefore, her 

 fatellite may be too fmall to be perceived at fuch a diftance. 

 When (lie is between us and the fun, her full moon has her 

 dark fide turned towards us ; and when Venus is at her 

 greateft elongation, we have but one half of the enlightened 

 fide of her full moon towards us, and even then it may be 

 too far diftant to be feen by us. But it was prefumed, 

 that the two tranfits of 1761 and 1769, would afford op- 

 portunity for determining this point ; and yet we find that, 

 although many obfervers direfted their attention to this 

 objeft, no fatellite was feen in the fun's difc ; and, there- 

 fore, it is reafonable to conclude, that Venus has no fa- 

 tellite. 



The phenomena of Venus evidently fhew the falfity of the 

 Ptolemaic fyftem : for that fyftem fuppofes, that Venus's 

 orb, or heaven, inclofes the earth, paffing between the fun 

 and Mercury. And yet all our obfervations agree, that 

 Venus is fometimes on this fide of the fun, and fometimes 

 on the other ; nor did ever any body fee the earth between 

 Venus and the fun ; which yet muil frequently happen, if 

 Venus revolved round the earth in an heaven below the fun. 



Dr. Defaguliers contrived a planetarium to reprefent the 

 phenomena of Venus, according to the difcoveries of Bian- 

 chini ; as did alfo Mr. Fergufon an orrery for the fame pur- 

 pofe. The principal properties of thefe machines are the 

 following : that the angle of the axis of the globe repre- 

 fenting Venus makes, with the ecliptic, an angle of 15°; 

 that the tropics are 75° from the equator ; that the tropics 

 are 1 5° from the equator ; that the plane of a folar horizon 

 for the longeft day cuts the plane of the equator at an angle 

 of 15° ; that the fun's greateft dechnation is 75^^ ; that there 

 are but 9:^: days in every revolution round the fun ; and that 

 to bring the days to an even reckoning, every fourth year 

 muft be a leap year, which, taking in the four quarters of 

 a revolution, will make the leap year in Venus confift of ten 

 of her days, equal to 7^ months of pur time ; and that the 

 long day for the north pole will contain 45 apparent diurnal 

 revolutions of the fun. For a detail and illuftration of the 

 phenomena refulting from thefe properties, fee Defaguliers's 

 Exp. Phil. vol. ii." p. 552, &c. Phil. Tranf. vol. xHv. 

 p. 127, &c. &c.' Fergufon's Attron. p. 8, &c. For the 

 hiftory and account of various inllruments of this kind, fee 

 Orrery and Plaketabium. 



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