Motion of the Earth – NDA Geography Notes

This is one of the most important chapters in World Geography for NDA. The chapter covers two fundamental movements of the Earth: rotation (spinning on its own axis) and revolution (moving around the Sun). Together, these two movements 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.

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The Shape of the Earth

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 farther from Earth’s centre 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, making the equatorial diameter slightly larger than the polar diameter. [NDA 2022-I]

The equatorial diameter is greater than the polar diameter, not smaller. Both of these points are directly tested in NDA. [NDA 2022-I]

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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.

Sidereal day = the time to complete one full rotation relative to distant stars = 23 hours 56 minutes 4 seconds.

Solar day = the time from one noon to the next noon = 24 hours. It is slightly longer than the sidereal day because while the Earth is rotating, it is also moving along its orbit. So it must 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]

Proof of Earth’s Rotation: The Foucault Pendulum: 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 fixed direction relative to the stars while the Earth rotated beneath it. [NDA 2008-II]

Why don’t we feel the Earth rotating? Three reasons: [NDA 2022-I]

The angular velocity is constant for each place on Earth’s surface. No sudden changes alert our senses. The atmosphere rotates with the Earth, so there is no wind resistance or relative motion between us and the air. There are no nearby stationary objects to use as a reference point for measuring our movement.

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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. The boundary between the lit and dark halves is called the Circle of Illumination. If the Earth stopped rotating, there would be no sunrise and sunset, but seasons would continue because seasons are caused by revolution and axial tilt.

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 is the Coriolis Effect (covered fully in Chapter 14 on Atmosphere).

★ IMPORTANT Effects of Earth’s rotation: Daily rhythm of daylight and air temperature: TRUE. Sideward deflection of air and water (Coriolis): TRUE. Movement of tides: FALSE. Tides are caused by the gravitational pull of the Moon and Sun, not by rotation. [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, linear velocity is zero. Among commonly tested cities, Kampala, Uganda (at approximately 0° latitude) has the greatest linear velocity of rotation. [NDA 2022-II]

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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. The extra 0.25 day accumulates over four years, giving us a leap year every four years (with February 29).

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: around January 3 (~147 million km from the Sun).

Aphelion = the point when Earth is farthest from the Sun: around July 4 (~152 million km from the Sun). [NDA tested via CDS]

Notice the critical implication: the Earth is closest to the Sun in January: which is winter in the Northern Hemisphere. This proves that distance from the Sun does NOT cause seasons.

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Kepler’s Laws of Planetary Motion

Kepler’s Second Law states that a planet sweeps out equal areas in equal times. This means Earth moves faster when closer to the Sun (perihelion, January) and slower when farther away (aphelion, July).

Kepler also demonstrated that planets move around the Sun in ellipses, not circles.

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The Tilt of the Earth’s Axis and Seasons

The Earth’s axis 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]

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

If the axis were not tilted (0° 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. The North Pole would NOT always be in darkness. It would receive sunlight half the day. [NDA 2011-I | NDA 2015-I]

Because the axis is tilted, as the Earth moves around the Sun, sometimes the Northern Hemisphere tilts toward the Sun (summer) and sometimes it tilts away (winter).

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Solstices and Equinoxes

Event	Date	Sun’s Position	Key Fact
Summer Solstice (NH)	June 21	Overhead at Tropic of Cancer (23.5°N)	Longest day in NH; North Pole = 24h daylight
Winter Solstice (NH)	December 22	Overhead at Tropic of Capricorn (23.5°S)	Shortest day in NH; North Pole = 24h darkness [NDA 2017-II]
Vernal Equinox	March 21	Overhead at Equator (0°)	Day = Night = 12 hours everywhere [NDA 2009-II]
Autumnal Equinox	September 23	Overhead at Equator (0°)	Day = Night = 12 hours everywhere [NDA 2016-I]

Common exam trap: The statement “In September, the midday Sun is overhead at the Arctic Circle” is FALSE. In September (autumnal equinox), the Sun is directly overhead at the Equator, not at the Arctic Circle. [NDA 2008-II]

Another trap: “Summer solstice occurs on December 22” 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]

On the summer solstice (NH), which latitude experiences the minimum Sun angle? Tropic of Capricorn (23.5°S), because it is winter there, with the Sun at its lowest angle for the year. [NDA 2025-I]

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Duration of Day and Night

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 and shortest day increases. At the poles, there is 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.

Cities at or near the Tropic of Cancer, such as Nagpur and Kolkata, have the midday Sun exactly overhead only once a year (on June 21). Cities north of the Tropic of Cancer never have the Sun directly overhead.

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Types of Year

Sidereal Year = the time the Earth takes to complete one orbit relative to the fixed stars = approximately 365 days 6 hours 9 minutes. [NDA 2008-II]

Tropical Year (Solar Year) = the time between two consecutive vernal equinoxes = approximately 365 days 5 hours 48 minutes. 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 (February 29) is added because the year is 365.25 days, not 365 exactly.

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Eclipses

Solar Eclipse happens when the Moon comes between the Sun and the Earth, blocking sunlight from reaching part of Earth’s surface. This can only happen on a New Moon day. [NDA 2016-II | NDA 2010-I]

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

Eclipse	Order	When
Solar Eclipse	Moon: between: Sun and Earth	New Moon
Lunar Eclipse	Earth: between: Sun and Moon	Full Moon

Syzygy is the technical term for when the Sun, Earth, and Moon are in a straight line, during both solar eclipses (conjunction) and lunar eclipses (opposition).

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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. Energy is needed only to change the orbit, not to maintain it. [NDA 2017-II]

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