- The earth makes two different movements;

1. Rotation
2. Revolution

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Rotation

Rotation of the earth is the movement of the earth on its own axis.

• The earth spins from West to East.
• The Earth takes 24 hours to make one complete spin or movement.
• The earth is tilted at an angle of 23 1/2 º

Effects of Rotation of the Earth on Time

Rotation of the earth affects time. Rotation of the earth causes day and night. It also causes tides.

Time is worked out from the Greenwich Meridian. The time at Greenwich Meridian is known as the Greenwich Mean Time (GMT).

Places to the East of the Greenwich Meridian are ahead in time while those to the West are behind in time. This is because the Earth rotates from West to East. Rotation of the earth causes day and night.

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How time is Calculated

It takes the earth one day to make a complete rotation on its own axis. One day has 24 hours. That is 12 hours of day and 12 hours of night.

If the earth takes 24 hours to rotate through 360º, how long will it take to rotate through 1º of longitude?

This can simply be expressed in the following way;

1. To rotate through 360º, the earth takes 24 hours.
2. Therefore to rotate through 1º of longitude, the earth will take; (24 hrs × 60 minutes) ÷ 360º = 4 minutes If the earth takes 4 minutes to rotate through 1o of longitude, it means that it will take (4 × 15) 60 minutes or 1 hour to rotate through 15º of longitude.

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Example 1

a) If the time at Greenwich is 12 noon, what will be the time at Town A which is along Longitude 30º East?

• 12.00 noon + (4 x 30) 120 minutes or 2 hrs
• 12.00 noon + 2 = 14.00 hrs or 2.00 pm

Because Town A is to the East of the Greenwich Meridian, its time will be ahead of that at the Greenwich Meridian.

Example 2

a) What is the time at Town B located along longitude 30º W, if the time at Greenwich Meridian is 12.00 noon?

• The time at point B is 12.00 – 2.00 = 10.00 a.m.

Because Town B is to the West of Greenwich Meridian, its time will be behind that at the Greenwich Meridian.

Revolution

Revolution of the earth is the movement of the sun around the earth.

The earth follows a specific path as it goes around the sun. This path is known as an orbit. The earth takes 365 1/4 days or one year to make a complete revolution round the sun.

However, only 365 days are recorded. The 1/4 day is added up to make a full day every fourth year. This year has 366 days. It is called a leap year. During a leap year, the month of February has 29 days.

The sun is overhead at the Tropic of Cancer on 21st June. The sun is overhead the Tropic of Capricorn in the Southern Hemisphere on 21st December.

The area around the Equator is always hot. However, the sun is overhead at the Equator on 21st March and 22nd September. This is when the highest temperatures are recorded along the equator.

This image shows the Revolution of the Earth

Effect of Revolution on the Earth

• It causes seasons.

• It causes aphelion and perihelion. Aphelion is when the earth is far from the sun. Perihelion is when the earth is near the sun.
• It causes different length in days and nights. During revolution, the length of days and night are equal at the equator. This is called equinox. In Northern and Southern hemisphere, length of days and nights vary. This is called solstices.