Of course such a traveler does not really live twenty-four hours more than his •friends who stay at home; therefore, in order to have his calendar correct he must skip a day, that is, move the date ahead one day to make up for the twenty-four hours which have been added bit by bit to his other days. In traveling eastward the days are shortened instead of lengthened, and the watch must be set ahead instead of back. A certain amount of the twenty-four hours is lost out of each day. As the days are shorter, they pass more rapidly than a home, and when the traveler gets back to his starting point his reckoning will be one day ahead of that of the people who have stayed at home. Therefore, he must set his calendar one day back, that is, repeat one date.
Where Days are Lost or Gained.—Whichever way one travels around the world the date must evidently be changed somewhere. If each person changed when he finished his journey, it would cause great confusion. The easiest place to change the date is the 180th meridian, for this lies almost wholly in the ocean, and com paratively few people cross it.
For convenience the actual International Date Line is a little zigzag, as shown in Fig. 6, for the Fiji and Chatham Islands prefer to have the same day as New Zealand, while the Aleutian Islands wish to be like the rest of Alaska. Whenever a ship crosses this line it adjusts its time, that is, drops the next day of the calendar if bound westward and adds a day usual ly called Meridian Day if bound eastward. Perhaps the only unhappy result of this arrange ment is to the boy whose birth day may fall upon a lost day.
How Navigators Find Their Location.—An exact knowledge of latitude and longitude and also of time is essential to the great art of navigation. The sea captain has no mile posts or signals to guide him as has the engineer on a locomotive. He cannot even measure his speed with perfect accuracy. His ship, to be sure, is equipped with a log or little wheel that drags in the water far astern and measures the distance like the speedometer of an automobile. The mariner also knows how many revolutions the propeller makes and how far each is supposed to send the ship forward. Yet since the speed and direction of ocean currents are not constant, one cannot be sure how much of the distance indicated by the log and pro peller is due to the actual movement of the ship and how much to the currents. Moreover, the mariner cannot always be sure that his ship is moving in the right direction. Its prow may always point right, but an unusually strong current or wind may carry the vessel many miles from its true course in spite of all the mariner's corrections.
When the navigator approaches land such mistakes are most hazardous unless he can correct them by determining his exact loca tion. In other words, he must be able to determine his latitude and
longitude, and thereby correct any mistakes that he has made in his "dead reckoning" from the log and propeller and from his direction as determined by the compass.
Latitude at Sea.—Wherever one may be, the number of degrees from the zenith to the noonday sun is always equal to the number of degrees between the observer's position and the part of the earth where the sun's rays are then falling vertically. The "Nautical Almanac"' tells the mariner exactly where the rays are vertical at any given time. Hence the only thing for him to do is to measure with his sex Lant the number of degrees in the angle between his zenith and the noon-day sun and add or subtract the number of degrees by which the vertical rays are north or south of the equator. For convenience, however, the mariner actually measures the distance from the horizon to the noon-day sun, and subtracts this measure ment from This is merely because the horizon can easily be located while the zenith is not definitely marked. At sea the zenith, of course, is always from all parts of the horizon.
Longitude at Sea.—While determining his latitude the mariner may determine the hour of local noon, by finding the exact moment when the sun is highest. Knowing this he can determine his longitude by simply comparing local noon with Greenwich time which is always kept on ships by clocks of remarkable accuracy, -ailed chronometers. In practice he makes another observation for this earlier in the day.
Since any place on the earth's surface rotates through in one hour, there is a difference of in longitude for every hour of differ ence between the mariner's local noon and the time indicated by the chronometer. Thus if local noon occurs when the chronometer reads 1 p.m. the ship is 15° from the prime meridian. Since local time is behind Greenwich time the ship is west of the prime meridian. Hence the longitude is 15° W. Suppose that local noon came at 9.40 a.m. by the chronometer. In this case the difference in time is two hours and twenty minutes, which is equivalent to 35°. Here local time is ahead of Greenwich time. Hence the ship is east of the prime meridian, and the longitude is 35° E. When the mariner has deter mined both latitude and longitude, he knows exactly where he is, and can locate the spot on his chart of the sea on which he is sailing. The more important points upon maps are first located by methods like those used by the mariner. Then when the latitude and longitude of many places are known, the map can be drawn.