Updated February 21, 2017 | Factmonster Staff
  • A day is measured by how long it takes the earth to rotate (turn) once: 24 hours.
  • The earth is divided into 24 time zones. It takes the sun an hour to cross each zone.
  • The time zones are marked by imaginary lines called meridians, which run north and south on the globe.
  • The prime meridian is at Greenwich, England. It is 0° longitude. From here, east and west longitude is calculated for the rest of the globe.
  • A new day begins at the international date line, which is halfway around the world from the prime meridian.
  • East of the international date line it is 1 day earlier than west of it. Remember, “Go ahead west to the new day, go back east to the old.”
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