404 REPORT — 1873. 



On the Visibility of the dark side of Venus. 

 By Professor A, Schaparik^ of Prague. 



[A Communication ordered by the General Committee to be printed in extenso.'] 



Ix is well known that the uuillumined side of the planet Yenus has been 

 sometimes seen shining with a faint grey light, like the dark side of the moon 

 when illumined by the earth, 



Schrciter in 1806 thought he had made for the first time this remark- 

 able observation ; but it was found afterwards that Harding had made it 

 almost simultaneously, and Olbers pointed out an old observation made by 

 A. Mayer at Gryphiswald in 1759. Arago found a still older observation of 

 the same kind made by Derham at a date not fixed, but certainly anterior to 

 1729, the date of publication of the Ereuch edition of his 'Astrotheology.' 

 - [Nevertheless this phenomenon is stated in the best text-books of astronomy 

 to be one of the utmost rarity. Madler knows only two observers of it, the 

 profoundly learned Humboldt only three, Arago only five ; and even re- 

 cently Dr. Winnecke, of Karlsruhe, believed that he was the only witness of 

 that phenomenon in daylight since the time of A. Mayer ; but under these 

 particular circumstances it has been seen by eleven observers, and by five 

 of them more than once. 



It was known to me for a long time that there were on record far more 

 observations of this phenomenon than is ordinarily svipposed ; and when, 

 some years ago, I happened to be a witness of it myself, I undertook to 

 collect all existing observations of it. 



This I have now done ; and as I have succeeded in collecting the sur- 

 prising number of twenty-two observations, many of them repeated more 

 than once, a short account of what I have found will perhaps be not unin- 

 teresting to astronomers. 



1. The first observation recorded is that of William Derham, Canon of 

 "Windsor, referred to in his 'Astrotheology ' as made in the perigeum of Venus, 

 probably in bright twilight, when he saw the dark side of the planet shining 

 with a dim reddish light. Arago, who mentions this observation, quotes 

 from a French translation published in 1729. It would be interesting to 

 know if this observation is found also in the first English edition published 

 in 1714. 



2. The second in order was Christian Kirch, first astronomer of the Eoyal 

 Academy of Sciences at Eerlin. He saw the phenomenon twice (June 7, 1721 , 

 and March 8, 1726), both times with moderate optical power and in bright 

 twilight. He remarked that the bright crescent was apparently a part of a 

 larger sphere than the faintly shining dark side. (Astrouomische Nachrichten, 

 No. 1586, vol.lxvii. p. 27.) 



3. Third came Andreas Mayer, Professor of Mathematics in the Gryphis- 

 wald University, who, on October 20, 1759, obsen-ed Venus, culminating 

 only 10° from the sun, with an unacliromatic transit-instrument of only 

 l|-inch aperture, and saw the whole disk "like the crescent moon which re- 

 flects the light of the earth." (Observationes Veneris Gryphiswaldenses, 1762, 

 p. 19. Schrciter, Beobachtungen des grossen Cometen von 1807, Appendix, 

 p. 74.) 



4. The fourth witness is Sir William Herschel, who about 1790 several 

 times saw a part of the limb of the daik side in a faint light. Neither date 

 nor time of day is given. (On the planet Venus, Philosophical Transactions 

 for 1793.) ^ 



