Maarg Manthan · Topic 1.3

Rotation of the Earth and Its Effects

Geography › Earth, Latitudes, Longitudes and Motions · Topic 1.3

Rotation is the spinning of the Earth on its axis from west to east once in about 24 hours (23 hours 56 minutes 4 seconds). It causes day and night, the deflection of winds and ocean currents, and a daily rise and fall of tides, and together with the tilt of the axis it gives the Earth its slightly flattened geoid shape.

6 min read · UPSC Prelims · Geography · Earth, Latitudes, Longitudes and Motions

At a Glance

Direction West to east
Period 23 hours 56 minutes 4 seconds
Axis tilt 23.5 degrees from the normal
Main effect Day and night
Shape Geoid, flattened at the poles
Exam link UPSC Prelims, Geography: Earth, Latitudes, Longitudes and Motions

Where Does It Fit?

This is the third topic of the category on the Earth, latitudes, longitudes and motions. The previous topic covered longitudes and time, and this one covers the spin of the Earth that causes day and night. The next topic covers revolution and seasons. Previous topic: Longitudes, Meridians and Time Zones. Next topic: Revolution, Inclination of the Axis and Seasons.

What Is the Rotation of the Earth?

In short: the Earth spins like a top on its axis, from west to east.

The spinning movement of the Earth is called rotation. The Earth rotates around its axis from west to east, which is why the Sun appears to rise in the east. The axis is the imaginary line that passes through the North Pole, the centre of the Earth and the South Pole. It takes approximately 24 hours, more exactly 23 hours 56 minutes and 4 seconds, to complete one rotation. The speed of rotation is greatest at the equator, about 1,670 km an hour, and falls to nearly zero at the poles. Days and nights occur because of the rotation, since only half of the Earth faces the Sun at any time. The circle that divides the day from the night on the globe is called the circle of illumination. The Earth’s axis is tilted: it makes an angle of 23.5 degrees with the normal, or 66.5 degrees with the orbital plane.

What Is the Shape of the Earth?

The Earth is not a perfect sphere. Its shape is called a geoid, although some sources call it an oblate spheroid. It is slightly flattened at the poles and bulges at the equator. The radius at the equator is larger than at the poles because the speed of rotation, and hence the centrifugal force, is greater at the equator than at the poles. The long-term effect of rotation has therefore pushed matter outward in the equatorial region.

Feature	Equator	Poles
Radius	Larger	Smaller
Speed of rotation	Greatest	Nearly zero
Centrifugal force	Greatest	Least
Gravity	Slightly less	Slightly more
Length of one degree of latitude	Shorter	Longer

Why Does Gravity Vary with Latitude?

The force of gravitation is not the same at different latitudes. It is greater near the poles and less at the equator, for two reasons. First, the poles are closer to the centre of the Earth because of the equatorial bulge, so the gravitational field there is stronger. Second, the speed of rotation is greater at the equator, so the centrifugal force is greater there. Centrifugal force and gravitation act in opposite directions, so gravity is slightly reduced at the equator compared with the poles. A person weighs slightly less at the equator than at the poles for this reason, although the difference is small.

What Are the Effects of Rotation?

Day and night: the side facing the Sun has day and the other side has night, so every place on Earth has a daily cycle.
Time: rotation gives the basis for local time and time zones, since the Earth turns 15 degrees in an hour.
Deflection of winds and currents: because of the rotation, moving air and water are deflected to the right of their path in the northern hemisphere and to the left in the southern hemisphere. This is the Coriolis effect, and it shapes cyclones, planetary winds and ocean currents.
Tides: the rotation of the Earth, with the pull of the Moon and the Sun, brings about the regular rise and fall of the sea level.
Shape: the equatorial bulge and polar flattening arise from rotation.

The deflecting effect is covered further in Forces Affecting Wind Movement and in Ocean Currents – Causes and Types.

Why Does Temperature Fall Towards the Poles?

Temperature falls at the surface of the Earth as one moves away from the equator towards the poles. This is because of the spherical or geoid shape of the Earth and the position of the Sun relative to it. The equator receives direct sunlight, while the Sun’s rays become slant or oblique as one moves poleward, so the energy received per unit area decreases. The details of this variation, with the heat zones, are given in Latitudes and Important Parallels. As an analytical point, rotation alone does not explain the temperature pattern; the shape of the Earth and the angle of the Sun’s rays are the direct causes, while rotation shapes the day-night cycle and the deflection of moving fluids.

Why Is This Topic Important?

Day and night Basic cause
Geoid and gravity Frequent Prelims concept
Coriolis effect Foundation of winds and currents

What Came Next?

The next topic describes the revolution of the Earth, the inclination of the axis and the seasons. Next topic: Revolution, Inclination of the Axis and Seasons.

Key People and Terms

Axis Line through the poles and the centre
Circle of illumination Line dividing day from night
Geoid Shape of the Earth, flattened at the poles
Centrifugal force Outward force from rotation
Coriolis effect Deflection of moving air and water
Equatorial bulge Extra radius at the equator

Exam Corner

Points to Remember

Rotation takes 23 hours 56 minutes 4 seconds.
Earth rotates from west to east.
The shape is a geoid.
Gravity is greater at the poles than at the equator.
Rotation causes day and night; the tilted revolution causes seasons.

Do Not Confuse With

Rotation and revolution Spin on the axis versus motion around the Sun
Day and night versus seasons Rotation versus the tilted revolution
Geoid and sphere Flattened at the poles versus perfectly round

Memory Hook

Spin gives day and night; bulge gives geoid; poles pull harder, equator pulls softer.

Mains Angle

Questions ask about effects of rotation. These points add depth.

Time Day, night and time zones
Shape Geoid and gravity differences
Fluids Coriolis deflection

A Question You May Face

An original practice question, not a past paper question.

“Explain the effects of the rotation of the Earth on its shape and on atmospheric and oceanic circulation.”

How to Answer

Define Rotation and its period.
Shape Geoid and gravity.
Circulation Coriolis deflection.
Link Day, night and time.

GS Relevance

Prelims: rotation, geoid, gravity variation. GS Paper 1: motions of the Earth and their effects.

Frequently Asked Questions

What is the rotation of the Earth?

Rotation is the spinning of the Earth on its axis from west to east. It takes about 24 hours, more exactly 23 hours 56 minutes and 4 seconds, and it causes day and night. The axis makes an angle of 23.5 degrees with the normal to the orbital plane.

What is the circle of illumination?

The circle of illumination is the circle on the globe that divides the day from the night. Only the half of the Earth that faces the Sun is lit, and as the Earth rotates, the circle moves over the surface, bringing the cycle of day and night.

What is the shape of the Earth?

The shape of the Earth is a geoid, sometimes called an oblate spheroid. It is slightly flattened at the poles and bulges at the equator, because the speed of rotation, and hence the centrifugal force, is greater at the equator than at the poles.

Why is gravity greater at the poles than at the equator?

Gravity is greater at the poles because the poles are closer to the centre of the Earth due to the equatorial bulge. Also, the greater centrifugal force at the equator acts against gravity, so gravity is slightly reduced there compared with the poles.

How does rotation affect winds and ocean currents?

Because of the rotation, moving air and water are deflected to the right of their path in the northern hemisphere and to the left in the southern hemisphere. This Coriolis effect shapes the direction of planetary winds, cyclones and ocean currents.

PYQ Practice — Statement Analysis

1 The Earth rotates from east to west.

False

It rotates from west to east.

2 The shape of the Earth is a geoid.

True

It is flattened at the poles and bulged at the equator.

3 Gravity is slightly greater at the equator than at the poles.