NDA Geography · Earth in Space, Maps and Coordinates
Earth's Shape, Rotation and Motion
The Earth is a slightly squashed ball (an oblate spheroid) that spins on a tilted axis once a day and circles the Sun once a year — its rotation gives day and night, its revolution plus tilt gives the seasons.
Why this matters
The largest subtopic here (7 PYQs) and the source of the chapter's trickiest reasoning. Three pictures earn most marks: oblate spheroid (bulges at the Equator, flattened at the poles), rotation gives day/night while revolution + tilt gives seasons, and linear velocity is fastest at the Equator. Multi-statement 'which is/are correct' questions dominate, so know which effect comes from rotation and which from revolution.
Concept 1 of 7
The shape of the Earth — oblate spheroid
Intuition
The Earth is not a perfect sphere. Because it spins, the material is flung outward a little at the Equator, so the planet bulges at the middle and is flattened at the poles. This squashed-sphere shape is called an oblate spheroid (or geoid). The bulge means the equatorial diameter is the LARGER one and the polar diameter is the smaller one.
Definition
- The Earth's true shape is an oblate spheroid — a sphere bulging at the Equator and flattened at the poles.
- The equatorial diameter is greater than the polar diameter (by about 43 km). The polar diameter is NOT the bigger one.
- The equatorial bulge is caused by the Earth's rotation (its daily spin), NOT its revolution around the Sun.
- A precise model of this shape that follows mean sea level is called the geoid.
Worked example
A student claims: 'The Earth bulges at the Equator because it revolves around the Sun.' Correct the error.
- The bulge comes from the Earth spinning on its own axis, which flings material outward at the middle.
- Spinning on its own axis is ROTATION, not revolution.
- Revolution is the yearly trip around the Sun and does not cause the bulge.
Answer:The bulge is caused by ROTATION (the daily spin), not revolution.
Practice this conceptself-check · 3 quick reps
Try it yourself
Which is larger — the Earth's equatorial diameter or its polar diameter?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Name the true shape of the Earth.
- 2.Which diameter is greater, equatorial or polar?
- 3.The equatorial bulge is caused by rotation or revolution?
From the bank · past-year question
In a group discussion on shape and size of the Earth, three students stated the following points:
1. Student 1: The shape of the Earth is basically an oblate spheroid.
2. Student 2: The polar diameter of the Earth is more than the equatorial diameter.
3. Student 3: Bulge along the equatorial region is due to revolution of the Earth.
Who among the above students is/are correct?
[Q108 · Apr · 2022]
Polar diameter is NOT greater
A classic trap states 'the polar diameter is more than the equatorial diameter'. It is the other way round — the equatorial diameter is greater because the Earth bulges at the Equator.
Bulge = rotation, not revolution
Another trap blames the equatorial bulge on revolution. The bulge comes from the daily spin (rotation); revolution is the yearly orbit and is unrelated to the bulge.
Concept 2 of 7
Rotation — day, night and its effects
Intuition
Rotation is the Earth spinning on its own axis, once every 24 hours, from west to east. The half facing the Sun has day; the half facing away has night; the boundary is the terminator. Spin is also why winds and currents get deflected (the Coriolis effect) — but the tides are caused by the Moon and Sun's gravity, NOT by rotation.
Definition
Rotation = the Earth spinning on its axis once in ~24 hours (west to east). Its effects:
- Day and night — the fundamental effect; the lit half has day, the dark half night.
- Diurnal (daily) rhythm of daylight and air temperature.
- Deflection of moving air and water (the Coriolis effect) — winds and currents turn consistently sideways (right in the Northern Hemisphere, left in the Southern).
- Tides are NOT an effect of rotation — they are caused by the gravitational pull of the Moon and Sun. (Rotation only affects the *timing* of high tides at a place.)
Worked example
Of these three, which are direct effects of the Earth's rotation: (i) day and night, (ii) deflection of winds, (iii) the ocean tides?
- Day and night come straight from the spin — yes.
- Deflection of winds and currents (Coriolis) comes from the spin — yes.
- Tides come from the Moon and Sun's gravity, not from the spin — no.
Answer:(i) and (ii) are effects of rotation; (iii) tides are not.
Practice this conceptself-check · 3 quick reps
Try it yourself
In which direction does the Earth rotate?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Rotation produces which most basic effect?
- 2.Are tides an effect of rotation?
- 3.Name the sideways deflection of winds caused by spin.
From the bank · past-year question
Which of the following is/are environmental effects of Rotation of the Earth?
1. Daily or diurnal rhythm in daylight and air temperature
2. Flow path of both air and water are turned consistently in a sideward direction
3. The movement of the tides
Select the correct answer using the code given below:
[Q91 · Apr · 2019]
Tides are not caused by rotation
A 'pick the effects of rotation' question lists the tides as a tempting third option. Tides come from the gravitational pull of the Moon and Sun — rotation only fixes *when* the tide arrives, not the tide itself.
Concept 3 of 7
Why we do not feel the Earth spin
Intuition
The Earth's surface moves very fast, yet nobody feels it. Three reasons combine: the spin is steady (no jerks because the angular velocity is constant), the atmosphere turns along with us so there is no rushing wind, and there is no nearby fixed object to compare against — everything around us moves with the Earth.
Definition
We are unaware of the Earth's rotation because ALL THREE of these hold:
- The angular velocity is constant for every place — the spin is smooth, with no sudden change to feel.
- The atmosphere rotates with the Earth — so the air does not blow past us as a constant gale.
- There is no nearby stationary or differently-moving object to notice the motion against; everything visible moves with the Earth.
Worked example
Give one reason a passenger on the spinning Earth feels no motion, even though the surface moves at hundreds of metres per second.
- Compare with a smoothly-cruising train: you only feel speed when it changes or when something rushes past.
- The Earth's spin is steady (constant angular velocity) and the air moves with us, so neither cue is present.
Answer:Because the spin is steady and the atmosphere moves along with us, there is no felt motion (and no nearby fixed object to compare against).
Practice this concept2 quick reps
Practice — Level 1 (2 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Why is there no constant wind from the Earth's spin?
- 2.Is the angular velocity of rotation constant for a place?
From the bank · past-year question
Inhabitants are unaware of the speed of rotation of the planet Earth because
1. the angular velocity is constant for each place on the Earth's surface
2. the atmosphere rotates with the Earth
3. there are no nearby objects, either stationary or moving at a rate different from that of the Earth
Which of the above is/are the correct explanation(s)?
[Q117 · Apr · 2022]
Concept 4 of 7
Linear velocity of rotation is fastest at the Equator
Intuition
Every point on Earth completes one full spin in the same 24 hours, but points near the Equator have a much bigger circle to travel, so they move faster in metres per second. A place on the Equator has the greatest linear (rotational) velocity; a place near a pole barely moves; at the poles the linear velocity is zero. So the lower the latitude, the faster you are whipped around.
Definition
- Angular velocity (degrees per hour) is the SAME everywhere — one rotation per day.
- Linear velocity (distance per second) DEPENDS on latitude: it is maximum at the Equator (~1670 km/h) and falls to zero at the poles.
- So among several cities, the one nearest the Equator (lowest latitude) has the greatest linear velocity of rotation.
- Rule of thumb: lower latitude → bigger circle of travel → faster linear speed.
Worked example
Of these cities, which has the greatest linear velocity due to the Earth's rotation: Singapore (~1 deg N), Cairo (~30 deg N), Oslo (~60 deg N)?
- Linear velocity is largest at the lowest latitude (nearest the Equator).
- Singapore is almost on the Equator; Cairo and Oslo are far from it.
- So Singapore, the lowest-latitude city, is fastest.
Answer:Singapore — it lies nearest the Equator, so it has the greatest linear velocity.
Practice this conceptself-check · 3 quick reps
Try it yourself
What is the linear velocity of rotation at the North Pole?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Where on Earth is linear velocity of rotation greatest?
- 2.Linear velocity at the poles is?
- 3.Lower latitude means faster or slower linear velocity?
From the bank · past-year question
Which one of the following cities of the world would represent the greatest linear velocity of rotation of the Earth?
[Q93 · Sep · 2022]
Angular speed is equal — LINEAR speed is not
Don't confuse the two. Every place finishes one spin in 24 hours (angular velocity equal). But linear velocity (km/h on the ground) is largest at the Equator and zero at the poles. The city question is about *linear* velocity.
Concept 5 of 7
Where the Sun can be overhead — the Tropics
Intuition
The Sun is only ever directly overhead (rays perpendicular) between the Tropic of Cancer (23.5 deg N) and the Tropic of Capricorn (23.5 deg S). Any place poleward of 23.5 degrees never gets a vertical Sun. In India this matters: the Tropic of Cancer runs through the middle of the country, so states LYING ENTIRELY NORTH of it (like Bihar) never have a perpendicular Sun, while states the Tropic crosses or that lie south of it can.
Definition
- The Sun's rays are perpendicular (Sun directly overhead) ONLY within the tropics — between 23.5 deg N and 23.5 deg S.
- Anywhere with latitude greater than 23.5 degrees (north or south) NEVER gets a vertical Sun.
- In India the Tropic of Cancer (~23.5 deg N) passes through eight states. A state lying wholly north of it (e.g. Bihar, Manipur) never has the Sun overhead; states it crosses or that lie south of it do.
- This is a consequence of the Earth's 23.5-degree axial tilt combined with its revolution.
Worked example
Can the Sun ever be exactly overhead at New Delhi (~28.6 deg N)? Explain.
- The Sun is overhead only between 23.5 deg N and 23.5 deg S.
- New Delhi lies at 28.6 deg N, which is north of 23.5 deg N.
- So the Sun's rays can never be perpendicular there.
Answer:No — Delhi is north of the Tropic of Cancer, so the Sun is never directly overhead.
Practice this conceptself-check · 3 quick reps
Try it yourself
Between which two parallels can the Sun be directly overhead?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.The Sun is overhead only within which latitude band?
- 2.Can the Sun be overhead at 40 deg N?
- 3.Which tropic passes through India?
From the bank · past-year question
In which among the following states in India, the Sun rays are never perpendicular ?
1. Bihar 2. Chhattisgarh 3. Manipur 4. Rajasthan
Select the answer using the code given below :
[Q136 · Sep · 2024]
Tilt limits the overhead Sun to 23.5 degrees
Because the axis is tilted 23.5 degrees, the overhead point shifts only between the two Tropics over a year. Any latitude beyond 23.5 degrees never sees a perpendicular Sun — that is the key to the 'which state never has overhead Sun' question.
Concept 6 of 7
Which places the Equator passes through
Intuition
The Equator (0 deg latitude) crosses several countries, and the NDA likes asking which island or state it touches. For Indonesia, the Equator runs across Sumatra, Kalimantan (Borneo) and Sulawesi but JUST MISSES Java, which lies a little south of it. Picturing the Equator on the map separates the touched islands from the untouched one.
Definition
- The Equator is the 0 deg parallel, dividing the globe into the Northern and Southern Hemispheres.
- In Indonesia, the Equator passes through Sumatra, Kalimantan (Borneo) and Sulawesi, but does NOT touch Java (which lies south of the Equator).
- Knowing roughly where 0 degrees runs on a map is enough to answer 'which place is/is not on the Equator'.
Worked example
Among the Indonesian islands Sumatra, Java, Kalimantan and Sulawesi, which one does the Equator NOT cross?
- The Equator runs across the northern-central Indonesian islands.
- Sumatra, Kalimantan and Sulawesi are all crossed by it.
- Java sits a little south of the Equator and is missed.
Answer:Java is not touched by the Equator.
Practice this concept2 quick reps
Practice — Level 1 (2 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which Indonesian island is NOT crossed by the Equator?
- 2.The Equator marks what latitude?
From the bank · past-year question
Which one of the following land territories of Indonesia is touched by the Equator?
[Q112 · Sep · 2019]
Concept 7 of 7
Solstices, equinoxes and day length
Intuition
Because the Earth's axis is tilted as it revolves, day length changes through the year. The shortest day in the Northern Hemisphere is around 22 December (the winter solstice), when the North Pole leans away from the Sun. The longest is around 21 June (summer solstice). On the two equinoxes (around 21 March and 23 September) day and night are equal everywhere.
Definition
Key dates (Northern Hemisphere):
- 22 December — winter solstice: the shortest day (North Pole tilted away from the Sun). In the Southern Hemisphere this is the longest day.
- 21 June — summer solstice: the longest day (North Pole tilted toward the Sun).
- 21 March (spring) and 23 September (autumn) — equinoxes: day and night equal everywhere; the Sun is overhead at the Equator.
- These arise from the 23.5-degree axial tilt combined with revolution — not from rotation alone.
Worked example
On which date does the Northern Hemisphere have its longest day, and why?
- Day length is longest when that hemisphere leans most toward the Sun.
- The North Pole leans most toward the Sun around 21 June (summer solstice).
Answer:Around 21 June — the summer solstice, when the North Pole is tilted toward the Sun.
Practice this conceptself-check · 3 quick reps
Try it yourself
Which date marks the SHORTEST day in the Northern Hemisphere?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Shortest day in the Northern Hemisphere is on?
- 2.Longest day in the Northern Hemisphere is on?
- 3.On the equinoxes the Sun is overhead at the?
From the bank · past-year question
The shortest day length that occurs in the Northern hemisphere is on
[Q98 · Sep · 2017]
Don't pick an equinox for the shortest day
21 March and 23 September are equinoxes (equal day and night), not the shortest day. The shortest day in the Northern Hemisphere is the 22 December solstice.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Watch out for (6)
- Polar diameter is NOT greater→ The shape of the Earth — oblate spheroid
- Bulge = rotation, not revolution→ The shape of the Earth — oblate spheroid
- Tides are not caused by rotation→ Rotation — day, night and its effects
- Angular speed is equal — LINEAR speed is not→ Linear velocity of rotation is fastest at the Equator
- Tilt limits the overhead Sun to 23.5 degrees→ Where the Sun can be overhead — the Tropics
- Don't pick an equinox for the shortest day→ Solstices, equinoxes and day length
Drill every past-year question on this subtopic
7 questions from the bank — paginated, with cart and Word-export support.