THE sun's spots. 61 



The apparent diameter of the Sun, at its mean distance 

 from the Earth, is 32' l"-8, and therefore only 54"' 8 greater 

 than the Moon's disk at its mean distance from us. In the 

 perihelion, when in winter we are nearest to the Sun, the 

 apparent diameter of the latter increases to 32' 34"*6 ; in the 

 aphelion, when in summer we are farthest from the Sun, its 

 apparent diameter is diminished to 31' 30"' 1. 



The Sun's true diameter is 770,800 geographical miles, oi 

 more than 112 times greater than that of the Earth. 



The mass of the Sun is, according to Encke's calculation of 

 Sahine's pendulum formula, 359,551 times that of the Earth, 

 or 355,499 times that of the Earth and Moon together ( Vierte 

 Abhandlung uber den Cometen von Pons in den Schr. der 

 Bed. Akad., 1842, p. 5) ; whence the density of the Sun is 

 only about one fourth (or, more accurately, 0-252) that of the 

 Earth. 



The volume of the Sun is 600 times greater, and its mass 

 (according to Galle) 738 times greater than that of all the 

 planets combined. It may assist the mind in conceiving a 

 sensuous image of the magnitude of the Sun, if we remem- 

 ber that if the solar sphere were entirely hollowed out, and 

 the Earth placed in its center, there would still be room 

 enough for the Moon to describe its orbit, even if the radius 

 of the latter were increased 160,000 geographical miles. 



The Sun rotates on its axis in 25^ days. The equator in- 

 clines about 7° 30' toward the ecliptic. According to Lau- 

 gier's very careful observations {Comptes Rendus de V Acad, 

 des Sciences, tom. xv., 1842, p. 941), the period of rotation 

 is 25yW days (or 25d. 8h. 9m.), and the inclination of the- 

 equator 7° 9'. 



The conjectures gradually adopted in modern astronomy re- 

 garding the physical character of the Sun's surface are based 

 on long and careful observations of the alterations which take 

 place in the self-luminous disk. The order of succession, and 

 the connection of these alterations (the formation of the Sun- 

 spots, the relation of the deep black nuclei to the surround- 

 ing ash-gray penumbras), have led to the assumption that the 

 body of the Sun itself is almost entirely dark, but surrounded 

 at a considerable distance by a luminous envelope ; that fun- 

 nel-shaped openings are formed in this en> elope, in conse- 

 quence of the passage of currents from below upward, and 

 that the black nucleus of the spot is a portion of the dark 

 body of the Sun which is visible through the opening. In oi 

 der to render this explanation, of which we here only briefly 



