SUN SPOTS. 



499 



shall find that the even surface we can plainly distinguish at sunset is 

 marked, and the brightness is greater towards the centre of the orb. We 

 can perceive various irregularities ; we shall find spots, faculae (little torches), 

 etc. These spots were discovered by Galileo and other astronomers, and 

 were, as we have stated, found to be surface markings, and not a series of 

 bodies passing between the earth and the sun. The rotation of the sun was 

 measured, and it was found that the orb revolved in about 253 days, and in 

 such a manner as to be slightly inclined to the plane of the ecliptic. 



Herschel observed a spot at least 50,000 miles in diameter, which is 

 more than six times the diameter of the earth. The sun spots are observed 

 to be constantly changing, and are naturally observed differently as the 

 revolution proceeds. The dark pole, or " nucleus " (ttmbra), as it is called, is 

 surrounded by a less dark surface called the penumbra, but the umbra is 

 not really dark ; it is extremely bright when viewed alone, as has been 

 proved by Professor Langley, while the heat is even greater in proportion. 

 But the umbra of a sun spot must be below the level of the penumbra, for the 

 shape changed as the sun revolved on its axis. The penumbra was wider 

 on the side nearest the edge of the solar disc, and the umbra may be due to 

 the uprushing or downpouring of gas or vapour like " whirlpools in the solar 

 atmosphere." 



Near the sun spots the long streaks, or faculce, are often observed by the 

 borders of the disc, and a transition of the luminous part of the photosphere* 

 into darkness has been observed, and bright bridges crossing the spots, 

 and then gradually getting dark, were seen by M. Chacomac. The sun spots 

 vary in direction, but the same general course is continued. Sometimes they 

 describe curves, sometimes lines. 



During solar eclipses the sun exhibits what are termed " red pro- 

 minences," which are the luminous vapours 

 existing around the sun. When the orb is 

 eclipsed, we can see the bright-coloured 

 vapours shooting out from underneath the 

 dark shadow, and this light is termed the 

 " coronal atmosphere" ; the vapours are called 

 the sun's chromosphere. In the coronal at- 

 mosphere are certain curious shapes of 

 vapour thrown up, and frequently changing, 

 projecting, in fact, from the gaseous enve- 

 lope. These red prominences were first 

 observed in 1842, and in 185 I it was proved 

 that they appertained to the sun, for the moon hid them as the eclipse began. 

 Before the prominences were discovered, the red light surrounding the. solar 

 disc was known, and called the "sierra" (now chromosphere), or chromato- 

 sphere. " The luminosity of these prominences is intense," says Secchi, " and 

 they rise often to a height of 80,000 miles, and occasionally to more than 



* The bright part of the sun. 



Fig. 549. Direction of sun spots. 



