The Romance of the Heavens 21 



about eleven years. During this period the spots increase to a 

 maximum in number and then diminish to a minimum, the varia- 

 tion being more or less regular. Now this can only mean one 

 thing. To be periodic the spots must have some deep-seated con- 

 nection with the fundamental facts of the sun's structure and 

 activities. Looked at from this point of view their importance 

 becomes great. 



It is from the study of sun-spots that we have learned that 

 the sun's surface does not appear to rotate all at the same speed. 

 The "equatorial" regions are rotating quicker than regions 

 farther north or south. A point forty-five degrees from the 

 equator seems to take about two and a half days longer to com- 

 plete one rotation than a point on the equator. This, of 

 course, confirms our belief that the sun cannot be a solid 

 body. 



What is its composition? We know that there are present, 

 in a gaseous state, such well-known elements as sodium, iron, 

 copper, zinc, and magnesium ; indeed, we know that there is prac- 

 tically every element in the sun that we know to be in the earth. 

 How do we know? 



It is from the photosphere, as has been said, that we have won 

 most of our knowledge of the sun. The instrument used for this 

 purpose is the spectroscope ; and before proceeding to deal further 

 with the sun and the source of its energy it will be better to de- 

 scribe this instrument. 



A WONDERFUL INSTRUMENT AND WHAT IT REVEALS 



The spectroscope is an instrument for analysing light. So 

 important is it in the revelations it has given us that it will be best 

 to describe it fully. Every substance to be examined must first 

 be made to glow, made luminous ; and as nearly everything in the 

 heavens is luminous the instrument has a great range in As- 

 tronomy. And when we speak of analysing light, we mean that 



