TO KNOW THE STARRY II1^:AVENS 



277 



variable star, Alira, there will (l()ul)t- 

 less be several, at least, who will be 

 surprised to learn what a great num- 

 ber of the stars in the sky are now 

 known to be variable suns. More than 

 five thousand have already been dis- 

 covered, and this number is constantly 

 being added to. 



The modes of variation of these dis- 

 tant suns vary greatly, many brighten- 

 ing or growing dim in the course of a 

 few days, or even hours, and others, 

 like Mira, suiTering their great out- 

 bursts of light and heat at much longer 

 intervals. Of the latter class there are 

 no less than ninety-three which during 

 the present year will become sufficient- 

 ly bright to be observed with a very 

 small telescope, or even with an opera 

 glass, and in many of them the range of 

 variation is much more remarkable 

 than in Mira itself. 



For example, just above the handle 

 of the Great Dipper, at the point A, 

 Figure i, almost equally distant from 

 the stars B and C, the observer will 

 find a variable star of this kind which 

 when at its faintest is of but the thir- 

 teenth magnitude, and hence only visi- 

 ble in the largest telescopes, but which 

 every 257 days becomes quite clearly 

 visible to the naked eye. This star is 

 now growing brighter and will attain 

 its greatest brightness of 5.5 magnitude 

 on March 13. Thus during the latter 

 part of February it will be very easily 

 visible in a small telescope. 



An even more striking variable of 

 this class is in the constellation Gemini, 

 in the position D, Figure i ; but this 

 is now wdiolly invisible in a small tele- 

 scope and will not become bright until 

 next October. The bright star at F 

 more than doubles in brightness everv 

 eight months. This is now approaching 

 a minimum ; it will be at its greatest 

 faintness next June. This remarkable 

 variable is also a double star, but the 

 presence of its companion sun has noth- 

 ing to do with its remarkable varia- 

 tions. 



While exploring this region with the 

 telescope the observer should not omit 

 to examine the remarkable star cluster 

 at H, just above the variable at F. This 

 is one of the finest objects of this kind 

 in the heavens ; though easily seen in 

 the opera glass, it requires a larger 

 telescope to reveal its multitude of 

 streaming and intermingled stars. It 



was near the star at F that Herschel 

 discovered the planet Uranus on March 

 13, 1781. During the 135 years which 

 have elapsed since that time this very 

 distant and slow moving world has pas- 

 sed one and two-thirds times around 

 the celestial sphere, and is now near 

 Mars in the constellation Capricornus, 



Figure 2. Showi"g the position of the variable star 

 at A, Fia;iire 1. The small crosses indicate the po- 

 sitions of nebulas or clusters. 



where during these February evenings 

 it is lost in the rays of the sun. 



The Planets in February, 



Mercury attains its greatest distance 

 west of the sun on February 12; for a 

 few days before and after this date it 

 will rise in the extreme south-east, 

 about one hour before sunrise, and may 

 then be detected in the glo\y of the 

 dawn. 



This strange little world, whose rad- 

 ius is but four hundred miles greater 

 that that of our moon, is revolving so 

 close to the sun that the light and heat 

 which it receives from that body are 

 seven times as great as with us on the 

 earth. The path which it follows 

 around the sun is very far from being 

 a perfect circle, so that at some times 

 it is but 28,000,000 of miles from the 

 sun and at others it is 48,000,000 of 

 miles away. Its year is but 88 days 

 long, so that when nearest the sun the 

 heat poured down upon this little world 

 is three times as great as when it at- 

 tains its greatest distance 44 days later. 

 We on the earth reached our least dis- 

 tance away from the sun on the third of 

 last January. Mercury will reach its 

 greatest distance on the 25th of the 

 present month. But few on our earth 

 ever hear the words Perihelion (near- 

 est the sun), and Aphelion (farthest 

 away), because our earth's path is so 

 nearly a circle that its varying distance 

 produces no appreciable efifect. But 

 upon a world situated as Mercury is 



