Before there were any clocks, people told the time of day by sundial (Fig. 11), which consisted of a vertical rod, the shadow of which fell upon a horizontalplane. From local noon, or the time the sun cast the shortest shadow on a certain day, until it cast the shortest shadow the next day, was consideredFig. 11.
a day"s time, or a solar day, and was
divided into twenty-four equal parts called hours.
The direction of the shortest shadow is a north and south line, since the sun must then be halfway between the eastern and westernhorizon. As the lengths of these solar days vary slightly, for reasons which cannot be explained here, we now divide the mean length of the solar days for the year into 24 parts to get the hours. The civil or conventional day begins at midnight, not noon. The determination of the exact time is very important; for the United States it is done at the Naval Observatories at Washington and at Mare"s Island, San Francisco, and telegraphed each day to different parts of the country.
A day may be measured by the interval between the successivepassages of a star across the zenith. This would be called a sidereal day, from the Latin word for star. It might also be measured by successive passages of the moon across the zenith. This would be called a lunar day, from the Latin word for moon.
If a person should start at noon and travel around the earth from east to west as fast as the sun does, the sun would be overhead all the time and no solar day would have passed for the traveler, even though 24 hours would be required for the trip. But when he reached homeMAP SHOWING INTERNATIONAL DATE LINE (Dotted Line).
he would find that it was the next day. Thus any one traveling around the earth must drop a day if going toward the west and add a day if going toward the east. The conventional place where this is done is at the International Date Line, a line extending through the Pacific Ocean and in general corresponding with the 180th meridian.
14.Standard Time. -When railways extending east and west be-came numerous in the United States and there were many through trains and numerous passengers, it became very inconvenient to use local time, since no two places had the same time. Each railway there- fore adopted a time of its own, and when several railways entered the same city these different times became very confusing. Therefore in 1883 the American Railway Association persuaded the Government to adopt Standard Time.
A certain meridian was adopted as the time meridian for a definite belt of country. The meridians adopted were 75° for Eastern, 90° for Central, 105° for Mountain, 120° for Pacific Time. These meridians run through the centers of the time belts and for 7?° on either side the time used is the local time of the central meridian. When a personMAP SHOWING STANDARD TIME BELTS.
crosses from one belt to another he finds that the time makes an abrupt change of an hour. This system has been extended to all the United States possessions, and is coming into general use over a large part of the world. In actual practice the changes of time are not made where the boundaries of the time belts are crossed, but at important places near these.
Experiment 10. -On a day when there appear to be indications of settled fair weather place a table covered with blank paper in an open space where the sun can shine upon it. Make the top of the table level and fix it firmly so that it cannot be moved. Fix vertically upon the table a knitting needle or a slender stick. Mark the line of the sun"s shadow and note accurately the time the shadow fell on this line. On the next day note the time the shadow falls upon the same line. If your watch is right, the difference in time it shows between the falling of the shadows the first and the second days is the difference between this particular solar day and the mean solar day. This may be nearly a minute. The shortest shadow of the day marks noon. It extends north and south. (Your watch keeps mean solar time. But twelve o"clock by your watch will probably not be midday or high noon, as your watch is set to Standard Time.)15.Meridians and Parallels of Latitude. -For purposes of mea- surement, circles of any size are divided into 360 equal parts called degrees. Thus the equatorial circle of the earth is divided into 360parts. Through each of these divisions there
is a semicircle drawn from pole to pole.
These semicircles are called meridians. Each meridian is divided into 180 parts called degrees of latitude, and through these points of division are passed circles parallel to the equa- tor. These circles gradually decrease in size as they approach the poles. They arecalled parallels of latitude and are num-
Fig. 12.
bered from 0 at the equator to 90 at the poles.
A certain one of the meridians, usually the one passing through Greenwich, England, is called the prime meridian and numbered1.East and west of this the meridians are numbered from 1 to 180.
The degrees thus numbered are called degrees of longitude. Thus we have a skeleton outline by means of which we are easily able to locate the position of any place upon the earth. To secure greater accuracy than could be obtained by giving merely the degrees of latitude and longitude, each of these degrees is divided into 60 equal parts called minutes, and each minute can be divided into 60 parts called seconds.
It will be seen at once that the lengths of the degrees of longitudedecrease as the pole is approached, since all the meridians pass through the poles and the distance between the meridians, which is considerable at the equator, becomes nothing at the poles. Of course these lines are simply imaginary lines and do not really exist, but in making a map or a globe we draw them as if they existed. The length of a degree of latitude at the equator is 68.7 miles, at the poles, 69.4. The difference is due to the flattening of the earth near the poles. The length of a degree of longitude at the equator is 69.65 miles, at the poles, 0.
