The Earth’s Rotation – SS1 Geography Lesson Note (First Term)

The rotation of the earth is the movement of the earth on its axis from west to east. The earth turns through 360° in 24 hours, which makes one day. It rotates through 15° in one hour, or 1° in four minutes. Its effects include day and night and differences in local time.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe the shape and size of the earth;
• give proofs that the earth is spherical;
• explain what the rotation of the earth means;
• state the speed of the earth’s rotation in degrees per hour;
• explain the effects of the earth’s rotation.

What are the shape and size of the earth?

What is the shape of the earth?

The earth has a spherical shape. Its shape can also be described as a geoid.

Though we walk on what seems a flat surface, the earth is almost a sphere and is slightly flattened at the two poles. There are many facts that prove that the earth is spherical.

What is the size of the earth?

The earth is the fifth largest planet in the solar system.

• Surface area: approximately 510 million square kilometres (197 million square miles).
• Polar diameter: about 12,722 km.
• Equatorial diameter: about 12,762 km.
• Circumference at the equator: about 40,085 km.
• Polar circumference: 39,955 km.
• Mean density: estimated at 5.5 grams per cubic centimetre.

What are the proofs that the earth is spherical?

1. Circumnavigation of the earth: The expedition led by Ferdinand Magellan sailed around the world between 1519 and 1522 and returned to its starting point. Several other people have done so since then, which confirms that the earth is spherical. If the earth were flat, they would have met an abrupt edge and fallen off.
2. Sunrise and sunset: Different parts of the world experience sunrise and sunset at different times. If the earth were flat, the sun would rise and set at the same time for all places. The earth rotates from west to east, so places in the east see the sun before places in the west. The sun is said to rise in the east and set in the west.
3. Aerial photographs: Photographs of the earth taken from high altitudes by rockets show that the earth is spherical. This is the most recent proof of the sphericity of the earth.
4. The lunar eclipse: During a lunar eclipse, the earth casts a circular shadow on the moon. Only a sphere, like the earth, can cast such a circular shadow.
5. Ships’ visibility: When a ship approaches a port, the top of the mast is seen first, then the hull, and later the rest of the ship’s body. In the same way, a ship leaving a harbour disappears gradually. If the earth were flat, a ship viewed from a distance would appear and disappear at once.
6. Shape of other planets and planetary bodies: The sun, moon, stars and other planetary bodies are all circular in outline when viewed from any angle. So the earth cannot be an exception.
7. Experimental proof (engineers’ surveys, driving poles of equal length): Three poles of equal length were driven to the same depth in level ground. The centre pole was found to project slightly above the poles on either side because of the curvature of the earth. If the earth were flat, all the poles would have been at the same height.

How does the earth move?

The earth on which we stand is constantly in motion. It revolves around the sun and turns its different sides to the sun at different times. When the sun emerges, we say the sun is rising, and when the sun recedes, we say the sun is setting.

The earth’s movement can be grouped into two: the rotation of the earth and the revolution of the earth.

What is the rotation of the earth?

The rotation of the earth is the movement of the earth on its axis. By turning on its axis from west to east, the earth makes a complete rotation, that is, it rotates through 360° in every 24 hours, which makes a day.

The earth rotates through 15° in 1 hour, or through 1° in 4 minutes.

What are the effects of the earth’s rotation?

1. Day and night: As the earth rotates, only the part of the earth’s surface facing the sun receives the sun’s rays and experiences day. The part backing the sun experiences darkness (night).
2. Time differences from place to place: The earth rotates from west to east, so local time differs between places. For every 15° we go eastward, the local time is ahead by 1 hour. For every 15° we go westward, the local time is behind by 1 hour.
3. Apparent sunrise and sunset: During the rotation of the earth, the part that emerges from darkness into the sun’s rays experiences sunrise, while the part moving away from the sun’s rays experiences sunset. This causes the apparent sunrise and sunset.
4. Deflection of wind and ocean currents: The earth’s rotation causes freely moving objects, such as wind and ocean currents, to be deflected. They are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
5. Daily rising and falling of the tide: This is the rising and falling of the level of water in seas and oceans. It takes place twice every day.

Evaluation

1. State three proofs that the earth is spherical.
2. Through how many degrees does the earth rotate in one hour?
3. Explain how the earth’s rotation causes day and night.
4. In which direction are winds deflected in the northern hemisphere?

Assignment

1. Explain the term earth rotation.
2. Mention three effects of the earth’s rotation.
3. The earth rotates on its own ______.