Observations of the Transit of Venus Over the Sun, Contained in a Letter to the Reverend Nevil...

Observations of the Transit of Venus Over the Sun, Contained in a Letter to the ReverendNevil Maskelyne, Astronomer Royal, from Dr. Alexander Wilson, Professor of Astronomy inthe Universitv of GlasgowAuthor(s): Alexander WilsonSource: Philosophical Transactions (1683-1775), Vol. 59 (1769), pp. 333-338Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/105841 .

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[ 333 ]

XLlII. Obpertartions of the 7stat# of Venus ozver t Sun, contained ir a Leter to thc ReverenJ{: Nevil MaielyneX XyYrozorer Royal, from Dr. Alexander V\tilfon, PrF fe/for of SIJ?ronofny ; the UxiverAtv of Glafgow.

Cofilege, GlaGgow, Sept. g; ty6g S I X,.

Read Dec, 7, T S E N 1:) you now the pasticulars of 1769 1 tny ob6et vation of the tranfit of Venus,

together with the obServations of thofe who aflced in concert with me. I chofe for the place of aly ob- lservations a: houSe at fome diIlance from sur ob6er- vatory, but in fight of it, and more free from the fmoke of the town; where I had two gentlemen to attend the clock, and mark the times. I carried with xne t^ro refleEtors of Mr. Sholt'sfi which are defcribed below. Three other in{truments werc made ufe of at theobSeruatsry; tlle firIt was an achromatic tube of IDollond's, 29 incil focus, by which antmage ofthe Sun was formedj of about fix incbes diameterj on a board spvered with paper. The tieScopa being nzounted uponSa frame, by which it could be turned about as the Sun movedj and the room properly darkened. This inRrument was managed by Dr. Williamfon

and



[ 334 3 -stsd¢Dr Reid, at the u7elE window of tle root3 of tbe obErvatorarn svlzereill tlle aRronomical clock ftood > tl3e other two inRruments were placed without, at the {oalth and north -windozYs c)f the fanxe room, one being a refrador of I 3 fects by which Dr. Irvine ob- ferved; the other a 2 inch refleEtorof Sllolt's, b5t ^rhich my fon obServed TheSe two obEerxrers looked

direEtly at the Sun, having their inllrpmeats arnled \Vitll flmoke-glaffies; anotlwr perSon Rood at tIze clock, and tounted t}e fieconds by corncident beats upon a piece of board, whlcb lze held in his hand for

tllat purpofe, and who nanzed every fiftll fecond, {o that all the obServers could hear hiel diflindcly. Tlze

tnotion of the clock) made by Shelton, was care- fully adjulled by nlany tranfits of the 8un and fixed Itars, over the meridian, lioth before and after the day

of the trannt; the clock br vvhich lny obfiervations

were made-was adjulled by Shelton's, by means of

ignals made every hour, for fome hours before arJd sfter the tranfit Ie zvas apprehended, that the fmoke of the tow] might hart the obEervations ; allds to pre srent thIs as tntlch as pofl5lble, arl advertiSement was put in the newspaper, begging the ial1ahtants, in caSes where it would not be very inconvenient, eo put otft their Stes fron three oclock that afterizoon tiEl Mun*Setting, the poltfenefis of the inSabxtants of Glaf- gow, in complyin:g vjth this reque(t, was far greater tllan could well be expetteds infomuch that there was not a rpire of ftoke to be percewed in that quar- eerfrom vvhich the obfiervations could be itlcom- moded. Having made thefe preparations, we thought we had nothing to fear but the clouds-; and xndeed the weflern p7wrt of the heavens was covered * with



[ 335 ] With chick clouds all the afternoon, till a ffiort time before the external conta2; but theg drove away tosdrards the nortlz, and left the Sun perfiflrly bright, excepting that {z-ow and then a cloud paffied o?er llilu. But we foon found:that the conilitution of the air was otherwsfie urfavourable to our obfervations ; tlre image of tlle Sun on tlle white board, mFade by tbe achromatic telefcape, zas bright enough; bue there was a remarkable u;ndulation ill the limb, which could be owing to notlaing elSe but the Itate o£ the air t-his inconstenience svas alfo fenElbly felt both by the other gentlemen and mySelf. Befides the undulation now menvtioned in the limb of the Sun, there was alfo a confildegal31e tremor roulzd the planet Venus, wben flze ssras Rea upon the Sun's elilE, arld} in confequence of this, an-indiSinEtneS; in lz-er limb, which made it inlpoi1ble to meafure her liameter by our objedx glaSs nwicrometer, or other- zviKe. After the cetlter of Venus had paffied the )(lnts limb, lBe appeared to us not to be circularF but oblong, the; longe{} dianzeter being tlwat whiclu paffied through the Sun's center. As ehe internal contad approacbed, Venus appeared to us to adhere to the Sun's limb, byaEdark protul3erance or neck, both the length and breadth of which varied every ulonzent by a conRant- und-tllation :: neither did this neck break o£$ inIlantantouny, but changed its coloalr from black to a dufley browlle till at la(} the interval betvixt VeniZs ands the Sun's- limb appeared quite elear.* Each Qf the oWervers wrote down his obSer vations on tlze fpot.. I reduced thezn?, together NYiti rny own, to apparent time, fxm tle oblirvations 1

11ad.



[ 336 3 Iz.xd tnade Oll tlae golng of tlle clock, and are §t folloslts .

h t ts 'External contad 6 y4 3t,4

Venusis centetjudged to bc on the litnb 7 1 33,4 Sun's Ii,>ht appeared betwixt Yenusand). p It 56,

y Door The btinning of the SoIar eclipfe next] ilfion. t mflrlang; obServed by Short's Is inchl 18 30 t4,z

MiddIew frox a feries of obfervations w!ith 1 theobjeEtglafs micronceter, {ittcd toa.Jlg 18 4.7,, nine inch refleftor of Short's

End ndt vifiblB

BylZr.Wil- rExtzernalcontaft 6 54 zS liamfon and 4 Internal COtl&, or whed the Sun's light 1 I Dr.-ReM. k appearedbetwlxtVenusandthelimb f 7 t2 24

Yenus's center jetdged to be on the iimb, 1 by Dt. Reid J 7 1 24

Dr. Reld marked the time when h@ conceisted the internal contaft would have happened, if the dalk protuberance upon Venus had been taken away, atld her diSe reduced to a circle, viz. 7h IOt <4*t. tie thinles it likewifie proper to mention; thatf feveral We- conds before the time above fet down as the time of the internal contaEt?- he iRaw a finall dint upon the Sun's limbs which he tooli to be the external contadr; thae he immediately mentioned this to Dt. William fon, who happened that inRant to have his eyes turned anotller way; but before he could look at lt it difappeared by the undulation in the Sun'slimb. Dr. Reid is the xnore perhvlladed that this was not mere imagination,- becauSe this dirlt on the limS of the Sun appeared to hiln much nearer to tlse Sun's

vertica'l



[ 337 :1 vertical clialneter tlean he expeEted it, btlt in the rery point, h>)vever, wllAlse it utas c'eally feell illlme- diat-ely after. -

h f r

P. WilSon. { InXtetnll conda f, 5+ 28

My fon d&res me to remark, that lais firS obSer- v.ltion Ihould be confidered as no other but a poRerior confirmation of I)r. \N illiamfon's arld Dr. Reid's external contad; the tatt sras, that wllen theSe gentlemen perceived the firk contad:t, their lieentleIi nzade tElem call out, and it was not till then that he faw the phanomvnon with perfeEt certainty. He stras conScious, however, that he fludluated concerning the realIty of the appearance for about twelve fecorlds before that tinzea durin<, which his determinations were fuEpellded, through an apprehenfion of antici- pating the real time, wlaich was heightened by rO clofe a neighbourhood svitll the other obServers, all of whom he could not help being fenflble were Rill ex- peEting the phanomenon. Upon the whole, he is rather of opinion that he would llalre put down the external contaA at leaR eight Seconds fooner had he been obferving apart. His fecond obServationt by ^thich he means the inRant when the interval be- tween Venus and the Sun's limb firR appeared ob- violls, was taken down without the leaI} knowledge of what was pading among the oeher gentlemen who - obServed. ilAr. Irvine has been out of town for fome confiderable tinze pall, and fbrgot to lodge his obSer- vations with me, but - I remember certainly that he made the external contaA three fieconds fooner than the reR; but his internal contaft was fome feconds

Vo. LIX. X x later,



[ 338 ] later, but how many I do not now remenlber. Mr. AnderSon, F. R. S. fitted up a clock and appa- ratus in the coliege Iteeple; his clock was regulated as above, by f1gnals from the obfiervatory; he ob- ferved tlle tranflt with a large refleEtor, and hls afEl0- ants obServed with refraftors: they were all of them uncertain about the external contati, owing to the Rate of the atmofphere, and a tremor given to the Ileeple by the wind; but none of their other obSer- vations varied, above three feconds> from my own, as related above.

O , ,,

Latitlldeoftheobfiervatory 55 51 32 tongitude by correfponding 1

, ,* . > O I 7 I X of tlme from Greenwlch Ws owrvatlons J

Iam, Slr,

with great elleem}

Your moft obedient fervants

Alexander Wilfon

I have obSerrrede during the collrfe of this year, feveral times, the Aurora 130realis form itielt into an arch, zo or 3o degrees above the horizon, which con- tinued permanent for fome time; and in this cai the vertex of the arch appeared always to bn well from

orth, by about the variation of the needle in 19 or zo degrees. Whether or not it is always SO, I cannot yet ray.

XtIV. An



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