348 POPULAR SCIENCE MONTHLY. 



ideas of what constitutes habitability for a planet that one hesi- 

 tates to accept it as proved, and almost hopes that it may turn out 

 to have no real existence. Venus, as the twin of the earth in size, 

 is a planet which the imagination, warmed by its sunny aspect, 

 would fain people with intelligent beings a little fairer than our- 

 selves; but how can such ideas be reconciled with the picture of a 

 world one half of which is subjected to the merciless rays of a 

 never-setting sun, while the other half is buried in the fearful 

 gloom and icy chill of unending night? 



Any amateur observer who wishes to test his eyesight and his 

 telescope in the search of shades or markings on the disk of Venus 

 by the aid of which the question of its rotation may finally be set- 

 tled should do his work while the sun is still above the horizon. 

 Schiaparelli adopted that plan years ago, and others have followed 

 him with advantage. The diffused light of day serves to take off 

 the glare which is so serious an obstacle to the successful observa- 

 tion of Venus when seen against a dark sky. Knowing the loca- 

 tion of Venus in the sky, which can be ascertained from the Ephem- 

 eris, the observer can find it by day. If his telescope is not per- 

 manently mounted and provided with " circles " this may not prove 

 an easy thing to do, yet a little perseverance and ingenuity will 

 effect it. One way is to find, with a star chart, some star whose 

 declination is the same, or very nearly the same, as that of Venus, 

 and which crosses the meridian say twelve hours ahead of her. 

 Then set the telescope upon that star, when it is on the meridian 

 at night, and leave it there, and the next day, twelve hours after 

 the star crossed the meridian, look into your telescope and you will 

 see Venus, or, if not, a slight motion of the tube one way or an- 

 other will bring her into view. 



For many amateurs the phases of Venus will alone supply suffi- 

 cient interest for telescopic observation. The changes in her form, 

 from that of a round full moon when she is near superior conjunc- 

 tion to the gibbous, and finally the half-moon phase as she ap- 

 proaches her eastern elongation, followed by the gradually nar- 

 rowing and lengthening crescent, until she becomes a mere silver 

 sickle as she swings in between the sun and the earth, form a suc- 

 cession of delightful pictures for the eye. 



Not very much can be said for Mercury as a telescopic object. 

 The little planet presents phases like those of Venus, and, accord- 

 ing to Schiaparelli and Lowell, it resembles Venus in its rotation, 

 keeping always the same side to the sun. In fact, Schiaparelli's 

 discovery of this peculiarity in the ca^se of Mercury preceded the 

 similar discovery in the case of Venus. There are perceptible 

 markings on Mercury which have reminded some astronomers of 



