At the international date line, the west side of the line is always one day ahead of the East side, No matter what time of day it is when the line is crossed. About This Quiz & Worksheet. The two natural cycles on which time measurements are based are the year and the day. Because the Earth is moving around the Sun as it rotates on its axis, the sidereal day is about 4 minutes shorter than the solar day, being equivalent to 23 hours, 56 minutes, and 4 seconds in mean solar time. The Date Line. Infoplease is part of the FEN Learning family of educational and reference sites for parents, teachers and students. International Date Line, also called Date Line, imaginary line extending between the North Pole and the South Pole and arbitrarily demarcating each calendar day from the next. As we go towards east from the International Date Line, we are reducing one day from our prior location in the west. A day may be defined as the interval between the highest point of the Sun in the sky on two successive days. The International Date Line defines the boundary between calendar dates. But the movement of Earth around the Sun is not regular because Earth's orbit is an ellipse. Brush up on your geography and finally learn what countries are in Eastern Europe with our maps. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. The 180th meridian–commonly called the International Date Line–splits the island of Taveuni from today and yesterday. The Date Line makes a zigzag in the north to incorporate the eastern tip of Siberia into the Siberian time system and then another one to incorporate a number of islands into the Hawaii-Aleutian time zone. Twenty five time zones were established to the east and west of Greenwich with the International Date Line lying along the 180° line of longitude. It begins when the Sun is invisible by being 12 hours from its zenith. Infoplease is a reference and learning site, combining the contents of an encyclopedia, a dictionary, an atlas and several almanacs loaded with facts. Updates? The International Date Line, established in 1884, passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. Earth's surface was divided into 24 zones. Check our encyclopedia for a gloss on thousands of topics from biographies to the table of elements. The International Date Line does not exist! Paris. The International Date Line, established in 1884, passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. The Newark clock would be slightly ahead of a clock in Trenton, N.J., which, in turn, would be ahead of a clock in Philadelphia. T he International Date Line is in the middle of the time zone defined by the 180° meridian. Need a reference? Time zones. The standard time of each zone is the mean astronomical time of one of 24 meridians, 15 degrees apart, beginning at the Greenwich, England, meridian and extending east and west around the globe to the International Date Line. Our editors will review what you’ve submitted and determine whether to revise the article. Astronomers use the so-called Julian Day, which runs from noon to noon; the concept was invented by the astronomer Joseph Scaliger, who named it after his father, Julius. The mean sun of time measurements, then, is a sun that moves regularly all year round; the real Sun will be either ahead of or behind the mean sun. Or, today and tomorrow, depending on what side of the line you’re standing on. To avoid the problems caused by leap-year days and so forth, Scaliger picked a conveniently remote date in the past (4713 B.C.) This informative animation shows how the IDL works. It’s drawn between those areas whose legal times are about 24 hours apart [sup]1 [/sup]. 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