Time zones of the world. Image credit icsm.gov.au

### ZIMSEC O Level Geography Notes: Transport:Longitude,time and the International Date Line

• Travelers witness a change in time as they fly for long distances.
• The time difference in different countries is caused by the rotation of the Earth.
• This refers to the earth spinning on its axis.
• The Earth rotates always from its west to east, one half of it facing the sun while the other half is in darkness.
• This causes day and night.
• As a result of this rotation, the sun always rises from the east and sets in the west.
• Hence, sunrise always occurs first in places to your east and the same applies for sunset.
• The Earth is a spherical in shape,
• one complete rotation of the Earth is makes up 360°, a full cycle, which is covered in 24 hours causing day and night.
• Therefore, to find the degrees covered by the rotation of the earth we simply divide 360 by 24 to get 15.
• This means that a place 15° east of another is always 1 hour ahead in time and that which is farther with 30° is 2 hours ahead.
• Similarly, because the earth rotates from the west to the east always, places to the west are always behind those to the east by 1 hour for each 15°.
• Lines of longitude are used to calculate local times at different points on the earth’s surface.
• Longitude is the angular distance east or west of the main or prime longitude line or meridian whose value is 0°.
• This prime meridian passes over the observatory of Greenwich outside London and is therefore often called the Greenwich meridian.
• Lines of longitude are measured and numbered from 0° at the Greenwich Meridian eastwards and westwards.
• All these meridians meet and cross at the North and South Poles.
• If we know the longitude of a place, we can calculate its local time using that of another place whose time is known.
• There’s one common line that is both 180° E and 180° W and is called the International Date Line.
• Time changes occur as we cross this line either from west to east or from east to west.

NB Sometimes time at a place is influenced by political considerations so even though a place A might have the same longitude as place B in another country they might be in different time zones.

To access more topics go to the O Level Geography Notes page