sc//::VCE-GOss//: 



M I 



(irAlTERS r-OK VOfNt; ASTKONOMKUS. 

 H\ Kkank C. Dkssktt. 



(( \*ltlillHi!tt/tVHt /VlA'»" 117.) 



VENUS. 



Thin Iwautifiil phini!l, whuse brilliance .sul)lt'lllUL'^ 

 adds Mich splendour l() the sky as even lo siarilf 

 those will* ha\c not studied aslrtmoiny, is the earth's 

 truest companion so far as si/e is ctmsirlered. \'eniis 

 hns a diameter of some 7,480 miles, and its voliinie i.s 

 therefore, in comparison to that of the earth, as 

 einhtyfour is to one hundred. Its density is slij,'lilly 

 less than the rarth's, so that compared with our 

 planet, if one hundred he used to signify the mass of 

 the latter, seventvi-ij^ht will .^ive that of the former. 

 Its apparent diaiiicler varies i;reatly, more so than 

 that of any other planet except Mars. When it is 

 in superior conjunction with tlie sun, and shining as 

 a little full moon, it appears to be only 9'5" in 

 diameter : whilst at the time of inferior conjunc- 



lot 



Venus at Ckkaiesi amj 1,i;asi Uij 



tion, as at the time of transit, it increases to 62", 

 the difference heing indicated l>y the accompanving 

 diagram. 



Like Mercury, \enus is never seen on the meridian 

 at night. When farthest from llie sun its greatest 

 elongation can never e.vcecd 47- 12', at which time it 

 presents very much the appearance of a little half- 

 moon, the phase heiiig readily visible with verv slight 

 telescopic power. \\ dues' not reach its great'est 

 brilliance as an evening star until more than a month 

 later, when, although its diameter has increased, the 

 form is that of a decided crescent. At such times, 

 when distant some 40' only from the sun, its bril- 

 liance is such that on a clear ilay it maj- be readily 

 seen with the naked eye. whilst at night it throws a 

 perceptible shadow. .\ singular phenomenon has been 

 observed by man_\- with respect to the phases : a 

 (lifierence of some few days occurring between the 

 calculated and the observed time of quadrature. 

 Schroeter found this difl'crence to amount to as much 

 as eight days, whilst I)e Vico found it at Irast three 

 days, and quite small instruments will show the dis- 

 crepancy. Another phenomenon sometimes noticed 

 when Venus is near inferior conjunction is that the 



horns ap|x'.ir to lie loo long, an effect of refraction, 

 prulmlily due to the atmosphere of the planet. 



The early observers with the ancient non-uchru- 

 malic refractors delected irregularities of the termina- 

 tor or lioundary between .sunshine and darknes*. 

 These gave the impression that the surface was' 

 mountainous. More recent observations with modern 

 instruments confirm this. .\t the end of last ceiilur) 

 Schroeter carefully watched these irregularities, and 

 from changes occurring in them came to the con- 

 clusion that the planet revolved on its axis in 

 23 h. 21 ni. 8 s. One feature that particularly struck 

 this observer was the frei|Uent blunling-otl of the 

 southern horn, whilst at times a small point ap|K-.tre<I 

 detached with an apparent interval. Sometimes the 

 northern horn appeared blunled, but more frequently 

 it was seen too pointed. These oliservations have 

 been confirine<l by many observers since, using in- 

 struments of the finest definition. 



The intense brilliance of \'enus makes it difficult to 

 observe well, seeing that it not only dazzles the 

 ol>server, but finds cmt all the defects of the telescope. 

 If [Xissible, observations in daylight and near the 

 meridian are the best, the plane! then being less 

 affected by the tremulous I'ondition of ouratmosphere, 

 whilst the brightness of the sky lessens the du7;rling 

 brightness of \'enus. Thus observed, the planet may 

 iic followed very near to the sun ; Dawes thought to 

 within 1' of his limb. When a very narrow crescent 

 it has been seen interrupted at three points. Schroeter 

 considered it would necessitate mountains twenty- 

 seven or twenty-eight miles in height to produce the 

 effects he observed. 



When viewed as a very r.arrow crescent a singular 

 ■ phenomenon is sometimes seen : the entire planet 

 becomes visible. As 1 write I have lying before me 

 the names of at least twenty-four observers who have 

 thus observed it during the past 1 70 years. Some 

 have considered that the dark side of the planet .shone 

 out with a pale ashen light, which made it just 

 brighter than the sky. Some few have fancied that 

 the planet was really dark, but seen projected uixin 

 the bright background of the solar corona. Refraction 

 by the atmosphere of \'enus may contribute some- 

 what in the matter. That the appearance is real 

 there .seems to be no doubt, both from the trust- 

 worthiness of many of the observers, and the pre- 

 cautions taken by some of them. The dark side was 

 intleetl visible when the bright cr'.scenl was hidden 

 behind a bar in the tield of view. One jjoint agreed 

 upon by many ol>servers is that the unilluminated 

 portion of the limb is appareiitlv a jiortion of a 



W'lncii IS THE Lahgf.u V 



smaller circle than is indicated by the bright crescent, 

 an effect doubtless due to irradiation. An illustra- 

 tion of this seeming enlargement occurs in the well- 

 know n pu/zle, " Which is the larger ? " 



( To be ccnlinuea. ) 



