Accepting this as the basis of tbe measurement of time, it remains to distinguish between " apparent solar time," " mean solar time," "local time," and " standard time," of which the first is determined by the order of nature and the rest by agreement among authorities for the conven ience of civilization.
Apparent solar time is, to speak exactly, the moment at which the me ridian of any place passes under the sun as the earth revolves. On ac count of the facts that the path of the earth about the sun is an ellipse, that the earth is accordingly farther from the sun at certain seasons of the year than others, and hence that it travels faster at one time than an other (and for other reasons) the in terval between two passages of the sun over the meridian at a given place, i. C solar day varies slightly from day to day. Hence, as a mat ter of convenience, an average is struck.
Mean solar time, as this average is called, is determined by the motion of an imaginary sun called the " mean sun," conceived as moving with per, feet uniformity. The hours as thus determined are those measured by an accurate timepiece. Hence apparent solar time is set aside and mean solar time becomes the universal standard.
Local time is the mean solar time of any place; that is, the moment at which the place passes under the " mean sun " as the earth revolves. This moment is called noon. The sun at noon passes around the earth at the rate of four minutes for each de gree of longitude. It takes about three hours to pass from New York to San Francisco. Hence at noon in New York it is later than noon on the side toward London, east; and earlier on the side toward San Fran cisco, west.
After the introduction of railroads and steamships the use of local time became a source of great confusion. The traveler's watch would vary from the local time four minutes for every degree of longitude traversed with the added modification caused by the time occupied in the journey. Prior to the year 1883 each railway system adopted its own standard of local time. Hence the traveler could not determine the time of the arrival of trains without adjusting his watch to the local time of each railway system he patronized.
Standard Time.—The introduction of standard time in 1883 was in tended to lessen this confusion. It is
based upon the selection of certain meridians, the mean solar local time of which differs by exactly one hour or multiples of one hour, and the agreement among railway authorities and others to adopt the local time of these meridians as a standard for in tervening sections. The first of these meridians passes through Philadel phia, the second near New Orleans, St. Louis, and Davenport, the third near Denver, the fourth near Sacra mento and Stockton. The moment that the sun crosses these meridians is taken for noon in all places not more than half an hour distant from it either east or west. Hence these meridians give rise to four belts gov erned by four different standards Of time; namely, Eastern, Central, Mountain, and Pacific time, respect ively. The lines dividing these are approximately halfway between the meridians of Philadelphia, St. Louis, Denver, and Sacramento, respectively. In each of these belts railway time is uniform, and this is practically adopted by the entire population. The traveler in passing from one belt to another merely changes his watch one hour to conform to the local time. The division into minutes re mains unaltered.
In practice the lines adopted for the demarcation of the time belts are not true meridians but conform to the location of centers of population and other local conditions. The first section, Eastern time, includes all the territory between the Atlantic Coast and a broken line drawn from De troit to Charleston, S. C.; which is the most southern point. The second section, Central time, includes the territory between this eastern line and a broken line running from Bis marck, N. D., to the mouth of the Rio Grande. The third section, Mountain time, includes the territory between this central line and a line representing nearly the western bor ders. of Idaho, Utah, and Arizona. The fourth section, Pacific time, runs thence to the Pacific Coast. Standard time is uniform inside each of these sections and the time of each section differs from that next to it by exactly one hour. Thus, at 12 noon in New York City, Eastern time, at Chicago, Central time, it is 11 A.M. ; at Denver, Mountain time, 10 A.m.; at San Fran cisco, Pacific time, 9 A.M.