- 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.”