iv The Earth a Spinning Ball 57 



time he had got half-way from the equator to the pole (45 

 N., at O Fig. 12) the pole star would appear to have 

 risen half-way from the northern horizon toward the zenith, 

 an elevation of 45. All the stars within 45 of the pole 

 would remain in sight all night, never rising or setting, but 

 circling round the pole ; a star exactly 45 from the pole 

 would describe a circle, passing through the zenith at its 

 highest point, and touching the northern horizon at the 

 lowest. Stars beyond that limit would rise in the eastern 

 part of the sky, describe oblique arcs, and set in the 

 western; while stars more than 135 from the north pole 

 of the heavens would never become visible. Finally, if it 

 were possible to reach the north pole of the Earth, the pole 

 of the heavens would appear in the zenith (altitude of 

 90). All the stars within 90 of the pole would be visible, 

 but no others. They would ne\ er rise nor set, but always 

 wheel round in horizontal circles, once in twenty-four hours. 

 Measuring with a sextant the altitude of the pole of the 

 heavens above the horizon thus gives the latitude of the 

 observer. In practice the altitude of some bright star or of 

 the Sun when at the highest point of its daily apparent 

 path is observed, and the relative position of the Sun 

 or star being given with proper corrections in the Nautical 

 Almanac, it is easy to calculate the latitude. Thus the 

 position of an observer on the Earth with respect to the 

 poles can be found by observations of the stars without 

 any measuring of distances on the surface, and the position 

 of a degree of the meridian can be fixed. A degree of the 

 meridian varies a little in length ( 82) but averages 69-09 

 miles ; the sixtieth part of this, or one minute of latitude, 

 measures nearly 6000 feet, and is called a sea-mile, or 

 nautical mile ; the second of latitude measures about 100 

 feet. 



93. Angular and Tangential Velocity of Rotation. 

 The Earth turns on its axis uniformly, and the rate of 

 turning or angular velocity is the same at all parts. A 

 line drawn perpendicularly from the equator to the Earth's 

 axis at C describes a whole turn in the same time as a 

 line drawn perpendicular to the Earth's axis at A from a 



