The Heavens 159 



In order, however, to get the exact location, you must also 

 know the longitude of the place, that is, on what meridian it is 

 located. Degrees are measured east and west from the meridian 

 of Greenwich. There is a meridian 1 east, another 2, and so on. 

 To the west the meridians are numbered in the same way. In 

 both directions they are numbered up to 180. By knowing both 

 the latitude and the longitude of a place one can readily establish 

 its location. 



Solar Time. The rotation of the earth on its axis gives us 

 our day and night and our means of telling time. Before clocks 

 were invented, people told time by means of a sun dial. This 

 consisted of a horizontal plate on which was an upright pointer 

 placed along a north and south line. When the sun caused the 

 pointer to cast the least shadow, the time was called noon. From 

 noon of one appearance of the sun until noon of its next appear- 

 ance was called a solar day. This solar day was later divided 

 into 24 periods or hours. 



As the length of the solar day varies slightly with the seasons, 

 an average uniform length of 24 hours was taken for it. To 

 avoid starting a new day in the middle of the common working 

 hours, this civil day was made to run from midnight to mid- 

 night. 



All places on the same meridian have their solar noon at the 

 same time. Places east of this meridian receive the sunlight 

 earlier, and places west of it receive the sunlight later. For 

 every 15 degrees of longitude east or west, there is a differ- 

 ence in time of one hour, because the earth rotates 360 in 

 24 hours, or 15 in one hour. 



Standard Time. Before the coming of railroads, each place 

 used its own solar time without any inconvenience. But when 

 people began to travel rapidly east or west, much confusion re- 

 sulted as they found that their watches were always wrong, being 

 too fast or too slow according to the direction they were travel- 

 ing, since the solar time was different in almost every town. To 

 avoid this confusion, and especially to aid the railroads in prepar- 

 ing their time tables, our government in 1883 adopted a system 

 of standard time. 



