124 



THE GUIDE TO NATURE 



planet Mars was seen to pass in the 

 year 1672, an observation which later 

 proved of very great use in determining- 

 the motion of this planet and also the 

 mass of our sun and the distance from 

 the sun to the earth. In 1643, when the 

 planet Jupiter was a little to the left of 

 the star at N, it was seen surrounded 

 by five little points of light and the dis- 



sePT, 

 3 



2+ 

 26 



Figure 3. Showing tlie relative position of tlie 

 planet Jupiter and its four brightest satellites as seen 

 in a small inverting telescope in the early mornings 

 of the dates indicated. (At 20 minutes before 1 A. 

 M., E. S. T.) 



covery was accordingly announced of 

 no less than five new Jovian moons 

 The slowly moving planet, however, in 

 time left its new companions behind, 

 and these distant little stars may be 

 seen still grouped together at any time. 

 At the point M (Figure 2) there is a 

 beautiful round nebula ; at R of Figure 

 I there is the "Saturn Nebula." while 

 a bright nebula from which a stream of 

 stars branch out will be found in Fig- 

 ure 2 at S. In this region also there are 

 many variable stars. At U there is a 

 sun which varies in brightness from 

 the sixth to the eleventh magnitude in 

 the course of thirteen months, while 

 the star at V constantly varies from 

 the eighth to the twelfth magnitude 

 in a period a little longer than 

 nine months. What causes are in 

 operation to make these and many 

 other distant suns suddenly and 

 regularly blaze out with hundreds of 

 times their usual brightness we do not 

 know. But if our own sun should vary 

 in this manner all life upon our earth 

 would be quickly destroyed 



^ :^ ^ * * 



The Planets in September. 



Mercury will attain its farthest dis- 

 tance east of the sun on September 9, 

 and for a few evenings before and after 

 this date may be seen shining brightly 

 in the southwest, low in the twilight 

 elow. It should be looked for an hour 



or less after sunset. This planet will 

 pass to the west of the sun, and so leave 

 the evening sky on October 5. 



Venus is a brilliant object in the 

 morning sky, where it will be seen ris- 

 ing in the northeast three hours before 

 sunrise. It reaches its greatest dis- 

 tance west of the sun on September 12 ; 

 at this time it has in the telescope the 

 shape of the moon when half full. Ven- 

 us will pass to the east of Saturn on the 

 morning of September 9 ; the two plan- 

 ets will then form a most attractive 

 figure in the morning sky. 



Mars during the month will move 

 from the eastern borders of Virgo al- 

 most to the bright southern star of the 

 Balance. It is now too near the sun 

 to come within the borders of our even- 

 ing map, and, though it may be seen 

 in the southwest for about two hours 

 after sunset, it is in very unfavorable 

 position. 



The beautiful and interesting Jupi- 

 ter is now seen well above the ground 

 in the east in the position indicated 

 in Figure i. The planet is now mov- 

 ing slowly westward (or retrograding) 

 in the constellation Aries. It is in ex- 

 cellent position for observation, but it 

 can be studied to best advantage when, 

 toward midnight, it has mounted high- 

 er in the sky. 



Saturn is shining near Venus in the 

 early morning. Its rings are now wide- 

 ly opened and it is in favorable position 

 for observation. 



On September 22, at 4 hours 14 min- 

 uts P. M. (Eastern Standard Time), 

 the center of our sun will cross the 

 celestial equator and at this instant 

 autumn will begin. Were it not for 

 the refraction of our atmosphere, which 

 causes all heavenly objects to appear 

 slightly higher above the ground than 

 thev otherwise would be and thus 

 slightly lengthens the apparent day 

 (from apparent sunrise to sunset), the 

 night of September 22 and the follow- 

 ing day would be of exactly the same 

 length. 



The Observation of Jupiter's Satellites. 



The possessor of a small telescope 

 will find it most interesting to study 

 the rapid motion of this planet's four 

 bright moons. If he will look at the 

 svstem at any time, carefully noting or 

 sketching the positions of the satellites, 



