THE EARTH’S ROTATION AND REVOLUTION
PHYSICAL GEOGRAPHY

THE EARTH’S ROTATION AND REVOLUTION

The earth moves in space in two different ways. These are:

  • Rotation
  • Revolution

 

ROTATION OF THE EARTH

Rotation of the earth is defined as the movement of the earth round the sun on its axis.

  • The earth moves or rotates on its axis from west to east.
  • The rotating earth is inclined at an angle of 23 1/2°along its axis.
  • It takes the earth twenty-four hours (i.e. a day) to complete one rotation (i.e. the earth rotates through 360° in every 24 hours).
  • The rotation of the earth causes day and night.
  • This is why it is said that there are twenty-four hours in a day. That is, twelve hours of daylight and twelve hours of darkness. However, this division does not occur at all times in all places.
  • It rotates through 15° in 1 hour or 1° in 4 minutes.

 

EFFECTS OF ROTATION OF THE EARTH

  1. Day and Night: As the earth rotates, only one part of the earth’s surface facing the sun receives the rays of the sun and experiences daylight (day) while the other part of the earth backing the sun’s rays will experience darkness (night).
  2. Deflection of winds and ocean current: The rotation of the earth causes winds and ocean current to be deflected to the right of the direction in which they move in the Northern hemisphere and, to the left of this direction in the Southern hemisphere.
  3. Apparent sunrise and sunset: During rotation of the earth, part of the earth that emerges from the darkness into the rays of the sun experiences sunrise while the part that is moving away from the sun’s rays to darkness will experience sunset.
  4. Time differences from place to place: The earth rotates through 360° in every 24 hours which means that for every one hour, it passes through 15°. Since the earth moves from west to east, it means that for every 15°, the east is always one hour ahead of time while the west is always one hour behind time.
  5. Daily rising and falling of the tides: During rotation of the earth, it results in the rising and falling in the level of water in the seas and oceans. Owing to rotation of the earth, the tide producing forces cause the surface of water to oscillate, resulting in the rising and falling in the level of water in the seas and oceans.
  6. Dawn and Twilight: Dawn refers to the brief period between sunrise and full daylight while twilight refers to the brief period between sunset and complete darkness. Dawn and twilight which are the results of rotation caused by the earth receive diffused or reflected light from the sun while it is still below the horizon.

 

REVOLUTION OF THE EARTH

Revolution of the earth is defined as the movement of the earth round the sun on its orbit.

  • The revolving earth is tilted or inclined at an angle of 66 1/2° to the plane of ecliptic during its revolution.
  • It takes the earth 365 1/4 days to complete one revolution (i.e. moves round the sun).
  • The revolution of the earth causes four seasons in year.
  • The day or 6 hours is added up every four years to make an extra day in a leap year. A leap year has 366 days while a normal year has 365 days.

 

EFFECTS OF REVOLUTION OF THE EARTH

  1. It determines a year: The time taken by the earth to move round the sun (i.e. complete its orbit) is fixed at 365 1/4 days which make up a year.
  2. Varying the length of day and night at different times of the year: The length of day and night varies depending on the position of the earth in relation to the sun. In the Northern hemisphere, during the winter around December 22, as we go northwards, the hours of darkness increases. At the Arctic Circle (66 1/2°N), the sun never rises and there is darkness for the whole day (December, 22). But in the Southern hemisphere, the period of day light increases towards the South Pole on December 22, and the region experiences summer. In the Northern hemisphere, during summer around June 21, the periods or hours of daylight increase as we go towards the North Pole. At the arctic circle (66 1/2°N), the sun never sets by June 21, and there is a complete twenty-four hour period of continuous daylight and this region (arctic circle) at this time is called The Land of the midnight sun. The sun is overhead at the Tropic of Cancer and no part is in darkness.
  3. It causes seasons: In tropical areas like West Africa, including Nigeria, there are two seasons – rainy season and the dry season. But in temperate countries, the revolution of the earth causes four seasons – winter, spring, autumn and summer.
  4. Changes in the altitude of the mid-day sun: As a result of the earth’s revolution, the altitude (height) of the mid-day sun changes, resulting in Equinoxes and Solstices.

 

Solstices

  • This is the time when the sun is vertically over-head at the Tropic of Cancer or at the Tropic of Capricorn.
  • The solstices occur on December 22 and June 21.
  • The solstices mean that the period of day and night is not equal. On June 21, the sun is over-head at the Tropic of Cancer, resulting in June and Summer solstice. This means that the northern hemisphere will have its longest day and shortest night. On December 22, the sun is over-head at the Tropic of Capricorn, resulting in December winter solstice which means that the Southern hemisphere will have its longest day and shortest night. The Tropic of Cancer and Tropic of Capricorn mark the limit of the overhead sun. Outside these two points, the sun is never over-head at any time of the year.

 

Equinoxes

  • This is the time when the mid-day sun is directly overhead at the equator.
  • The equinoxes occur on March 21 and September 23.
  • The equinoxes mean there is equal day and equal night i.e. on March 21 and September 23.
  • There are twelve hours of daylight and twelve hours of darkness.
  • March 21 is also called Spring Equinox and September 23 is called Autumn Equinox.

 

  1. Changes in seasonal temperature: It is observed that summers are usually warm and bright, while winters are cold and dark in the arctic region. The reasons include:
  • During summer, the zenith angle of the sun is higher.
  • The concentration of heat over a small area leads to high temperatures.
  • In winter, the oblique rays of the sun fall faintly over a large area.
  • The little heat makes the temperature to remain low.
  • There are twenty hours of daylight in summer solstice and twenty hours of darkness in winter.
  • Days are longer than nights in summer while in winter, nights are longer.

 

REASONS WHY ALL AREAS NORTH OF THE ARCTIC CIRCLE HAVE 24 HOURS OF DAYLIGHT ON JUNE 21

On June 21,

  • The sun is over-head at the Tropic of Cancer.
  • In Arctic Circle, the sun is seen everywhere at the same time.
  • The earth’s axis is inclined to the elliptical plane at an angle 66 1/2°.
  • No part is in darkness.

 

REASONS WHY AREAS WITHIN THE TROPICS HAVE MORE SUNSHINE THAN AREAS OUTSIDE THE TROPICS

There is more sunshine in tropics because:

  • Apparent movement of the sun is always from one tropic to the other and back.
  • The sun is always over-head within the tropics.
  • The sun is never over-head outside the tropics.
  • Within the tropics, the angle of incidence is vertical while it is slanting pole wards.
  • There is more insolation in the tropics than outside the tropics.
  • Difference in length of day and night is hardly noticeable within the tropics.

 

ECLIPSE

Eclipse is formed when the three bodies – sun, earth and the moon are in straight line during the movement of the earth. The moon revolves around the earth once in every twenty-eight days (a month), the earth and the moon travel together, making a complete movement around the sun once a year. During this movement, there will be a time when these three bodies (earth, sun and moon) will be in straight line. When this occurs, the light from the sun is obscured from reaching either the earth or the moon. This phenomenon is known as an Eclipse.

 

TYPES OF ECLIPSE

There are two types of eclipse. These are;

  1. Eclipse of the Moon (Lunar eclipse): This occurs when the earth comes in-between the moon and the sun. The earth, being larger than the moon can completely obscure or cover the sun’s light from reaching the moon. This condition is known as eclipse of the moon or lunar eclipse. This type of eclipse can lead to full darkness of the moon described as Total or annular type of eclipse.
  2. Eclipse of the Sun (Solar eclipse): This occurs when the moon comes in-between the sun and the earth, in a perfect straight line. The moon is capable of blocking the light from the sun. The moon then casts its shadow of darkness on the earth. This condition is known as the eclipse of the sun or solar eclipse. This type of eclipse can lead to partial darkness of the earth called partial eclipse.

 

  • Eclipse is described as total eclipse if the body, e.g. moon is completely covered or shaded from the sun by the earth.
  • Eclipse is described as partial eclipse if the body, e.g. earth is half covered or shaded from the sun by the moon.
  • Eclipse is also described as annular eclipse if the body, e.g. earth is covered at the centre with darkness while the outer part is not.
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