LIWA

Rotation and Revolution

ICSE Class 9 · Geography · Chapter 3

What this chapter covers

Key definitions

Rotation is the spinning of the earth on its own axis from west to east, one complete turn taking about 24 hours.

Revolution is the movement of the earth around the sun in a fixed path, one complete round taking about 365¼ days.

Angular speed of rotation is the angle through which the earth turns in a given time, equal to 15° every hour, or 1° every 4 minutes, at every place on the earth.

Linear speed of rotation is the actual distance a point on the earth's surface travels in a given time because of the spin, and it is greatest at the equator and zero at the poles.

Day is the period during which a place lies in the half of the earth facing the sun and receives sunlight.

Night is the period during which a place lies in the half of the earth turned away from the sun and receives no direct sunlight.

Apparent motion of the sun is the movement of the sun across the sky from east to west that an observer on the earth sees, which is not a real movement of the sun but the result of the earth rotating from west to east.

Coriolis force is the apparent force, arising from the earth's rotation, that is used to describe this deflection of moving winds and currents.

Perihelion is the position of the earth in its orbit when it is nearest to the sun, at about 147 million km, which occurs around 3 January.

Aphelion is the position of the earth in its orbit when it is farthest from the sun, at about 152 million km, which occurs around 4 July.

Parallelism of the axis is the property by which the earth's axis remains inclined in the same direction at every position of the earth in its orbit, so that the axis at any one position is parallel to the axis at any other position.

Key points

Questions answered

Define rotation and revolution, and state the time each takes.
Rotation is the spinning of the earth on its axis from west to east, taking about 24 hours. Revolution is the movement of the earth around the sun in its orbit, taking about 365¼ days.
In which direction does the earth rotate, and how would the rotation appear from above the North Pole?
The earth rotates from west to east. Seen from above the North Pole it turns anticlockwise.
Why is the linear speed of rotation greatest at the equator and zero at the poles, although the angular speed is the same everywhere?
The whole earth turns through 15° an hour, but a point on the equator travels round the longest circle, about 40,000 km, in 24 hours, while points at higher latitudes travel round smaller parallels in the same time. At a pole the point only turns on the spot and covers no distance.
Calculate the linear speed of rotation of a place on the 30° parallel, which is about 34,700 km long.
Speed = (34700 km)/(24 h) ≈ 1446 km/h, about 1,450 km per hour.
Explain how day and night occur. What is the circle of illumination?
The spherical earth is lit by the sun on only one half at a time; rotation carries each place in turn through the lit half (day) and the dark half (night). The circle of illumination is the imaginary circle dividing the lit half from the dark half.
Give a reason: the sun appears to rise in the east and set in the west.
The earth rotates from west to east, so the sun, which does not itself go round the earth, appears to move in the opposite direction, from east to west, just as trees seem to move backwards when seen from a moving train.
Define the Coriolis effect and state Ferrel's law.
The Coriolis effect is the deflection of freely moving winds and ocean currents caused by the earth's rotation. Ferrel's law states that they are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Why do cyclones over the Bay of Bengal spin anticlockwise, and why do cyclones rarely form at the equator?
Winds blowing towards the low-pressure centre are deflected to the right in the Northern Hemisphere, which sets them spiralling anticlockwise. At the equator the Coriolis effect is zero, so the air is not turned into a spin.

Past board exam questions

ICSE 2019 sample paper · 2 marks

(i) What is meant by the inclination of the Earth’s axis? (ii) At what angle is the Earth’s axis inclined?

Show answer
(i) The Earth's axis is not perpendicular to the plane of its orbit around the sun; it is tilted (inclined), and this tilt always points in the same direction in space. This tilt is the inclination of the Earth's axis. (ii) The axis is inclined at 66½° to the plane of the orbit, i.e. 23½° from the perpendicular to the orbital plane.

ICSE 2019 sample paper · 2 marks

State two effects of the rotation of the earth.

Show answer
Any two: 1. Day and night occur one after another. 2. Winds and ocean currents are deflected (to the right in the Northern hemisphere and to the left in the Southern hemisphere - Coriolis effect). 3. Tides occur twice a day. 4. The sun appears to rise in the east and set in the west; there is a difference of time between places at different longitudes.