Another point to remember is that high latitudes are not only designated by high numbers both north and south of the equator, but are those which would be highest on the umbrella of our illus tration. The continent of Antarctica is in high latitudes, while Ecuador, which means Equator Country, is in low latitudes. Our own country is in the middle latitudes, which are the best parts of the world.
A good example of the use of latitude and longitude is the way it enables ships at sea to state their exact position. When the Lusitania, for example, was torpedoed by a German submarine in 1915, it sent out a wireless message that it was in 51° N. latitude, 9° W. longitude. Instantly the ships receiving the message hastened to the exact spot even though it was far beyond their vision. Though the Lusitania sank in twenty minutes many people were saved. An airplane in crossing the ocean asks the latitude and longitude of every ship that it passes, in order that the aviators may know just where they are.
How Time is Determined.—In order to ascertain the location of a place on the earth's surface, time as well as latitude and longitude must be employed.
The sun is the natural time-keeper for all the world. When people first made careful measurements of time they based their reckonings on the hour at which the shadow cast by the sun is shortest. This gave the simplest form of " local " time, but it was ultimately found that the days measured in this way vary in length, because of the earth's varying rate of motion around the sun. By mak ing corrections on this basis, they obtained what is called " mean " time, according to which twelve o'clock is the average time at which the sun reaches its highest point throughout the year. This was an important step. Today, however, local mean time is rarely used in progressive regions. It is not convenient in these days of railroads, automobiles, air planes and rapid travel, for no two places have the same local time unless they happen to be on the same meridian. When railroads were introduced each railroad used the time of the city where its headquarters were located. At railroad junctions there thus were often different times. In one town five systems were in use as late as 1880, while in the United States as a whole the rail roads ran on 53.
To obviate this confusion the present system of " standard " time was adopted with only four belts. These belts are based on longitude. Since the earth rotates an hour, the railroads in 1883 agreed to use only the local time of certain " standard " meridians that are multiples of 15°. The country is divided into belts lying on either side of these meridians.
The most easterly belt uses the time of the meridian 75° west of Greenwich. Eastern Time, as it is called, is therefore five hours
behind that of London. The next, or central time belt, uses the time of the 90th meridian, six hours behind London; while the moun tain time of the 105th meridian and the Pacific time of the 120th are respectively seven and eight hours behind London. This system is very convenient, for people do not need to change their watches ex cept on passing from one belt to another, and then the change is al ways exactly one hour. The standard time belts of the United States are shown in Fig. 5. As the railroads prefer to change time at division points where new trains are made up, rather than in the middle of long runs, the belts are irregular in shape.
How Travelers Gain or Lose Time.—On a journey around the world the time changes twenty-four hours. In 1519 Magellan left Spain with five ships to make the first voyage round the world. When his sole surviving vessel reached Spain three years later the crew cound not understand why their reckoning made the date September 6, while the people at home said it was September 7. No mistake could be found in the ship's records, and the travelers were much puzzled until Paoli Sarpi told them that during their adventurous voyage they had lost a day by going around the world with the sun. If they had gone eastward, they would have gained a day, and would have recorded the date of their return as September 8.
The explanation is simple. Suppose a traveler starts from London at noon on Monday, and travels westward at the rate of a day. Since of longitude are equal to an hour of time, the sun at the second noon will reach the zenith an hour later than at London. Therefore at noon by the sun on Tuesday, the traveler's watch will say 1 P.M., and he will have to set it back an hour and will actually have twenty-five hours in his day. Such changes in the clock are made regularly on ships that cross the Atlantic. The change, how ever, is generally less than an hour, for only in high latitudes can ships travel through of longitude in a day. On Wednesday, if the traveler continues westward at the rate of a day, a change of another hour will be necessary. London time will now be ahead of the traveler's by two hours, for each of the traveler's days has been twenty-five hours long instead of twenty-four. If he keeps on around the world he will traverse of longitude, and change his time twenty-four hours. Whether he travels slowly or rapidly he will gain the same amount of time in traveling the same number of degrees, and when he gets back to London he will have gained a whole day.