The Heavens 151 



the nucleus, or center, consisting of gas in a highly condensed 

 condition; the photosphere, a covering of burning gas several 

 thousand miles thick, the visible part of the sun; the chromosphere, 

 made up of luminous gas, mostly hydrogen ; and the corona, a halo 

 of pale light around the sun. 

 The corona is never visible ex- 

 cept during a total eclipse when 

 the sun itself is hidden from 

 our view by the moon. During 

 a total eclipse the corona ap- 

 pears as ribbon-like bands 

 reaching out at times several 



thousand miles. Of these four r Brown 



parts of the sun, only the photo- FLAMES OF THE SuN 



J Streamers of fire 75,000 miles m length 



sphere is usually seen by the 



eye and the telescope; the other parts may be seen during an 



eclipse, or can be studied by the aid of an instrument called the 



spectroscope. 



Function. The function of the sun is to provide radiant 

 energy, that is, heat and light. It is the ultimate source of most 

 of the energy in the solar system. Heat and light waves go out 

 from the sun into space equally in every direction. 



The Earth. Our earth is naturally the planet in which we 

 are most interested. Its diameter is about 8,000 miles and its 

 volume is far less than one-millionth of that of the sun, around 

 which, like all other planets, it revolves. The time taken by a 

 planet to cover its path, or orbit, around the sun is its year. No 

 two planets have the same length of year. Our earth requires a 

 year of 365^4 days to make a revolution. Hence we call our 

 regular calendar year 365 days, adding one day every fourth 

 year, or leap year. 



Experiment to Show the Motions of the Earth and Its Moon 

 in Their Orbits, and Their Relation to the Sun. Use a board 

 15 inches wide and 24 inches long with holes bored along the 

 line of an ellipse, a small globe mounted on a stick that fits into 

 the holes, and a lighted candle. The ellipse represents the orbit 

 of the earth around the sun, the globe represents the earth, and 



