Determining direction

Eastern Hemisphere

Western Hemisphere

Use a globe or a map in your atlas and make sure you understand what each of these facts means.

• The Greenwich meridian (0° line of longitude) forms a half-circle on a globe. If it is continued around the back of the globe, it forms another half-circle. The half-circle at the back is called the antipode of the 0° line of longitude and its number is 180°. A line of longitude and its antipode always form a full circle. Therefore, from the 0° line of longitude, one can move 180° to the west and 180° to the east.

In this way, each line of longitude has an antipode and if the numbers of the two lines’ grades are added together, the answer is always 180°.

But now we have a problem: If we move eastward from the 0° line of longitude and westward from the 0° line of longitude, we reach the 180° line of longitude from both sides. Now our times are going to differ a lot!

• To solve this problem, the International Date Line (IDL) was developed.

It follows the 180° line of longitude, but never cuts across land.

The date east of the IDL is one day earlier;west of the IDL is one day later.

From east to west, you lose a day if you move across the IDL.From west to east, you win a day if you cross the IDL.

• A place’s longitude is also an indication of time .

The earth revolves from west to east around its own imaginary axis. So the sun rises earlier in places that are nearer to the east.

The sun rises in Durban (31°E) approximately 50 minutes earlier than in Cape Town (18½° E). In South Africa such a difference isn’t a problem, because we use only one time zone (30° E). In a country like the USA, however, it becomes a problem. The sun rises in New York (74°W) approximately three hours earlier than in San Francisco (122½°W). So in the USA you need to have different time zones.

• You may have noticed that when rugby matches are played in Australia or New Zealand and we watch them on TV, it’s very early in the morning over here. This is the case because those countries lie very far east of South Africa and so the sun rises “earlier” over there.
• Calculating the time difference isn’t so difficult. It all has to do with the way in which the earth revolves around its own axis.
• How long does it take for the earth to revolve around its own axis?It takes 24 hours.
• Through how many degrees does the earth revolve in these 24 hours? The earth revolves through 360 ° .
• If we now divide the 360° by 24 hours, we find that the earth revolves through an angle of 15° every hour. If two places are 15 degrees of longitude from one another, the sun will rise precisely one hour earlier at the one lying further east.

Assessment