What is International Time line and International Date Line?


  • GS Prelims, GS mains paper I
  • Geography, International Date Line, Longitude and Latitudes

Longitude and Latitude

  • The earth is divided into horizontal and vertical imaginary lines
  • The parallels of latitudes are horizontal, while the meridians of longitudes are vertical
  • There are 360 degrees of longitude because one circle covers the earth from both the sides covering the entire globe.
  • There are 180 degrees of latitudes.
  • One easy way to remember the orientation of latitudes and longitudes is longitudes are long, and latitudes are lateral.
  • The centre point of the meridian lies at the Greenwich Observatory England is known as Prime Meridian,making it Greenwich Mean Time (GMT). Greenwich is a city located in London (UK)
  • The International Date Lineis located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, the reference point of time zones. It is also known as the line of demarcation

Longitudes and Latitudes

How is time calculated through longitudes and latitudes?

  • When the earth is divided into longitudes and latitudes, it forms a grid.
  • With the help of these girds, the longitude and latitudes are divided into degrees and time to find an exact location.
  • Each degree can be broken into 60 minutes, and each minute can be further divided into 60 seconds.
  • To be precise, there are 60 minutes in one degree and each degree has a distance of 69 miles. Each minute is divided into 60 seconds.
  • As we go towards east from the International Date Line, we are reducing one day from our prior location in the west. In short, every location falling on the east of this line is one day earlier in time if compared to the locations in the west.
  • If we calculate by latitude, each minute is 1/60th of a degree, i.e. approximately one mile.
  • The equator lies at 0 degrees.

Degrees of latitude

  • The distance between the degrees of latitude is somewhat same. However, the earth is slightly elliptical in shape which creates minute variations between the degrees as we go from the equator to the north and south poles.
  • Each degree of latitude is approximately 111 kilometres apart.
  • The distance at the equator is 110.567 kilometres.
  • The distance is 110.948 kilometres at the Tropic of Cancer and Tropic of Capricorn (23.5° north and south).

Degrees of Longitude

  • Unlike latitude, lines of longitudes are not equidistant each other. These may vary when they converge at the poles.
  • 0 degrees longitudes are at Prime Meridian
  • A degree of longitude is widest at the equator with a distance of 111.321 kilometres.

International Date Line

The Dateline Is Not Straight

  • The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere.
  • It is not straight but zigzags to avoid political and country borders and to not cut some countries in half.

What Happens When You Cross the Dateline?

  • When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.
  • Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours.
  • For example, if you travel the 1061 kilometers across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.

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