356 VENUS IN DAYLIGHT — CAMERON. 



I was keeping the glass and my eye in practice with daylight 

 peeps at Sirius, and I find that my latest one was had at 4 in the 

 afternoon of tlie Stli, when the sun was T'' higher in tlie sky than 

 Sirius was. I have no note of a try at Venus on the 8th, 9th or 

 10th, probably because the sky was not clear about noon. On 

 the 11th :— 



" 12.50 to 1 15 p m. — Venus in A. distinct enough, but not 

 fi-ood ; looks like a lengthened streak and should not. Tried N- 

 but no good. Again in A , very easy and fine and round Tried N. 

 again, all right this time. Several distinct glimpses, though 

 unsteady." 



From one superior conjunction to the nert makes an average 

 period of 584 days. As an opera glass can see Venus 9J days 

 after one of these, it should be as easj^ for it to see her d\ days 

 before the next. And as it can see her even on the day of the in- 

 ferior conjunction that falls between, there are only 19 days of 

 invisibility to be taken out of the 584 days in the whole synodic 

 period. This makes the opera glass record of 97 days out of 

 every 100, which I mentioned above. " Under favorable condi- 

 tions" I said, because at inferior conjunction Venus is not always 

 as far as 4|'' from the sun, and because at both conjunctions she 

 does not always mount as high above our horizon as at those 

 that happen during the months of May and July. 



» * * * 



The mention of Venus's altitude in the last sentence is the first 

 occurrence of this condition of visibility since I di-opped it out of 

 sight near the beginning of the paper. Since it has come up 

 ao-ain, perhaps it might be as well to give it a few words of at- 

 tention before closing. When dropping it, I said that when the 

 other two conditions (brilliancy and elongation) were favorable, 

 the minimum value of Venus's maximum daily altitude could 

 not be much under 20° in this latitude, and that at such times it 

 was quite easy to see her with the naked eye in daylight in a 

 clear sky. The mean value of her meridian altitude is 46° here, 

 and to see her at that there is no need of ver}^ favorable elongation 

 or brilliancy. An opera glass can see her 10^ lower when these 



