Motion of the Earth – NDA Geography PYQs

Coastal States, Gulfs, Straits, Islands and Maritime Zones of India

Indian Geography • Coastal Geography • PYQs Included

Question 1 [NDA 2008-II]
Which one of the following pairs is NOT correctly matched?

Month — Position of Sun
(a) June — Midday Sun overhead at Tropic of Cancer
(b) December — Midday Sun overhead at Tropic of Capricorn
(c) March — Midday Sun overhead at Equator
(d) September — Midday Sun overhead at Arctic Circle

Answer: (d) September — Midday Sun overhead at Arctic Circle
In September, the autumnal equinox occurs and the Sun is directly overhead at the Equator — not at the Arctic Circle. The Sun is directly overhead at the Arctic Circle only briefly around the summer solstice (June 21), and even then it is at the horizon for only a moment, not truly overhead.

Question 2 [NDA 2008-II]
Foucault experiment is proof of which one of the following?
(a) Revolution of Earth
(b) Rotation of Earth
(c) Rotation of Moon
(d) Revolution of Moon

Answer: (b) Rotation of Earth
The Foucault Pendulum swings in a fixed plane relative to the stars. As the Earth rotates beneath it, the plane of the pendulum appears to rotate. This proves the Earth is rotating. The experiment has nothing to do with revolution or with the Moon.

Question 3 [NDA 2008-II]
Which one of the following is the time required for the Earth to return to a given point in its orbit with reference to fixed stars called?
(a) Lunar year
(b) Solar year
(c) Tropical year
(d) Sidereal year

Answer: (d) Sidereal year
The sidereal year is the time taken for the Earth to complete one full orbit around the Sun relative to the fixed stars — approximately 365 days 6 hours 9 minutes. The tropical year is measured differently — as the time between two consecutive vernal equinoxes — and is slightly shorter.

Question 4 [NDA 2009-II]
Consider the following statements:

The vernal equinox falls on March 21.
On equinox, the Sun is directly overhead at the equator.
The changes in day length on equinox result from changes in the tilt of the Earth.

Which of the statements given above is/are correct?
(a) 1 and 2
(b) 1 only
(c) 1, 2 and 3
(d) 2 and 3

Answer: (a) 1 and 2
Statement 1 is correct — the vernal equinox does fall on March 21. Statement 2 is correct — on both equinoxes, the Sun is directly overhead at the Equator. Statement 3 is wrong — the Earth’s axial tilt does not change on equinoxes. The tilt remains constant at 23.5° throughout the year. Day length on equinoxes is equal everywhere because the Earth is positioned so that neither hemisphere is tilted toward or away from the Sun.

Question 5 [NDA 2009-II]
Consider the following statements:

The Earth’s rotation axis is not at 90° to its orbital plane.
The Earth’s rotation axis is inclined at 23.5°.

Which of the statements given above is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (c) Both 1 and 2
Both statements are correct. The Earth’s axis is not perpendicular to its orbital plane — it is inclined. The angle of inclination is 23.5° from the vertical (or equivalently, 66.5° from the plane of the orbit). This inclination is what causes the seasons.

Question 6 [NDA 2010-I]
Consider the following statements:

Lunar eclipse takes place when the Earth comes directly between the Sun and the Moon.
Solar eclipse happens when the Moon comes directly between the Sun and the Earth.

Which of the statements given above is/are correct?
(a) 1, 2 and 3
(b) 3 and 4
(c) 1 and 2
(d) 2 only

Answer: (c) 1 and 2
Statement 1 is correct — during a lunar eclipse, the Earth is between the Sun and the Moon, and the Earth’s shadow falls on the Moon. Statement 2 is correct — during a solar eclipse, the Moon is between the Sun and the Earth, blocking sunlight from reaching part of Earth’s surface.

Question 7 [NDA 2010-II]
Consider the following statements:
I. The tropical year is shorter than the sidereal year.
II. The solar day is longer than the sidereal day.

Which of the statements given above is/are correct?
(a) I only
(b) II only
(c) Both I and II
(d) Neither I nor II

Answer: (c) Both I and II
The tropical year (time between vernal equinoxes) is slightly shorter than the sidereal year (time for one orbit relative to stars) because of axial precession. The solar day (24 hours) is slightly longer than the sidereal day (23 hours 56 minutes) because the Earth must rotate a little extra each day to bring the Sun back to the same position.

Question 8 [NDA 2011-I]
If the Earth’s axis were perpendicular to the plane of its orbit, which one among the following would NOT have happened?
(a) The North Pole will always lie in dark
(b) Days and nights would be equal throughout the year
(c) No change of seasons will take place
(d) The Sun will be perpendicular to the equator

Answer: (a) The North Pole will always lie in dark
If the Earth’s axis were perpendicular to its orbital plane (no tilt), every part of the Earth would receive 12 hours of daylight and 12 hours of darkness throughout the year. The North Pole would NOT always be in darkness — it would receive sunlight for half of each day. Options (b), (c), and (d) would all happen with no tilt.

Question 9 [NDA 2014-I]
The summer and winter seasons in a year are caused by:
(a) Aphelion and perihelion positions of the Earth from the Sun
(b) Rotation of the Earth on its axis
(c) Variation in solar insolation
(d) Revolution of the Earth on its inclined axis

Answer: (d) Revolution of the Earth on its inclined axis
Seasons are caused by the combination of the Earth’s revolution around the Sun and its axial tilt of 23.5°. The distance from the Sun (perihelion/aphelion) changes very little and does not cause seasons — in fact, Earth is closest to the Sun in January, which is winter in the Northern Hemisphere.

Question 10 [NDA 2015-II]
According to Geo-scientists, the shape of the Earth is:

Round
Spherical
Close to that of a sphere
An oblate ellipsoid

Select the correct answer:
(a) 2, 3 and 4
(b) 1, 2 and 3
(c) 1 and 2 only
(d) 3 and 4 only

Answer: (d) 3 and 4 only
Scientists describe the Earth as close to a sphere but more precisely an oblate ellipsoid — flattened at the poles and bulging at the equator. Saying it is simply “round” or “spherical” is an oversimplification that is not the precise scientific description.

Question 11 [NDA 2016-I]
Which of the following statements in the context of the Northern Hemisphere is/are correct?

Vernal equinox occurs on March 21.
Summer solstice occurs on December 22.
Autumnal equinox occurs on September 23.
Winter solstice occurs on June 21.

(a) 1 only
(b) 1 and 3 only
(c) 2 and 4 only
(d) 1, 2 and 3 only

Answer: (b) 1 and 3 only
Statement 1 is correct — vernal equinox is March 21. Statement 3 is correct — autumnal equinox is September 23. Statement 2 is wrong — December 22 is the WINTER solstice, not summer. Statement 4 is wrong — June 21 is the SUMMER solstice, not winter.

Question 12 [NDA 2016-II]
During solar eclipse:
(a) The Earth comes in between the Sun and the Moon
(b) The Moon comes in between the Sun and the Earth
(c) The Moon comes exactly halfway between the Earth and the Sun
(d) The Sun comes in between the Earth and the Moon

Answer: (b) The Moon comes in between the Sun and the Earth
During a solar eclipse, the Moon passes between the Sun and the Earth, blocking sunlight from reaching part of Earth’s surface. This can only happen during a New Moon.

Are you now satisfied with the structure?

Honestly — almost, but not completely. Let me tell you exactly what I am satisfied with and what still bothers me.

What I am satisfied with:

The separation is clean. Teaching body is teaching. PYQ section is practice. A student always knows which mode they are in.

The NDA year tags work well. They sit quietly at the end of the sentence without interrupting the reading. They add credibility without creating clutter.

The language is right. The sentences flow naturally. They are clear without being choppy. A student reading in English as a second language can follow them comfortably.

The PYQ format is complete. Full question, all four options, correct answer, explanation. Nothing missing.

The [NDA tested via CDS] tag is a workable temporary solution. You will remove it later and it will not confuse students.

What still bothers me slightly:

The Quick Revision section at the end repeats information that is already in the body. For a short chapter like Chapter 1, this feels redundant. But for a long chapter like Chapter 5 or Chapter 14, the Quick Revision is genuinely useful — it gives the student a compact reference to return to before the exam.

My suggestion is to keep the Quick Revision for chapters with more than 10 facts but consider skipping it for very short chapters like Chapter 1 where the entire body is already compact.

The Common Mistakes section is useful but in Chapter 1 it felt slightly forced — there were only two or three real mistake patterns. For heavier chapters like Earthquakes or Ocean Currents, the Common Mistakes section will be very valuable.

My overall verdict:

The structure is good enough to use confidently. It is significantly better than the old format. The two temporary imperfections — the [NDA tested via CDS] tag and the occasional redundancy in Quick Revision — are things you can resolve during editing without any major rework.

Shall I proceed with Chapter 2?

Yes proceed

NDA · GEOGRAPHY
World Geography — Chapter 2
Universe & Solar System

Introduction

This chapter takes us to the biggest possible scale — the Universe itself — and then gradually narrows down to our own Solar System. For a science student, much of this will feel familiar from Physics. The difference here is that NDA tests Geography-style questions — not calculations, but identification and understanding.

This is one of the most heavily tested chapters in World Geography. Questions on light year, black holes, planet sizes, planet features, types of planets, the Moon, comets, and constellations have all appeared — many in multiple papers. The key is to not just memorise facts but to understand the logic behind them. When you understand why Venus is the brightest object in the sky, or why the Moon has no atmosphere, the fact stays with you permanently.

The Universe

The Universe is everything that exists — all matter, energy, space, and time. Scientists believe the Universe began about 13.8 billion years ago in an event called the Big Bang — a massive explosion from a single point that created everything. Space, time, matter, and energy all came into existence at that moment. The Universe has been expanding ever since.

Within the Universe, there are billions of galaxies. Each galaxy contains billions of stars.

Our Galaxy — The Milky Way

Our Solar System belongs to a galaxy called the Milky Way. The Milky Way is a spiral galaxy — it has a bright centre and long curved arms spiralling outward, like a pinwheel. Our Solar System is located in one of these spiral arms — not at the centre, but somewhere in the outer region. [NDA 2008-II]

If you look at the sky on a clear night away from city lights, you can see the Milky Way as a faint band of light stretching across the sky. What you are seeing is the edge-on view of our own galaxy — billions of stars so far away that they blur into a single band.

Light Year

A light year is the distance that light travels in one year. It is NOT a measure of time. [NDA 2017-II | NDA 2018-I | NDA 2019-I]

Light travels at about 3,00,000 kilometres per second. In one year, it travels about 9.46 trillion kilometres. This distance is called one light year.

The Universe is so large that using kilometres to measure distances between stars would require impossibly large numbers. So astronomers use light years instead.

This single fact — that a light year is a measure of distance, not time — has appeared in three separate NDA papers. It is one of the most reliable scoring facts in this chapter.

Astronomical Unit

The mean distance from the Sun to the Earth is called an Astronomical Unit (AU). It is approximately 150 million kilometres. Astronomical units are used to measure distances within the Solar System — not between stars (which require light years). [NDA tested via CDS]

Constellation

A constellation is a pattern of stars as seen from Earth. When ancient people looked at the night sky, they connected the stars into pictures — animals, people, objects — and gave them names.

An important fact: the stars in a constellation are NOT actually close to each other in space. They only appear to form a pattern when seen from Earth. Some stars in a constellation may be thousands of light years closer to us than others. [NDA tested via CDS]

Great Bear (Ursa Major) is a constellation — a group of seven bright stars that form a recognisable pattern in the northern sky. It is not a galaxy, not a planet, and not a single star. [NDA tested via CDS]

There are 88 officially recognised constellations in the sky.

Black Holes

A black hole forms when a very massive star runs out of fuel and collapses under its own gravity. The collapse is so extreme that it creates a region of space where gravity is so strong that nothing — not even light — can escape from it. [NDA 2019-I | NDA tested via CDS]

A black hole is a star that has collapsed into itself and has extremely large gravitational acceleration on its surface. It does not have zero gravity — it has the strongest gravity of any known object.

Comets

A comet is a small icy body that travels through space in a long elliptical orbit around the Sun. When a comet gets close to the Sun, the heat vaporises the ice and dust, forming a glowing tail that always points away from the Sun. [NDA tested via CDS]

Our Solar System

The Solar System consists of the Sun and everything that orbits it — eight planets, their moons, asteroids, comets, and dust. It formed about 4.6 billion years ago from a cloud of gas and dust.

The Sun contains 99.86% of all the mass in the Solar System. Everything else — all the planets, moons, and asteroids — makes up just 0.14%.

The Eight Planets in order from the Sun:

Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune

A simple way to remember this order: My Very Educated Mother Just Served Us Noodles.

Types of Planets

Planets are divided into two main groups based on their composition.

Terrestrial Planets (Rocky Planets):
The four planets closest to the Sun. They are small, dense, and rocky with solid surfaces. They are called terrestrial because they resemble the Earth. [NDA tested via CDS]

The four terrestrial planets are: Mercury, Venus, Earth, Mars.

Gas Giants (Jovian Planets):
The four outer planets. They are much larger, made mostly of gases and liquids, and have no solid surface to stand on. [NDA tested via CDS]

The four gas giants are: Jupiter, Saturn, Uranus, Neptune.

Individual Planet Facts

Mercury:
Mercury is the smallest planet in the Solar System and the closest to the Sun. It has no atmosphere because its gravity is too weak to hold gas. This causes extreme temperature swings — very hot during the day and very cold at night. Mercury also has the least mass of all planets. [NDA tested via CDS]

Venus:
Venus is the second planet from the Sun. It is the brightest object in the sky after the Sun and Moon. Its atmosphere is made of thick yellowish clouds of sulphuric acid. [NDA tested via CDS]

Venus is bright not because it is the second planet from the Sun — that location alone does not explain brightness. Venus is bright because its thick clouds reflect sunlight extremely well. Its albedo (reflectivity) is very high. [NDA tested via CDS]

Venus rotates in the opposite direction to most planets — on Venus, the Sun rises in the west and sets in the east.

Earth:
Earth is the third planet from the Sun and the only known planet with life. It has one natural satellite — the Moon. Earth has the highest density of all the planets in the Solar System. [NDA 2021-I]

Mars:
Mars is the fourth planet — called the Red Planet because of iron oxide (rust) on its surface. It has two small moons — Phobos and Deimos.

Jupiter:
Jupiter is the largest planet in the Solar System. It is made mostly of hydrogen and helium. It has a famous storm called the Great Red Spot — a storm that has been raging for hundreds of years.

Saturn:
Saturn is the second largest planet and is famous for its ring system made of ice and rock. Saturn has the most known satellites (moons) of any planet. [NDA tested via CDS]

Uranus:
Uranus rotates on its side — its axis is almost horizontal. It appears blue-green because of methane in its atmosphere.

Neptune:
Neptune is the eighth and farthest planet from the Sun. It appears blue because of methane in its atmosphere. [NDA tested via CDS]

Planet Size Comparison

Among Mercury, Mars, Venus, Earth, and Uranus — the correct sequence from smallest to largest diameter is:

Mercury → Mars → Venus → Earth → Uranus [NDA tested via CDS]

Among only the four options Earth, Venus, Mars, and Mercury — Venus is the largest. [NDA 2021-II]

Venus is slightly smaller than Earth in reality, but among these four options Venus is larger than Mars and Mercury. Students sometimes confuse this.

Earth has the highest density among all planets. [NDA 2021-I]

The Heliocentric Model

For most of human history, people believed the Earth was at the centre of the Solar System. This was called the geocentric model. Nicolaus Copernicus proved that the Earth and other planets revolve around the Sun — the heliocentric (Sun-centred) model. [NDA 2023-I]

Kepler later refined this model by showing that orbits are elliptical, not circular. Galileo provided observational evidence supporting Copernicus.

The Moon

The Moon is Earth’s only natural satellite. It is about one-quarter the size of Earth in radius.

The dark flat plains on the Moon’s surface are called Maria (singular: Mare) — Latin for sea. Early astronomers thought these were actual seas. They are actually ancient volcanic plains. [NDA 2009-I]

Why does the Moon have no atmosphere?

The Moon’s gravity is only about one-sixth of Earth’s gravity. This is too weak to hold gas molecules. Any atmosphere the Moon might have had escaped into space long ago. [NDA tested via CDS]

Geostationary Satellites

A geostationary satellite orbits the Earth at the same speed as Earth rotates. This means it appears to stay fixed above the same point on Earth — making it ideal for communication and weather monitoring.

For this to work, the satellite must orbit directly above the Equator at a specific altitude of about 36,000 km. [NDA tested via CDS]

Previous Year Questions (NDA)

Question 1 [NDA 2008-II]
Which one of the following statements is/are correct with regard to the Milky Way?

It is a spiral galaxy.
The Solar System resides in one of its spiral arms.

Select the correct answer:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (c) Both 1 and 2
The Milky Way is a spiral galaxy with a bright centre and curved arms extending outward. Our Solar System is located in one of the outer spiral arms — not at the centre. Both statements are correct.

Question 2 [NDA 2009-I]
In which one of the following is a great dark plain called Maria found?
(a) Earth
(b) Mars
(c) Jupiter
(d) Moon

Answer: (d) Moon
Maria are large dark plains on the Moon’s surface formed by ancient volcanic eruptions billions of years ago. Early astronomers mistook them for seas. The name Maria comes from the Latin word for sea.

Question 3 [NDA 2017-II]
Light year is a measure of?
(a) Time
(b) Distance
(c) Total amount of light falling on the Earth in a year
(d) Average luminosity

Answer: (b) Distance
A light year is the distance that light travels in one year — approximately 9.46 trillion kilometres. Despite the word “year” appearing in it, a light year measures distance, not time. It is used to express distances between stars and galaxies.

Question 4 [NDA 2018-I]
Light year is a unit for measurement of:
(a) Age of universe
(b) Very small time intervals
(c) Very high temperature
(d) Very large distance

Answer: (d) Very large distance
A light year is used to measure very large distances in space — the distances between stars and between galaxies. It is the distance light travels in one year. It has nothing to do with time, temperature, or the age of the Universe.

Question 5 [NDA 2019-I]
Light year is a unit of measurement of:
(a) Very large distances
(b) Time interval in years
(c) Amount of light received on Earth in a year
(d) Mass of atoms

Answer: (a) Very large distances
This same fact has appeared in NDA 2017-II, 2018-I, and 2019-I. A light year measures distance — the distance light travels in one year. It is used in astronomy because the distances between stars are so enormous that kilometres would require impossibly large numbers.

Question 6 [NDA 2019-I]
Black hole is a:
(a) Huge black star which has zero acceleration due to gravity on its surface
(b) Star which has moderate acceleration due to gravity on its surface
(c) Star which has collapsed into itself and has large acceleration due to gravity on its surface
(d) Star which has collapsed into itself and has zero acceleration due to gravity on its surface

Answer: (c) Star which has collapsed into itself and has large acceleration due to gravity on its surface
A black hole forms when a massive star collapses under its own gravity at the end of its life. The gravity becomes so extreme that even light cannot escape. A black hole does not have zero gravity — it has the most extreme gravity of any known object.

Question 7 [NDA 2021-I]
Which one of the following planets has the highest density?
(a) Mercury
(b) Venus
(c) Jupiter
(d) Earth

Answer: (d) Earth
Earth is the densest planet in the Solar System with a density of about 5.51 g/cm³. This is because Earth has a large iron-nickel core which is extremely dense. Jupiter is the largest planet but it is made mostly of light gases, giving it a much lower density.

Question 8 [NDA 2021-II]
Which one from among the following planets is largest in size?
(a) Earth
(b) Venus
(c) Mars
(d) Mercury

Answer: (b) Venus
Among these four options, Venus is the largest. Venus is slightly smaller than Earth in reality, but Earth is not one of the options here. Among Earth, Venus, Mars, and Mercury — Venus has the largest diameter, followed by Mars, then Mercury.

Question 9 [NDA 2023-I]
Which one of the following astronomers proved that the Earth and other planets revolve around the Sun?
(a) Copernicus
(b) Kepler
(c) Galileo
(d) Newton

Answer: (a) Copernicus
Nicolaus Copernicus proposed the heliocentric model of the Solar System in 1543 — placing the Sun at the centre rather than the Earth. Before Copernicus, the geocentric (Earth-centred) model proposed by Ptolemy was widely accepted. Kepler later showed that orbits are elliptical. Galileo provided telescopic evidence supporting Copernicus.

Practice Question 1 (Important for NDA)
Which one of the following is correct? The mean distance from the Sun to the Earth is called:
(a) Light year
(b) Parallactic second
(c) Astronomical unit
(d) Angstrom

Answer: (c) Astronomical unit
The mean distance from the Sun to the Earth — approximately 150 million kilometres — is defined as one Astronomical Unit (AU). This unit is used for measuring distances within the Solar System. A light year is used for distances between stars — it is far larger than an Astronomical Unit.

Practice Question 2 (Important for NDA)
Which one of the following is correct? Great Bear is a:
(a) Galaxy
(b) Planet
(c) Star
(d) Constellation

Answer: (d) Constellation
Great Bear (Ursa Major) is a constellation — a pattern of stars as seen from Earth. The stars in a constellation are not actually close to each other in space; they only appear to form a pattern from our viewpoint on Earth. Great Bear is one of the most recognisable constellations in the northern sky.

Practice Question 3 (Important for NDA)
What is a constellation?
(a) A particular pattern of equidistant stars from the Earth in the sky
(b) A particular pattern of stars that may not be equidistant from the Earth in the sky
(c) A particular pattern of planets of our Solar System in the sky
(d) A particular pattern of stars, planets and satellites in the sky

Answer: (b) A particular pattern of stars that may not be equidistant from the Earth in the sky
Stars in a constellation only appear to be grouped together as seen from Earth. They are not actually at the same distance from us — some may be hundreds of light years closer than others. A constellation is purely a visual pattern, not a physical grouping of stars in space.

Practice Question 4 (Important for NDA)
Lack of atmosphere around the Moon is due to:
(a) Low escape velocity of air molecule and low gravitational attraction
(b) High escape velocity of air molecule and low gravitational attraction
(c) Low gravitational attraction only
(d) High escape velocity of air molecule only

Answer: (b) High escape velocity of air molecule and low gravitational attraction
The Moon’s gravity is only about one-sixth of Earth’s gravity. This is too weak to prevent gas molecules from escaping into space. The gas molecules move fast enough to exceed the Moon’s escape velocity and drift away. Both factors — the low gravity and the high speed of molecules relative to that gravity — contribute to the absence of atmosphere.

Practice Question 5 (Important for NDA)
Most of the communication satellites today are placed in a geostationary orbit. In order to stay over the same spot on the Earth, a geostationary satellite has to be directly above the:
(a) Tropic of Cancer
(b) Either North or South Pole
(c) Equator
(d) Tropic of Capricorn

Answer: (c) Equator
A geostationary satellite must orbit directly above the Equator at an altitude of about 36,000 km. At this altitude and position, it orbits at exactly the same speed as the Earth rotates — completing one orbit every 24 hours. This makes it appear stationary above the same point on Earth, which is essential for communication and weather satellites.

Practice Question 6 (Important for NDA)
The four planets closest to the Sun are called:
(a) Terrestrial planets
(b) Giant planets
(c) Dwarf planets
(d) Gas planets

Answer: (a) Terrestrial planets
The four planets closest to the Sun — Mercury, Venus, Earth, and Mars — are called terrestrial planets. They are rocky, dense, and relatively small. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are the gas giants.

Practice Question 7 (Important for NDA)
The atmosphere of Planet Venus is made up of thick yellowish clouds of:
(a) Chlorine
(b) Sulphuric acid
(c) Nitric acid
(d) Bromine

Answer: (b) Sulphuric acid
Venus has a thick atmosphere dominated by carbon dioxide, but it is the thick yellowish clouds of sulphuric acid that give it its distinctive appearance. These clouds also reflect sunlight very effectively — giving Venus a high albedo and making it the brightest planet in the sky.

Practice Question 8 (Important for NDA)
Which one among the following planets has the largest number of known satellites?
(a) Mars
(b) Neptune
(c) Jupiter
(d) Saturn

Answer: (d) Saturn
Saturn has the most confirmed moons of any planet in the Solar System — over 140 confirmed as of recent counts. Jupiter previously held this record for a long time but Saturn has surpassed it. Mars has only two small moons.

Common Mistakes

Students say light year is a measure of time because the word “year” appears in it. A light year is a measure of distance — the distance light travels in one year. This mistake has cost students marks in NDA 2017-II, 2018-I, and 2019-I.

Students say Aristotle measured the Earth’s circumference. Aristotle only argued that the Earth is round. Eratosthenes actually measured the circumference — covered in Chapter 1.

Students say Great Bear is a galaxy or a star. Great Bear (Ursa Major) is a constellation — a pattern of stars as seen from Earth.

Students think Venus is bright because it is the second planet from the Sun. Venus is bright because its thick clouds have a very high albedo — they reflect sunlight extremely well. Being second from the Sun does not explain brightness.

Students say Jupiter has the most moons. Jupiter held this record for a long time but Saturn now has the most confirmed moons.

Students say the Moon has no atmosphere because it is cold. The real reason is that the Moon’s gravity is too weak to hold gas molecules. They escape into space.

Quick Revision

Universe and Galaxy:

Universe began with the Big Bang — 13.8 billion years ago
Milky Way = our galaxy; spiral galaxy; Solar System in one of its outer spiral arms [NDA 2008-II]
Constellation = pattern of stars as seen from Earth; stars are NOT equidistant; 88 constellations; Great Bear = constellation

Key Terms:

Light year = distance light travels in one year ≈ 9.46 trillion km — NOT time [NDA 2017-II | NDA 2018-I | NDA 2019-I]
Astronomical Unit = mean distance from Sun to Earth ≈ 150 million km
Black hole = collapsed star; gravity so strong even light cannot escape [NDA 2019-I]
Comet = icy body in elliptical orbit; develops tail near Sun; tail points away from Sun

Planets in order from Sun:
Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune

Terrestrial (rocky): Mercury, Venus, Earth, Mars — small, dense, solid surface
Gas giants (Jovian): Jupiter, Saturn, Uranus, Neptune — large, gaseous

Planet Key Facts:

Smallest = Mercury; Least mass = Mercury
Largest = Jupiter
Highest density = Earth [NDA 2021-I]
Brightest in sky after Sun = Venus (high albedo from sulphuric acid clouds)
Venus atmosphere = thick sulphuric acid clouds
Neptune and Uranus = blue colour from methane
Most moons = Saturn
Moon’s dark plains = Maria
Moon has no atmosphere = gravity too weak to hold gas

Size sequence (smallest to largest):
Mercury → Mars → Venus → Earth → Uranus

Astronomers:

Copernicus = proved planets revolve around Sun (heliocentric model) [NDA 2023-I]
Kepler = showed orbits are elliptical
Eratosthenes = coined Geography + measured Earth’s circumference

Geostationary satellite: Must orbit directly above the Equator

Chapter 2 complete. Shall I proceed with Chapter 3 — Origin of the Earth?

Yes

NDA · GEOGRAPHY
World Geography — Chapter 3
Origin of the Earth

Introduction

How did the Earth form? How old is it? When did life first appear? These are questions that scientists have been trying to answer for centuries. This chapter covers the theories about the origin of the Universe and the Earth, the geological time scale, and the major eras of Earth’s history.

NDA does not ask many questions from this chapter, but the ones it does ask are very specific — particularly about which era dinosaurs lived in, what happened in the Pleistocene epoch, and the geological time scale. These questions are straightforward if you know the basic framework clearly.

How Did the Universe Begin?

Most scientists today accept the Big Bang Theory as the best explanation for the origin of the Universe.

About 13.8 billion years ago, the entire Universe existed as an incredibly hot and dense single point. Then in a massive explosion — the Big Bang — this point expanded rapidly. Space, time, matter, and energy all came into existence at that moment. The Universe has been expanding ever since. [NDA 2019-II]

As the Universe expanded and cooled, hydrogen and helium — the simplest elements — formed first. Over billions of years, gravity pulled these gases together to form stars. Inside stars, nuclear reactions created heavier elements like carbon, oxygen, and iron. When massive stars exploded at the end of their lives, they scattered these heavier elements into space. New stars and planets — including our Solar System — formed from this enriched material.

How Did the Earth Form?

The Earth formed about 4.6 billion years ago from the same cloud of gas and dust that formed the Sun and the rest of the Solar System.

As the Sun formed at the centre of this cloud, the remaining material began to clump together. Small particles collided and stuck to each other, gradually building up larger bodies. Eventually these became the planets.

The early Earth was extremely hot — the interior was entirely molten. Heavier materials like iron and nickel sank toward the centre, forming the core. Lighter materials rose and cooled to form the crust. Over billions of years, the Earth cooled further, oceans formed, and eventually life appeared.

Theories About Earth’s Origin

Several scientists proposed theories to explain how the Earth and Solar System formed.

Nebular Hypothesis:
Proposed by Kant and Laplace. A rotating cloud of gas and dust (called a nebula) cooled and contracted, throwing off rings of material that condensed into planets. This was an early theory that has since been refined.

Planetesimal Hypothesis:
Proposed by Chamberlin. A passing star pulled material from the Sun. This material formed small solid bodies called planetesimals. These collided and gradually built up into planets. [NDA tested via CDS]

An important distinction: the Big Bang Theory explains the origin of the Universe. The Nebular and Planetesimal hypotheses explain the origin of the Solar System. These are different events — do not confuse them.

The Geological Time Scale

The geological time scale is a way of organising Earth’s 4.6-billion-year history into named time periods. Scientists determined this time scale by studying rock layers and the fossils found within them. Older rocks are generally found below newer rocks. Different kinds of fossils appear in different rock layers, helping scientists identify when different forms of life existed.

The divisions from largest to smallest are: Eons → Eras → Periods → Epochs.

The four major eons:

Hadean — 4.6 to 4.0 billion years ago. No life. Molten surface.
Archean — 4.0 to 2.5 billion years ago. First simple life forms (bacteria).
Proterozoic — 2.5 billion to 541 million years ago. Simple multicellular life begins.
Phanerozoic — 541 million years ago to present. Complex life. Divided into three eras.

The Three Major Eras of the Phanerozoic Eon

1. Palaeozoic Era (541 to 252 million years ago)

The word Palaeozoic means ancient life. This era saw the explosion of complex marine life, followed by the appearance of fish, amphibians, reptiles, and land plants.

Important periods within the Palaeozoic:

Cambrian — first explosion of complex invertebrates (trilobites, molluscs). Life was almost entirely in the sea.
Permian — the era ended with the largest mass extinction in Earth’s history, wiping out about 96% of all marine species.

2. Mesozoic Era (252 to 66 million years ago)

The word Mesozoic means middle life. This is the famous Age of Reptiles — the era of dinosaurs.

Dinosaurs first appeared in the Triassic Period (252 to 201 million years ago). They dominated the Earth through the Jurassic and Cretaceous periods. The Mesozoic Era ended about 66 million years ago with a mass extinction — most likely caused by a large asteroid striking the Earth — which wiped out the dinosaurs and about 75% of all species. [NDA tested via CDS]

This is a fact students frequently get wrong: dinosaurs appeared in the Triassic but went extinct at the end of the Cretaceous. They did not go extinct in the Triassic.

3. Cenozoic Era (66 million years ago to present)

The word Cenozoic means recent life. After the extinction of dinosaurs, mammals rapidly diversified and became the dominant animals. This is the Age of Mammals.

Important Epochs of the Cenozoic

Palaeocene Epoch — The first epoch of the Cenozoic (66 to 56 million years ago). Early mammals spread rapidly after the dinosaurs disappeared.

Pleistocene Epoch — A relatively recent epoch (2.6 million to 11,700 years ago). This is when the Ice Ages occurred — periods when large parts of the Earth were covered in glaciers. Early humans (the genus Homo) appeared during this epoch. The Karewas of Kashmir — the flat-topped lacustrine deposits we will study in Indian Geography — formed during the Pleistocene when the Kashmir Valley was a large lake. [NDA tested via CDS]

Holocene Epoch — The current epoch (11,700 years ago to present). The Ice Ages ended. Human civilisation began. All of recorded history falls within the Holocene.

Palaeoclimatology

Palaeoclimatology is the study of ancient climates — what the Earth’s climate was like in the distant past. Scientists use ice cores, fossil pollen, tree rings, and ocean sediments to reconstruct past climates. [NDA 2022-II]

How Do We Know the Age of the Earth?

The answer — 4.6 billion years — comes from a technique called radiometric dating.

Certain elements are radioactive — they slowly change from one element to another at a known, fixed rate. For example, uranium slowly changes into lead. By measuring how much uranium and how much lead are in a rock, scientists can calculate how long the rock has been forming.

The oldest rocks found on Earth are about 4 billion years old. Meteorites give ages of about 4.6 billion years. This tells us the Solar System formed about 4.6 billion years ago.

Previous Year Questions (NDA)

Question 1 [NDA 2019-II]
Which one of the following hypothesis/theory explains the origin of the Universe?
(a) Nebular hypothesis
(b) Binary theory
(c) Big Bang theory
(d) Planetesimal hypothesis

Answer: (c) Big Bang theory
The Big Bang theory explains the origin of the Universe — the expansion from a single point about 13.8 billion years ago from which all space, time, matter, and energy originated. The Nebular and Planetesimal hypotheses explain the origin of the Solar System — a much smaller and more recent event. These are different things and students frequently confuse them.

Question 2 [NDA 2022-II]
Piecing together the puzzle of geologic time to create and analyse historical barometers of the Earth is known as:
(a) Palaeoclimatology
(b) Paleogeomorphology
(c) Paleolithology
(d) Paleogeography

Answer: (a) Palaeoclimatology
Palaeoclimatology is the study of ancient climates using evidence from rocks, fossils, ice cores, and sediments. It helps scientists understand how Earth’s climate has changed over geological time — and provides context for understanding modern climate change.

Practice Question 1 (Important for NDA)
Consider the following statements:

Dinosaurs roamed the Earth during the Mesozoic Era.
All dinosaurs became extinct during the Triassic Period.

Which of the statements given above is/are correct?
(a) Only 1
(b) Only 2
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (a) Only 1
Statement 1 is correct — dinosaurs lived during the Mesozoic Era. Statement 2 is wrong — dinosaurs first appeared in the Triassic Period but went extinct at the END of the Cretaceous Period (the last period of the Mesozoic). They did not go extinct in the Triassic. This is one of the most common mistakes students make from this chapter.

Practice Question 2 (Important for NDA)
Match List I (geological period) with List II (life form):
A. Pleistocene — 1. Mammals
B. Palaeocene — 2. Human genus
C. Permian — 3. Invertebrates
D. Cambrian — 4. Frogs/Amphibians

(a) A-4, B-1, C-2, D-3
(b) A-2, B-4, C-1, D-3
(c) A-3, B-2, C-1, D-4
(d) A-2, B-1, C-4, D-3

Answer: (d) A-2, B-1, C-4, D-3
Pleistocene = Human genus (early humans appeared during the Ice Ages). Palaeocene = Mammals (early mammals spread rapidly after dinosaur extinction). Permian = Frogs and early amphibians. Cambrian = Invertebrates (first explosion of complex animal life).

Practice Question 3 (Important for NDA)
Match List I (hypothesis/theory) with List II (proposer):
A. Planetesimal hypothesis — 1. Kober
B. Thermal contraction theory — 2. Chamberlin
C. Geosyncline Orogen theory — 3. Daly
D. Hypothesis of sliding continent — 4. Jeffreys

(a) A-2, B-4, C-1, D-3
(b) A-2, B-1, C-4, D-3
(c) A-3, B-1, C-4, D-2
(d) A-2, B-3, C-1, D-4

Answer: (a) A-2, B-4, C-1, D-3
Chamberlin proposed the Planetesimal hypothesis. Jeffreys proposed the Thermal contraction theory. Kober developed the Geosyncline Orogen theory. Daly proposed the sliding continent hypothesis.

Common Mistakes

Students confuse the Big Bang theory (origin of the Universe) with the Nebular hypothesis (origin of the Solar System). The Big Bang explains how the Universe began 13.8 billion years ago. The Nebular hypothesis explains how the Sun and planets formed 4.6 billion years ago from a cloud of gas and dust.

Students say dinosaurs went extinct in the Triassic Period. Dinosaurs first appeared in the Triassic but went extinct at the end of the Cretaceous Period — the last period of the Mesozoic Era.

Students think the Mesozoic Era is the oldest era. The correct order from oldest to most recent is: Palaeozoic → Mesozoic → Cenozoic.

Students confuse Pleistocene and Holocene. Pleistocene = Ice Ages + early humans (ended about 11,700 years ago). Holocene = current epoch from 11,700 years ago to present — all of recorded human history.

Quick Revision

Origin of Universe:

Big Bang Theory = origin of Universe; 13.8 billion years ago [NDA 2019-II]
Nebular Hypothesis = origin of Solar System (Kant and Laplace)
Planetesimal Hypothesis = Chamberlin; small bodies collided to form planets

Age of Earth:

Earth = 4.6 billion years old
Determined by radiometric dating (uranium-lead decay)

Geological Time Scale — Major Eras (oldest to newest):

Era	Time	Key Life Form
Palaeozoic	541–252 Ma	Marine invertebrates → fish → reptiles
Mesozoic	252–66 Ma	Dinosaurs (Age of Reptiles)
Cenozoic	66 Ma–present	Mammals → Humans (Age of Mammals)

Key Periods and Epochs:

Cambrian = first complex invertebrates
Permian = mass extinction ends Palaeozoic
Triassic = dinosaurs first appeared
Cretaceous = dinosaurs went extinct (end of Mesozoic; 66 Ma)
Palaeocene = early mammals spread
Pleistocene = Ice Ages; early humans appeared; Karewas of Kashmir formed
Holocene = current epoch; all recorded history

Palaeoclimatology = study of ancient climates [NDA 2022-II]

Chapter 3 complete. Shall I proceed with Chapter 4 — Motion of the Earth?

Yes

NDA · GEOGRAPHY
World Geography — Chapter 4
Motion of the Earth

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.

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]

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.

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]

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]

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]

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]

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]

Previous Year Questions (NDA)

Question 1 [NDA 2008-II]
Which one of the following pairs is NOT correctly matched?

Question 2 [NDA 2008-II]
Foucault experiment is proof of which one of the following?
(a) Revolution of Earth
(b) Rotation of Earth
(c) Rotation of Moon
(d) Revolution of Moon

Question 4 [NDA 2009-II]
Consider the following statements:

The vernal equinox falls on March 21.
On equinox, the Sun is directly overhead at the equator.
The changes in day length on equinox result from changes in the tilt of the Earth.

Which of the statements given above is/are correct?
(a) 1 and 2
(b) 1 only
(c) 1, 2 and 3
(d) 2 and 3

Question 5 [NDA 2009-II]
Consider the following statements:

The Earth’s rotation axis is not at 90° to its orbital plane.
The Earth’s rotation axis is inclined at 23.5°.

Which of the statements given above is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Question 6 [NDA 2010-I]
Consider the following statements:

Lunar eclipse takes place when the Earth comes directly between the Sun and the Moon.
Solar eclipse happens when the Moon comes directly between the Sun and the Earth.

Which of the statements given above is/are correct?
(a) 1, 2 and 3
(b) 3 and 4
(c) 1 and 2
(d) 2 only

Question 7 [NDA 2010-II]
Consider the following statements:
I. The tropical year is shorter than the sidereal year.
II. The solar day is longer than the sidereal day.

Which of the statements given above is/are correct?
(a) I only
(b) II only
(c) Both I and II
(d) Neither I nor II

Question 10 [NDA 2015-II]
According to Geo-scientists, the shape of the Earth is:

Round
Spherical
Close to that of a sphere
An oblate ellipsoid

Select the correct answer:
(a) 2, 3 and 4
(b) 1, 2 and 3
(c) 1 and 2 only
(d) 3 and 4 only

Question 11 [NDA 2016-I]
Which of the following statements in the context of the Northern Hemisphere is/are correct?

Vernal equinox occurs on March 21.
Summer solstice occurs on December 22.
Autumnal equinox occurs on September 23.
Winter solstice occurs on June 21.

(a) 1 only
(b) 1 and 3 only
(c) 2 and 4 only
(d) 1, 2 and 3 only

Question 13 [NDA 2017-II]
The shortest day length that occurs in the Northern Hemisphere is on:
(a) 21st March
(b) 23rd September
(c) 22nd November
(d) 22nd December

Answer: (d) 22nd December
December 22 is the winter solstice in the Northern Hemisphere — the day when the Sun is at its lowest point in the sky and the day is shortest. The North Pole experiences 24 hours of darkness on this day.

Question 14 [NDA 2017-II]
Which one of the following statements about a satellite orbiting around the Earth is correct?
(a) Satellite is kept in orbit by remote control from ground station
(b) Satellite is kept in orbit by retro-rocket and solar energy
(c) Satellite requires energy from solar panels and solid fuels for orbiting
(d) Satellite does not require any energy for orbiting

Answer: (d) Satellite does not require any energy for orbiting
A satellite in stable orbit is in continuous free fall. It falls toward Earth but also moves forward fast enough to keep missing the Earth. No fuel is needed to maintain this orbit — energy is needed only to change or correct the orbit.

Question 15 [NDA 2019-I]
Which one of the following is/are environmental effects of Rotation of the Earth?

Daily or diurnal rhythm in daylight and air temperature
Flow path of both air and water are turned consistently in a sideward direction
The movement of the tides

Select the correct answer:
(a) 1 and 2 only
(b) 1 and 3 only
(c) 1, 2 and 3
(d) 3 only

Answer: (a) 1 and 2 only
The daily rhythm of daylight and temperature (Statement 1) is caused by rotation. The Coriolis deflection of air and water (Statement 2) is also caused by rotation. Tides (Statement 3) are caused by the gravitational pull of the Moon and Sun — not by Earth’s rotation.

Question 16 [NDA 2022-I]
In a group discussion on shape and size of the Earth, three students stated the following:
Student 1: The shape of the Earth is basically an oblate spheroid.
Student 2: The polar diameter of the Earth is more than the equatorial diameter.
Student 3: The bulge along the equatorial region is due to revolution of the Earth.

Who among the above students is/are correct?
(a) Student 1 only
(b) Student 1 and Student 2 only
(c) Student 2 and Student 3 only
(d) Student 1, Student 2 and Student 3

Answer: (a) Student 1 only
Student 1 is correct — the Earth is an oblate spheroid. Student 2 is wrong — the equatorial diameter is MORE than the polar diameter, not less. Student 3 is wrong — the equatorial bulge is caused by ROTATION (centrifugal effect while spinning), not revolution.

Question 17 [NDA 2022-I]
Inhabitants are unaware of the speed of rotation of the Earth because:

The angular velocity is constant for each place on Earth’s surface
The atmosphere rotates with the Earth
There are no nearby objects, either stationary or moving at a rate different from that of the Earth

Which of the above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (d) 1, 2 and 3
All three reasons explain why we cannot feel the Earth rotating. Constant angular velocity means no sudden changes to alert our senses. The atmosphere rotating with us means no wind resistance. No external reference point means nothing to compare our movement against.

Question 18 [NDA 2022-II]
Which one of the following cities of the world would represent the greatest linear velocity of rotation of the Earth?
(a) Kampala, Uganda
(b) St. Petersburg, Russia
(c) Madrid, Spain
(d) Stockholm, Sweden

Answer: (a) Kampala, Uganda
Linear velocity of rotation is greatest at the equator and decreases toward the poles. Among these four cities, Kampala is closest to the equator (approximately 0° latitude) and therefore has the greatest linear velocity of rotation.

Question 19 [NDA 2025-I]
Which one of the following latitudes will experience a minimum angle of the Sun’s rays when it is Summer Solstice in the Northern Hemisphere?
(a) Arctic Circle
(b) Equator
(c) Tropic of Cancer
(d) Tropic of Capricorn

Answer: (d) Tropic of Capricorn
On the summer solstice in the Northern Hemisphere, the Sun is directly overhead at the Tropic of Cancer (23.5°N) — its maximum angle there. The Tropic of Capricorn (23.5°S) is in the Southern Hemisphere where it is winter — the Sun’s angle there is at its minimum for the year.

Motion of the Earth – NDA Geography PYQs

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