352 VENUS IN DAYLIGHT — CAMEllOX. 



ness is equal to If times a star " magnitude," and is exactly the 

 difference between Sirius and Rigel (according to the Harvard 

 Photometry), and more than the difference between the brightest, 

 and the faintest of the seven chief stars in Ursa Major. If any 

 observer, who knows exactly where to look, will look for these 

 two pairs of stars in strong twilight, he will get a much more 

 lively sense of wdiat this difference in brightness means than by 

 looking at them at night. And after a few such experiments he 

 will find it easy to believe that, if Rigel could be seen with the 

 naked eye about noon at an angular distance of 7° from the sun, 

 Sirius could be much more easily seen when only 5° distant, if 

 the seeing was equally good. My star-gazing experience assures 

 me of the reasonableness of this conclusion, and, as a corollary, 

 of the reasonableness of the conclusion that, whereas I have seen 

 Venus with the naked eye in daylicfht when only 7° degrees 

 from the sun at inferior conjunction, it is highly probable that 

 an eye no better than mine can see her when nearer than 5° at 

 superior conjunction. 



But I have never yet so seen her, and, in summing up the case 

 in favor of her visibility as a general rule, I shall keep within 

 the limits prescribed by my observations. These more than jus- 

 tify one in saying that Venus can be seen with the naked eye 

 in daylight when not less than 10* from the sun if the sky 

 is clear. At this limit she has proved herself easy to see. When 

 farther out she is easier. She is farther out, on the average, dur- 

 ing 246 out of every 292 days. Therefore, on the average, out 

 of every 100 days there are 84 on which any star-gazer with a 

 fairly good eye can see Venus in daylight, if the weather permits 

 and if he knows where to look for her. Near the limit, and 

 especially' inside it, both of these " if's " are very emphatic, but 

 they become less and less so as the elongation increases ; and 

 during full}^ one-half of the period of daylight visibility just 

 mentioned, the conditions of brilliancy and elongation are so very 

 favorable that none but those of defective vision need go without 

 their daily look at her, even when the sky is not at its clearest. 

 * * * * 



When Venus is near the limit of daylight-eye-visibility, the 



