Motion of the Earth – NDA Geography Notes

Introduction

This is one of the most important chapters in World Geography for NDA. It has produced over 20 questions across NDA papers and over 15 across CDS papers. That makes it one of the most reliable chapters to score from — if you understand it well.

The chapter covers two fundamental movements of the Earth — rotation (spinning on its own axis) and revolution (moving around the Sun). These two movements together explain day and night, the change of seasons, why days are longer in summer, why the poles experience six months of daylight and six months of darkness, and many other things we experience every day but rarely stop to think about.

For a science student, this chapter connects directly to Physics — concepts like angular velocity, gravity, and elliptical orbits will feel familiar. The difference is that here we apply these ideas to understand how the Earth behaves as a whole.

Understand the Topic

The Shape of the Earth

Before discussing the Earth’s movements, it helps to understand its shape.

The Earth is not a perfect sphere. It is slightly flattened at the poles and bulges outward at the equator. The scientific term for this shape is oblate spheroid or oblate ellipsoid. [NDA 2015-II]

The equatorial diameter is slightly larger than the polar diameter — by about 43 kilometres. The equator is about 21 km higher than the poles.

Why does the Earth bulge at the equator?

The bulge is caused by the Earth’s rotation — not its revolution. As the Earth spins, the centrifugal effect pushes material outward at the equator. This makes the equatorial diameter slightly larger than the polar diameter. [NDA 2022-I]

Students often say the bulge is caused by revolution. That is wrong. The bulge is caused by rotation.

The equatorial diameter is greater than the polar diameter — not smaller. Students sometimes reverse this. [NDA 2022-I]

Rotation of the Earth

Rotation is the spinning of the Earth on its own axis — the imaginary line running from the North Pole to the South Pole.

The Earth rotates from west to east — this is why the Sun appears to rise in the east and set in the west.

The Earth completes one full rotation in approximately 23 hours 56 minutes 4 seconds. This is called the sidereal day — the time taken to complete one rotation relative to distant stars.

The solar day — the time from one noon to the next noon — is 24 hours. It is slightly longer than the sidereal day because while the Earth is rotating, it is also moving along its orbit around the Sun. So it has to rotate a little extra each day to bring the Sun back to the same position in the sky.

The sidereal day is shorter than the solar day. [NDA 2010-II | NDA tested via CDS]

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Why Don’t We Feel the Earth Rotating?

We do not feel the Earth spinning because of three reasons. [NDA 2022-I]

First, the angular velocity is constant for each place on Earth’s surface — there are no sudden changes in speed to alert our senses.

Second, the atmosphere rotates with the Earth — so there is no wind resistance or relative motion between us and the air around us.

Third, there are no nearby objects that are stationary or moving at a different rate — so we have no reference point to measure our movement against.

Proof That the Earth Rotates — The Foucault Pendulum

The most famous scientific proof of Earth’s rotation is the Foucault Pendulum. [NDA 2008-II]

In 1851, French physicist Léon Foucault hung a very long pendulum from the ceiling of a building. As the pendulum swung, its plane of swing appeared to rotate slowly over time. In reality, it was the Earth beneath the pendulum that was rotating — the pendulum kept swinging in the same direction relative to the stars while the building and the Earth rotated beneath it.

Effects of Rotation

Day and Night:
The side of the Earth facing the Sun is in daylight. The side facing away is in darkness. As the Earth rotates, different parts move into and out of sunlight — creating the daily cycle of day and night.

The boundary between the lit and dark halves of the Earth is called the Circle of Illumination. It divides the Earth into a day hemisphere and a night hemisphere. [NDA tested via CDS]

If the Earth stopped rotating, there would be no sunrise and sunset, and no cycle of day and night. But seasons would continue — because seasons are caused by revolution and axial tilt, not rotation. [NDA tested via CDS]

Deflection of Winds and Currents — Coriolis Effect:
Because the Earth rotates, anything moving across its surface — wind, ocean currents — gets deflected. In the Northern Hemisphere, moving objects deflect to the right. In the Southern Hemisphere, to the left. This deflection is called the Coriolis Effect. It will be covered fully in Chapter 14.

The environmental effects of Earth’s rotation include the daily rhythm of daylight and air temperature, and the sideward deflection of air and water (Coriolis effect). The movement of tides is NOT caused by rotation — tides are caused by the gravitational pull of the Moon and Sun. [NDA 2019-I]

Linear Velocity and Latitude:
The Earth’s linear speed of rotation is fastest at the equator and decreases toward the poles. At the poles, the linear velocity is zero.

Among cities in the world, the one closest to the equator has the greatest linear velocity of rotation. Kampala, Uganda — located near the equator at approximately 0° latitude — has the greatest linear velocity of rotation among commonly tested cities. [NDA 2022-II]

Revolution of the Earth

Revolution is the movement of the Earth around the Sun in its orbit. The Earth takes approximately 365.25 days to complete one orbit. This is why we have a leap year every four years — to account for the extra 0.25 day that accumulates. [NDA tested via CDS]

The Earth’s orbit is an ellipse — an oval shape — not a perfect circle. The Sun sits at one of the two focal points of this ellipse.

Perihelion — the point when Earth is closest to the Sun. This happens around January 3. At perihelion, Earth is about 147 million km from the Sun.

Aphelion — the point when Earth is farthest from the Sun. This happens around July 4. At aphelion, Earth is about 152 million km from the Sun. [NDA tested via CDS]

Notice something important: the Earth is closest to the Sun in January — which is winter in the Northern Hemisphere. This tells us that the distance from the Sun is NOT what causes seasons. Seasons are caused by something else — the tilt of the Earth’s axis.

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Kepler’s Laws of Planetary Motion

Johannes Kepler, a German astronomer, described three laws of planetary motion in the early 1600s.

His second law states that a planet sweeps out equal areas in equal times. This means the Earth moves faster when it is closer to the Sun (perihelion) and slower when it is farther away (aphelion). [NDA tested via CDS]

Kepler also demonstrated that planets move around the Sun in ellipses, not circles. [NDA tested via CDS]

The Tilt of the Earth’s Axis

The Earth’s axis is not straight up and down — it is tilted at an angle of 23.5° from the vertical (or 66.5° from the plane of its orbit). This tilt is the single most important reason for the change of seasons. [NDA 2009-II]

If the axis were straight up with no tilt, the Sun would always shine equally on both hemispheres. There would be no seasons. Every place would have 12 hours of day and 12 hours of night throughout the year. [NDA 2011-I | NDA 2015-I]

Because the axis is tilted at 23.5°, as the Earth moves around the Sun, sometimes the Northern Hemisphere tilts toward the Sun and sometimes it tilts away. When it tilts toward the Sun — summer. When it tilts away — winter.

Seasons are caused by the revolution of the Earth on its inclined axis — NOT by the distance from the Sun. [NDA 2014-I]

Solstices and Equinoxes

Because of the Earth’s tilt and revolution, the Sun appears to move between the Tropic of Cancer and the Tropic of Capricorn over the course of a year. This movement creates solstices and equinoxes.

Summer Solstice (Northern Hemisphere):

• Date: June 21
• The Northern Hemisphere is tilted most toward the Sun
• The Sun is directly overhead at the Tropic of Cancer (23.5°N) at noon
• Longest day and shortest night in the Northern Hemisphere
• The North Pole has 24 hours of daylight [NDA tested via CDS]

Winter Solstice (Northern Hemisphere):

• Date: December 22
• The Northern Hemisphere is tilted most away from the Sun
• The Sun is directly overhead at the Tropic of Capricorn (23.5°S) at noon
• Shortest day and longest night in the Northern Hemisphere
• The North Pole has 24 hours of darkness [NDA 2017-II | NDA tested via CDS]

Vernal Equinox (Spring Equinox in Northern Hemisphere):

• Date: March 21
• The Sun is directly overhead at the Equator
• Day and night are equal (12 hours each) everywhere on Earth
• [NDA 2009-II | NDA 2016-I]

Autumnal Equinox:

• Date: September 23
• Again the Sun is directly overhead at the Equator
• Day and night are equal everywhere [NDA 2016-I]

A common exam trap: September — Sun overhead at Arctic Circle. This is FALSE. In September, the Sun is overhead at the Equator (autumnal equinox), not at the Arctic Circle. [NDA 2008-II]

Another common trap: Summer solstice occurs on December 22 — this is FALSE for the Northern Hemisphere. December 22 is the WINTER solstice in the Northern Hemisphere. June 21 is the summer solstice. [NDA 2016-I]

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Duration of Day and Night

The length of day and night varies with latitude and season.

At the equator, day and night are always close to equal throughout the year — about 12 hours each. The equator is never strongly tilted toward or away from the Sun.

As you move toward the poles, the difference between the longest day and the shortest day increases. At the poles, you get six months of continuous daylight and six months of continuous darkness.

The difference in day length is least near the Equator and progressively increases away from it. [NDA tested via CDS]

In summer, the duration of day is more in the Northern Hemisphere — because it is tilted toward the Sun. [NDA tested via CDS]

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Which Indian Cities Have the Sun Overhead Only Once a Year?

The Sun is directly overhead only at locations between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). Cities north of the Tropic of Cancer never have the Sun directly overhead. Cities exactly on the Tropic of Cancer have the Sun overhead only once a year — on June 21.

Cities on or very near the Tropic of Cancer — such as Nagpur and Kolkata — have the midday Sun exactly overhead only once a year. [NDA tested via CDS]

Types of Year

Sidereal Year — the time the Earth takes to complete one orbit relative to the fixed stars. This is the true orbital period — about 365 days 6 hours 9 minutes. [NDA 2008-II]

Tropical Year (Solar Year) — the time between two consecutive vernal equinoxes — about 365 days 5 hours 48 minutes. This is slightly shorter than the sidereal year because of a slow wobble of Earth’s axis called precession.

The tropical year is shorter than the sidereal year. [NDA 2010-II]

Leap Year — every four years an extra day is added to February because the year is not exactly 365 days. This keeps our calendar aligned with Earth’s actual orbit. [NDA tested via CDS]

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Eclipses

Solar Eclipse:
A solar eclipse happens when the Moon comes between the Sun and the Earth. The Moon blocks sunlight from reaching part of the Earth. This can only happen on a New Moon day. [NDA 2016-II | NDA 2010-I]

Lunar Eclipse:
A lunar eclipse happens when the Earth comes between the Sun and the Moon. The Earth’s shadow falls on the Moon. This can only happen on a Full Moon day. [NDA 2010-I]

Syzygy is the technical term for when the Sun, Earth, and Moon are in a straight line. It occurs during both solar eclipses (conjunction — Moon between Sun and Earth) and lunar eclipses (opposition — Earth between Sun and Moon). [NDA tested via CDS]

Which Latitude Experiences Minimum Sun Angle on Summer Solstice?

On the summer solstice in the Northern Hemisphere, the Sun is directly overhead at the Tropic of Cancer (23.5°N). The Tropic of Capricorn (23.5°S) is the farthest point in the Southern Hemisphere — here the Sun’s angle is at its minimum because it is winter in the Southern Hemisphere. [NDA 2025-I]

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Satellites in Orbit

A satellite in stable orbit needs no fuel to keep orbiting. It is in a state of continuous free fall — falling toward Earth but also moving forward at high speed, so it keeps missing the Earth and stays in orbit. Energy is needed only to change the orbit, not to maintain it. [NDA 2017-II]

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