ICSE Class 6 Geography Chapter 3 Rotation and Revolution

Chapter Three: Rotation And Revolution

Syllabus

Rotation and Revolution: Rotation - direction, speed and its effects occurrences of day and night, the sun rising in the east and setting in the west, Coriolis effect

Revolution of the earth and its inclined axis - effects - the variation in the length of the day and night and seasonal changes with Equinoxes and Solstices.

The earth, like all other planets in the Solar System revolves round the sun in an elliptical orbit and it also rotates on its axis. These two motions of the earth are known as revolution and rotation, respectively. Many phenomena on earth like day and night, seasons, winds and ocean currents depend on these motions.

Rotation Of The Earth

Rotation is the spinning of the earth on its axis from west to east once in 24 hours. The axis is the imaginary line passing through the centre of the earth as it does on the globe. The two end points through which the axis passes are known as the North Pole and the South Pole. The axis is tilted at an angle of 23\(\frac{1}{2}\) degrees from the vertical. This means that if a line were drawn parallel to the plane of the earth's orbit, the axis would make an angle of 66\(\frac{1}{2}\) degrees to this line as shown in Fig. 3.1.

Characteristics Of Rotation

1. Direction: The earth completes one rotation from west to east on its axis in 23 hours, 56 minutes, 4.09 seconds (roughly in 24 hours).

2. Inclination of Axis: The earth is also tilted in the same direction (towards the Pole Star) as it moves round the sun. The angle at which the earth is tilted is known as the inclination of the earth's axis.

3. Speed: The speed of earth's rotation at the Equator is 1670 km per hour (circumference of the earth is 40,075 km, which is covered in 24 hours). The speed decreases towards the poles.

Effects Of The Rotation Of The Earth

1. Occurrence of Day and Night are caused by the rotation of the earth on its axis. The sun rises in the east and the sun sets in the west is because parts of the earth which face the sun have day and the parts which do not face the sun have night. This happens with precision and progression and not suddenly. The time when the sun begins to cast its light in the sky is known as dawn. At noon, the sun is overhead. At dusk, it is twilight and the sun is seen disappearing in the sky. At night, it is completely dark. Duration of the day and night is not equal at all places on the earth because of the inclined axis of the earth. The length of days varies with respect to the seasons as well as latitude.

2. Sunrise and Sunset are caused by the rotation of the earth from west to east. This happens because due to the earth's rotation, every part of the earth's surface faces the sun at some time or the other. The part of the earth that moves from darkness into the sun's rays, experiences sunrise and when it moves away from the sun's rays, it experiences sunset.

3. Coriolis Effect refers to the force experienced by any object moving over the surface of a rotating body such as the earth. The Coriolis Effect plays an important role in the movement of air (winds) and water (ocean currents) on the earth. Due to its effect, the winds and ocean currents move clockwise and are deflected to the right in the Northern Hemisphere and move in the anticlockwise direction are deflected to the left in the Southern Hemisphere.

4. The sun, the moon and the stars seem to move from east to west. This is because the earth spins from west to east. This effect is similar to what one experiences while looking at trees from a moving train.

5. Centrifugal force is an apparent outward force that acts on a rotating object. It is always directed outward, i.e., away from the axis of rotation. The speed of rotation has created a centrifugal force resulting in a bulge in the middle portion of the earth and flattened top at the poles.

6. Rotation causes difference in time over various places on the earth. The difference in time is 4 minutes for each degree of longitude [24 hours (1440 minutes) divided by 360'].

7. The side of the earth towards the sun constantly gains heat and the side away from the sun constantly loses heat by radiating it into outer space. In this respect rotation of the earth in a tilted position on its axis has the following advantages:

(i) The short rotational period makes the variation of temperature between day and night moderate.

(ii) Rotation in combination with Revolution (earth's movement round the sun) causes seasons and variations in the lengths of day and night over different places on earth.

Inclination Of Earth's Axis And Its Significance

As we have said rotation is the spinning of the earth on its axis from west to east. The axis is the imaginary line passing through the centre of the earth on which it rotates. The earth's axis is not vertical. It makes an angle of 23\(\frac{1}{2}\) degrees with the vertical or 66\(\frac{1}{2}\) degrees with the plane of the Earth's orbit.

Let us imagine that the earth had a vertical axis. Then at all places on the earth, the length of day and night would have been equal as shown in the Fig. 3.2.

(i) Place L at the Equator would be at position L' after 12 hours.

Distance LO = Distance OL'

Therefore, Length of day = Length of night.

(ii) Similarly at all places on the earth, the length of day and night would be equal if the axis of the earth was not inclined.

The effect of the inclined axis on the duration of day and night is that their duration differs as shown in the Fig. 3.3.

(i) At the Equator place C would be at position C' after 12 hours.

Distance CO = Distance OC'

Therefore, Length of day = Length of night.

(ii) On the Tropic of Cancer place B will be at B' after 12 hours.

But distance BX is greater than B'X.

Therefore, Days are longer than nights.

(iii) On the Arctic Circle place A will be at position A' after 12 hours. AA' remains in continuous light in summer for six months.

Therefore, Day of 24 hours near the North Pole in summer.

(iv) Place D on the Tropic of Capricorn in the Southern Hemisphere will be at position D' after 12 hours.

Distance DY (in light) is less than the distance D'Y (in darkness)

Therefore, Days are shorter than nights.

(v) Place E on the Antarctic Circle remains in continuous darkness in winter.

Therefore, Night for 24 hours near the South Pole in winter for six months.

Effect Of The Inclined Axis On The Duration Of Day And Night

- The duration of day and night is not equal at all places.

- The time of the day at any place is determined by the height of the sun in the sky.

- If the axis of the earth was vertical instead of inclined, places on earth would have the same temperature throughout the year.

- If the earth did not revolve around the sun even then with its inclined axis there would be no seasons.

Effect Of The Inclined Axis On Seasons

The path of the earth around the sun is known as elliptic. It passes through the centre of both the earth and the sun. The earth's axis is not perpendicular to the plane of the elliptic, but is tilted at an angle of 66\(\frac{1}{2}\) degrees to it. The axis is, therefore, inclined at an angle of 23\(\frac{1}{2}\) degrees from the perpendicular to this plane. As a result of this, in the course of the earth's movement around the sun, the North Pole remains inclined towards the sun for a part of the year while the South Pole is tilted away from it. For the remaining part of the year, the situation is reversed, the North Pole being tilted away from the sun and the South Pole being inclined towards it. On earth this gives rise to the changing seasons and the varying lengths of day and night. If the earth's axis were perpendicular to the plane of orbit, there would be equal nights and days at all places on the earth, at all times of the year and there would also be no seasons.

Why We Do Not Feel The Motions Of The Earth?

Rotation and revolution are the two chief motions of the earth. If we were to feel these motions it would make life uncomfortable. The force of gravity keeps everything attached to the earth. This keeps all objects at rest with reference to the motions of the earth. Using this principle and Newton's First Law of Motion (which states that a body remains at rest or uniform motion unless compelled to change that state by an external force acting on it), scientists have proved that the earth rotates on its axis.

Effects Of Revolution

1. Seasonal Changes: The amount of heat that a place receives on earth depends on the angle at which the sun's rays reach the place. The place receives it when it receives vertical sun rays the place receives heat in a small area, whereas the same amount of heat is spread over a larger area, in the case of the oblique rays. We have already noted that the tilt of the earth's axis causes the seasons.

Revolution causes different seasons i.e., the changing weather conditions due to the heating of the earth. Revolution is an annual cycle; so, the seasons repeat year after year.

2. Inclination of axis and seasonal changes: Since the earth is tilted, the seasonal changes are not abrupt.

3. Perihelion and Aphelion positions: The earth's orbit is the path along which the earth travels around the sun.

This path is elliptical in shape. Due to this elliptical orbit of the earth, the distance between the earth and the sun varies. When the earth is closest to the sun the position is called perihelion and when it is farthest the position is called aphelion.

At aphelion the distance of the sun is 152 million km. While at its closest at perihelion the earth is 147.3 million km away from the sun.

Owing to the elliptical orbit, the minimum distance of the earth from the sun is reached on or about January 3rd every year. It is winter in the Northern Hemisphere when the axis is tilted away from the sun. While the tilt makes of the Northern Hemisphere cool in winter, its comparative nearness to the sun helps to make the Southern Hemisphere warm in summer.

On January 3, the earth in its orbit is closest to the sun. This time of the earth is said to be Perihelion.

On about July 4, the earth is farthest from the sun, at this time the earth is said to be Aphelion.

Creation Of Heat Zones:

The spherical shape of the earth along with its movements around the sun causes the difference in the angles at which the sun's rays reach the earth's surface. This causes difference in the distribution of heat on the earth's surface. As a result the world has been divided into three distinct heat or temperature zones.

(a) The Torrid Zone: This zone stretches between 23\(\frac{1}{2}\) degrees N and S latitudes and more or less receives the vertical rays of the sun. As a result, this region experiences a moderately warm climate throughout the year.

(b) The Temperate Zone: In between the Torrid and the Frigid zones i.e., between 23\(\frac{1}{2}\) degrees North and South lies the Temperate Zone. In this zone the sun's rays fall obliquely and hence the region experiences a moderate climate.

(c) The Frigid Zone: This zone extends from 66\(\frac{1}{2}\) degrees N to 90 degrees N (or the North Pole) and 66\(\frac{1}{2}\) degrees S to 90 degrees S (or the South Pole). Since the sun's rays fall extremely obliquely in this region, the region experiences extremely cold climate.

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