The Date Line

If a person were to start at Greenwich at the instant of noon and travel westward at the rate of about 600 miles per hour, i.e., rapidly enough to keep the sun always on his own meridian, he would arrive at Greenwich 24 hours later, but his own (local) time would not have changed at all; it would have remained noon all the time. His date would therefore not agree with that kept at Greenwich but would be a day behind it. When travelling westward at a slower rate the same thing happens except that it takes place in a longer interval of time. The traveller has to set his watch back a little every day in prder to keep it regulated to the meridian at which his noon occurs. As a consequence, after he has circumnavigated the globe, his watch has recorded one day less than it has actually run, and his calendar is one day behind that of a person who remained at Greenwich. If the traxeller goes east he has to set his watch ahead every day, and, after circumnavigating the globe, his calendar is one day ahead of what it should be. In order to avoid these discrepancies in dates it has been agreed to change the date when crossing the i8o° meridian from Greenwich. Whenever a ship crosses the i8o° meridian, going westward, a day is omitted from the calendar; when going eastward, a day is repeated. As a matter of practice the change is made at the midnight occurring nearest the i8o° meridian. For example, a steamer leaving Yokohama July 16th at noon passed the i8o° meridian about 4 p.m. of the 2 2d. At midnight, when the date was to be changed, the calendar was set back one day. Her log therefore shows two days dated Monday, July 22. She arrived at San Francisco on Aug. 1 at noon, having taken 17 days for the trip.

The international date line actually used does not follow the 1800 meridian in all places, but deviates so as to avoid separating the Aleutian Islands, and in the South Pacific Ocean it passes east of several groups of islands so as not to change the date formerly used in these islands.

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