iv The Earth a Spinning Ball 59 



this is the apparent time occupied by the Earth in making 

 one rotation. It is in many ways more convenient, and 

 also more exact ( 1 1 1), to determine the period of rotation 

 of the Earth by observing the successive transits of con- 

 spicuous stars. By this means the exact period of the 

 Earth's rotation has been fixed as 23 hours, 56 minutes, 

 4 seconds. The name Sidereal Day is given to the rota- 

 tion period of the Earth as measured by the stars, and 

 astronomers divide it into 24 hours, subdivided into minutes 

 and seconds of sidereal time. 



95. Time. The uniform rotation of the Earth is the 

 only standard of time which is practically employed, and 

 for common purposes the solar day of 24 hours is every- 

 where used as the unit. The Sun crosses the meridian of 

 any place midway between its hour of rising and of setting, 

 and the name meridian (mid-day) was given to the north 

 and south line on this account. Mid-day or noon can be 

 determined exactly by measuring the altitude of the Sun 

 by a sextant or transit circle, or roughly by watching the 

 shadow cast by a stick or a pillar. As the Sun is rising the 

 shadow gradually becomes shorter, and at noon the Sun 

 being at its highest the shadow is at its shortest, and 

 marks out on the ground the north and south line or 

 meridian of the place. The movement of mechanism 

 actuated by a falling weight or an uncoiling spring, and 

 regulated by a pendulum or a balance-wheel, is uni- 

 versally employed for time -measuring ; but all clocks, 

 watches, and chronometers must be adjusted according to 

 astronomical determinations of the rotation period of the 

 Earth. 



96. Local Time. As the Earth turns, the Sun appears 

 successively on every meridian. It is always noon some- 

 where, but it can never be noon on two meridians at the 

 same moment. The rate of angular rotation is 360 in 24 

 hours, or 15 in I hour, or i in 4 minutes. Thus when 

 the Sun is on the meridian of Greenwich it is 1 2 hours since 

 it shone on the meridian of the Fiji Islands (180), where 

 it is consequently midnight. Two towns 15 apart differ I 

 hour in their local noon, so that it is necessary in describing 



