INTERNATIONAL DATE-LINE.—The line at which dates change, being made later by one day by those who cross the line from east to west, and earlier by one day by those crossing it from west to east. • If a person start at midday, that is, when the sun was shining perpendicularly on the meridian that passes through the place of starting, and travel westward, keeping pace with the sun, thus keeping the sun directly over the meridian of the place at which he might he, he would make a complete journey around the globe in twenty-four hours, and return to his place of starting at noon the next day. Twenty-four hours would have passed, but to the traveler the sun would have been shining perpendicularly as at noon all the time: and the question arises, when or at what point did the traveler change from noon of one day to noon of the next? For instance, if he should start at Monday noon and keep the sun in the zenith, he would arrive at the place of starting Tuesday noon—it would be noon-day to hint during the whole journey of twenty.four hoots— Monday noon would change to Tuesday noon without an intervening night: where would the change occur ? It is to him apparently still :Monday noon, and to obtain the correct date he must drop a day. The reason for dropping a day can he more fully shown as follows:—Remembering that the earth makes one complete revolution on its axis in twenty-four hours, and thus the sun in its diurnal revolution moves over 800 degrees of space in twenty-four hours, it thus moves over 15 degrees of space in one hour, from which it is evident that the diffel'etice in longitude which causes the difference in the relative time, may be estimated in time, allowing 15 degrees to an hour, or one 'degree to four minutes. Therefore, suppose a man starting from any given point, travel one degree w., 'his watch, instead of marking 12 o'clock at noon, according to the correct time at that place, would mark four minutes after twelve. Let him travel w. 15 degrees, and he will find.that 1 o'clock by his watch will be noon day by the sun. Let him go on to 120 degrees, and when the sun is in the zenith his watch will indicate eight o'clock P.m. Completing his journey around the globe, he will have gained, in this manner, twenty-four hours. From this it wilt be seen that in order to obtain the correct date twenty-four hours must be subtracted from his time. On the other hand, if a person could travel eastward at the same speed with which the sun apparently travels westward (the same rate of speed with which the earth revolves on its axis), if he should start on his journey at noon-day, lie would meet the sun when exactly on the opposite side of the earth from the place of starting, and continuing the journey would again meet the sun at the place of starting, thus seeing three noon-days within the twenty-four hours, or apparently gaining a day. This we know to be
impossible, since only twenty-four hours of time have passed; while in reality an extra period of light has been gained, and thus to obtain the correct local date a clay must be added to your time.
Front this we see that, for every time a person travels around the earth in either direction. there is a difference in time of one day, and the result is the same regardless of the rate of speed. To avoid the confusion of dates which must necessarily result from this constant gain on one side and loss on the other, it has been proposed to determine upon some line at which eastern bound 'travelers shall add a day, and westward bound travelers shall drop a day from their reckoning, and thus prevent a disagreement in regard to the day of the week. The line at which this addition or subtraction shall be made is what is meant by the dateline.
The fact and necessity of such a.date-line may be shown in a way with which all are familiar. Take a simple problem in arithmetic on "longitude and time." " When it is 9 o'clock A.m., May 1, at Singapore, long. 104' c., what time is it at Manila, long. 121° 30' c.?" The difference of longitude estimated from Singapore is 17° 30. The application of the ordinary rules of arithmetic gives for an answer, 1() h. 10 m. May 1. But the difference of longitude estimated westicard from Singapore is 342' 30', giving for an answer 10:10 A.M., April 30. This shows that when the time of clay at one place is known, and the longitudes of hot]] places known, the time of day at the other may be obtained in two ways, viz.: by using the difference of longitude estimated west, or estimated east. But the dates thus obtained differ by one day; which is correct? Some times the one and sometimes the other. In the problem just considered the latter result is correct. In such problems the difference of longitude must be taken in such a direc tion as not to conic across the date line; or if the date-line be crossed, the dates must be chawred in accordance with the above definition.