76 The Realm of Nature CHAP. 



real rotation of the Earth in 25,000 years round an axis per- 

 pendicular to its orbit. The axis of diurnal rotation thus 

 describes a slow conical motion like the mast of a boat which 

 is pitching and rolling equally, and the north pole, instead of 

 pointing steadily to the pole star, traces out a circle on the 

 star-dome about 47 in diameter in the course of 25,000 years. 

 The horizontal axis of a gyroscope at rest is at once drawn into 

 a perpendicular position by attaching a lightweight to one end. 

 But if the fly-wheel is in rapid rotation, the angle which the 

 axis makes with the perpendicular remains constant, and 

 the weight attached merely sets up a slow rotation of the gyro- 

 scope about the perpendicular, the axis of spinning tracing 

 out a circular cone ( 51). The differential attraction of the 

 Sun and Moon on the protuberant region about the Earth's 

 equator (82) exerts a force tending to pull the equator 

 into the plane of the ecliptic and make the axis of diurnal 

 rotation perpendicular. Rotation sets up resistance as in 

 the gyroscope, and the attempt to make the Earth sit up- 

 right results in the very slow rotation about the perpendic- 

 ular, to which the axis of diurnal rotation preserves the 

 nearly constant angle of 23^. 



1 1 6. The Sun's Surface. The bright disc of the Sun 

 which we see is termed the Photosphere, and although it 

 appears uniform in texture to the eye, the telescope shows that 

 it is finely mottled with brilliant granules separated by a less 

 luminous network. The Sun rotates in about 25 days, but 

 not like a solid globe, and the fact that marks on different 

 parts of the surface move at different rates proves that the 

 photosphere is the surface of a dense and intensely heated 

 atmosphere in which the bright granules are vast luminous 

 clouds. During a total solar eclipse red flames of fantastic 

 form are usually seen projecting beyond the black disc of the 

 Moon, and these Prominences may also be observed without 

 an eclipse by an ingenious arrangement of the spectroscope. 

 They consist of great outbursts of intensely heated gas, 

 mainly hydrogen. Prominences have been seen rising to 

 the height of 400,000 miles above the Sun's surface in a 

 few hours, against gravity 27 times as powerful as that of 

 the Earth. This gives us some idea of the terrific violence 



