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THE GUIDE TO NATURE 



ing weeks in order to witness its grad- 

 ual fading away. It will be seen to 

 diminish in brightness much less rap- 

 idly than it rose, its general behavior 

 being well illustrated by Figure 3. This 

 figure represents the actual measures 

 of its light during one variation. It 

 shows that Mira increased from great- 



Figure 3. Observations of Mira. The dots show 

 the observed brightness and the line drawn through 

 them is the "Light Curve." 



est faintness, at A, to its greatest 

 brightness, at B, in about thirty days. 

 During the next forty days it dimin- 

 ished but little in brightness, to C, 

 while to completely fade to the faint- 

 ness D required no less than eighty 

 days more. The suddenness of the rise 

 in brightness is very remarkable, but 

 this is a characteristic of all long-per- 

 iod variable stars. 



The Planets in December. 



Mercury reaches its greatest distance 

 east of the sun on January 3, and there- 

 fore may perhaps be detected during 

 the last few days of December, shin- 

 ing far to the south in the sunset glow 

 for a short time after sunset. It is, 

 however, far below the celestial equa- 

 tor, and therefore in an unusually un- 

 favorable position for observation. 



Venus is still brilliant in the morn- 

 ing sky, rising in the southeast two and 

 one-half hours before sunrise on De- 

 cember I, which time is diminished to 

 but two hours by December 31. This 

 planet is also far below the celestial 

 equator, and is therefore always low 

 in the sky. 



' Throughout the month Mars sets 

 very far toward the south, only an 

 hour after sunset ; it is therefore in a 

 very unfavorable position for satisfac- 

 tory observation. On December 22 

 Mercury will pass to the east of this 

 planet, and the two may be seen in 



the same field in a small telescope, 

 Mercury being at this time i degree 10 

 minutes to the north of Mars. 



Jupiter and Saturn will readily be 

 found with the help of Figure i. In- 

 teresting phenomena of Jupiter's moons 

 will occur on December 3, 4, 5, 11, 12, 

 i<S, 19 and 28. 



Neptune is in Cancer, between Sat- 

 urn and the Prasepe, but it is not at 

 present near any bright star. 



On December 21 at 10 hours 59 min- 

 utes P. M. (Eastern Standard Time), 

 the sun will reach its lowest point in 

 the hea\'ens. This will therefore be 

 the shortest day of the year ; it will be 

 no less than 5 hours 34 minutes shorter 

 than the following night. 



-T* 'K -1- -J* 'I* 



An Eclipse of the Christmas Midnight 

 Sun. 



A very remarkable eclipse of the sun 

 will occur on the afternoon of Decem- 

 ber 24, but it will be wholly invisible 

 from all northern stations on the earth. 

 The great shadow of the moon will, in 

 fact, miss the turning earth altogether, 

 stretching out into space many thous- 

 ands of miles below the South Pole. 

 Hence from nowhere on the earth will 

 the sun be seen to be completely hid- 

 den, but from a very small region the 

 black edge of the moon may be seen 

 to cover a very little of the disc of the 



Figure 4. The solar eclipse of December 24 to 

 December 25, 1916. This will be visible only from 

 within the small region M A B C. 



sun. Yet by so little does this eclipse 

 miss being wholly invisible from the 

 earth that at no time will more than a 

 mere one-hundredth part of the sun's 

 diameter be seen to be covered. 



