EARTH’S MOVEMENTS

WEEK 4                                                                                        DATE………………

SUBJECT: GEOGRAPHY

CLASS: SS 1   

TOPIC: EARTH’S MOVEMENTS

CONTENT: 

1.   Meaning and effects of rotation of the earth.
2. Meaning and effects of revolution of the earth.
3. Differences between rotation and revolution of the earth. 

EARTH’S MOVEMENT

The earth moves in space in two different ways. They are (a) Rotation (b) Revolution.

SUB-TOPIC 1: Meaning and effects of rotation of the earth   

Meaning of Rotation of the Earth

The earth rotates on its axis from west to east in every 24hours through 3600. This is why it is said that there are twenty four hours (24) in a day. It rotates through 150 in 1hour or 10 in 4 minutes. The rotating earth is inclined at an angle of 23½0 along it axis.

EFFECT OF ROTATION OF THE EARTH

1. DAY AND NIGHT: As the earth rotates from west to east only one part of the earth facing the sun experiences daylight while other part of the earth backing the sun’s ray will experience darkness (night).
2. DEFLECTION OF WIND AND OCEAN CURRENT: The rotation of the earth causes the deflection of winds and ocean current to the right in the northern hemisphere and to the left in the southern hemisphere. This principle was based on the fact that during the earth’s rotation, the equatorial region moves at a greater velocity or speed than any other higher latitudes because the equator is longer in circumference. The man who established this principle is called Ferrell and at times it is referred to as Ferrell’s law of deflection.     
3. APPARENT SUNRISE AND SUNSET: During the earth’s rotation, the part of the earth that emerges from the darkness into the rays of the sun experience sunrise while the other part going away from the sun’s rays to darkness will experience sunset.
4. TIME DIFFERENCE FROM PLACE TO PLACE: There is a difference of one hour at every 150 apart. To the west an hour is lost and to the east an hour is gained.
5. DAILY RISING AND FALLING OF THE TIDES: The rotation of the earth causes the rising and falling in the level of water in the sea and oceans.
6. IT CAUSES DAWN AND TWILIGHT: Dawn is a brief period between the sunrise and complete daylight while twilight is the brief period between sunset and complete darkness.

EVALUATION:

1. Explain the rotation of the earth.
2. State five effects of rotation of the earth.

SUB-TOPIC 2: Meaning and effects of revolution of the earth

Meaning of Revolution of the Earth

The earth revolves around the sun on its orbit. It takes 365¼ days to complete one revolution. The ¼ day or 6hours is added up every four years to make an extra day in a leap year. A leap year has 366days while a normal year has 365days. Leap years are all divisible by 4.

The revolving earth is tilted or inclined at an angle of 66½ 0 to the plane of ecliptic orbit during its revolution.

EFFECTS OF REVOLUTION OF THE EARTH

1. IT DETERMINES A YEAR. The time taken by the earth to move round the sun is fixed at 365 ¼ days which make up a year.
2. VARYING LENGTH OF DAY AND NIGHT: The length of day and night varies depending on the position of the earth. During winter, in the northern hemisphere, around December 22, there is increase in the number of dark hour towards the northern pole. Within the Arctic Circle 66 ½ 0N, there is a complete absence of sunrise, hence a whole day of darkness is experienced. While in the southern hemisphere, the period of day light increases towards the South Pole on December 22 but the region experiences summer.

In the summer period around June 21st, the northern hemisphere experiences longer daylight towards the poles while the Arctic Circle 66½oN the sun never sets, hence they experience 24hours of continuous daylight. This region (Arctic Circle) at this time is called ‘Land of mid-night sun’. The sun is overhead at the tropic of cancer and no part is in darkness.

In the southern hemisphere, the period of darkness increases towards the South Pole on June 21st, but the region experience a winter.

Position of the Earth in Relation to the Sun

1.  IT CAUSES SEASONS: The tropical areas like West Africa have two seasons- rainy and dry seasons. But the temperate region countries experience four seasons- Spring, Summer, Autumn and Winter.  During spring, March 21st, the sun will be overhead at the equator giving all places in the northern hemisphere spring equinox. This is the period when plants begin to grow. The summer period occurs in the northern hemisphere between late May till late July. On June, 21st, the sun is overhead at the tropics of cancer resulting in summer solstice. This is when the northern hemisphere will have its longest day and shortest night. September 23rd is the period referred to as Autumn Equinox. In the northern hemisphere, leaves fall off the trees and their colour turn yellow-brown from September to October. December 22nd is the period referred to as a winter. This is the period when weather is very cold in the northern hemisphere because of the generally decreased sunshine. It is also known as winter solstice. Summarily, summer is a hot season; winter is a cold season, while spring and autumn are warm or cool.

The Four Seasons of the Earth

1. CHANGES IN THE ALTITUDE OF THE MID-DAY SUN: The revolution of the earth cause a change of the mid-day sun resulting in equinoxes and solstices.

1. EQUINOXES: Equinox means equal day and night. It is the time when the mid-day sun is directly overhead at the equator. This occurs on March 21st and September 23rd.
2. SOLSTICES: This means that the period of day and night is not equal. It is the time when the sun is vertically overhead at the tropic of cancer. This normally occurs on December 22nd and June 21st.

1. CHANGES IN SEASONAL TEMPERATURE: There is a warm and bright summer while winters are cold and dark in the arctic region

EVALUATION:

1. Describe the revolution of the earth.
2. Explain four effects of the revolution of the earth.

SUB-TOPIC 3: DIFFERENCES BETWEEN ROTATION AND REVOLUTION OF THE EARTH

1. During rotation the earth rotates on its axis while during revolution it revolves round the sun along its orbit.
2. The earth rotates through 3600 in 24 hours (one day) whereas it revolves round the sun in 365¼ days (one year).
3. The rotating earth is inclined at an angle of 23½0 along its axis while the revolving earth is tilted or inclined at an angle of 66½ 0 to the plane of ecliptic.
4. Rotation causes day and night whereas revolution causes seasons.
5. Rotation causes the difference of one hour between two meridians – 150 apart whereas revolution causes changes in the altitude of the mid-day sun.

EVALUATION:

1. Differentiate between rotation and revolution of the earth.

GENERAL EVALUATION

1. (a) Describe the effects of rotation of the earth.

(b) Distinguish between rotation and revolution of the earth.

(c) With the aid of a suitable diagram, describe an eclipse of the sun. (SSCE August, 1991)

1. Give reasons why: (i) all areas north of the Arctic Circle have 24hours of day light on June 21st. (ii) Area within the tropics have more sunshine than areas outside the tropics. (SSCE NOV.,1994)
2. (a) With the aid of diagram, describe the four positions of the earth during its revolution around the sun.

(b) Explain why summers are usually warm and bright while winters are cold and dark in the Arctic region. (SSCE NOV., 1994)

1. Write explanatory notes on each of the following: (a) Twilight (b) Solstices (c) Equinoxes

WEEKEND ASSIGNMENT:

Objective Test. Choose the correct answer from letter a-d

1. The rotation of the earth causes (A) changes in the altitude of the mid-day sun (B) day and night (C) variations in the length of day and night (D) differences in the length of one year.
2. Which of the following conditions occurs during equinox? (A) The tropic of Cancer experiences longer day than the tropic of Capricorn (B) All places across the globe experience equal lengths of day and nights (C) The length of day at the Equator is shorter than the tropic of Cancer (D) The moon is star-shaped all through the night all over the world. 
3. When the sun is directly overhead at the tropic of Capricorn (23½0S) on 22nd December, the Northern Hemisphere experiences its (A) twilight (B) winter solstice (C) summer solstice (D) spring equinox (E) autumn equinox.
4. In the southern hemisphere, the autumnal equinox occurs on (A) 21st March (B) 21st June (C) 23rd September      (D) 22nd December (E) 4th January.
5. Which of the following is a leap year?  (a) 1978 (b) 2001 (c) 1988 (d)1990

ESSAY

1. Explain the following: (a) Twilight (b) Dawn (c) Eclipse.
2. With the aid of diagram describe (i) Eclipse of the Sun (ii) Eclipse of the Moon.