Shape and Size of the Earth

Why This Topic Matters for NDA

The shape and size of the Earth is the very first chapter of physical geography, and the NDA General Ability Test treats it as guaranteed, low-effort marks. The facts here never change with current affairs, so once you memorise them they stay useful for every attempt.

Questions are usually direct: the term for the Earth's shape, the difference between equatorial and polar diameter, or a proof that the Earth is round. Because the wording is simple, candidates who have read this chapter score in seconds and save time for the tougher map and calculation questions later in the paper.

This topic also forms the foundation for almost every chapter that follows in physical geography. Latitudes and longitudes, the system of time zones, the seasons, day and night, and even ocean tides all depend on understanding that the Earth is a rotating, slightly flattened body. So the effort you put in here pays off many times over across the whole syllabus, which is why The Cavalier teaches it first and tests it repeatedly.

From Flat Earth to Round Earth

For much of history people believed the Earth was flat, because in daily life the ground looks level and the horizon seems to end in a straight line. The Greeks were the first to argue otherwise. Pythagoras suggested a spherical Earth on philosophical grounds, and later Aristotle gave physical evidence — he noticed that the Earth always casts a circular shadow on the Moon during a lunar eclipse.

Around 240 BCE the Greek scholar Eratosthenes went further and actually measured the Earth. He knew that at noon on the summer solstice the Sun shone straight down a well at Syene (modern Aswan), casting no shadow, while on the same day at Alexandria, far to the north, a vertical pole did cast a shadow. By measuring that shadow angle and knowing the distance between the two cities, he calculated the full circumference with remarkable accuracy for his time. This clever experiment is why he is often called the “father of geography”.

Why the Earth Is Not a Perfect Sphere

A perfect sphere is the same distance from its centre in every direction. The Earth is almost that, but not quite. As the Earth spins on its axis, the rotation produces a centrifugal effect that throws material outwards most strongly at the equator, where the spin speed is highest. Over billions of years this caused the planet to bulge at the equator and become slightly flattened at the poles.

This shape — a sphere squashed top and bottom — is called an oblate spheroid (also written oblate ellipsoid). It was the great physicist Isaac Newton who first predicted, from his laws of motion and gravitation, that a rotating Earth must be flattened at the poles. Later precise surveys confirmed he was right.

The Geoid - Earth's Real Shape

Even “oblate spheroid” is a simplification. The real surface of the Earth has mountains, ocean trenches and uneven distribution of mass, so gravity is not exactly the same everywhere. The true mathematical shape of the Earth is therefore named after the planet itself: the geoid, meaning literally “Earth-shaped”.

The geoid is defined as the shape the Earth's surface would take if the oceans were free to flow under gravity alone, ignoring tides and winds, and extended in imaginary channels through the continents. It is a slightly irregular, bumpy surface — the reference level from which heights and ocean depths are measured. So we have three levels of accuracy: a rough sphere, a better oblate spheroid, and the most accurate geoid.

An important point for the exam is that the geoid does not follow the visible land surface. Where there is extra mass underground — for example a dense mountain range — gravity is a little stronger and the geoid rises slightly; where mass is missing, it dips. These bumps are tiny compared with the size of the planet, but they matter for precise surveying, for sea-level measurement and for the satellite navigation systems we use today. This is why “mean sea level”, the zero from which the height of mountains like Everest is measured, is really a point on the geoid.

The Key Dimensions of the Earth

NDA frequently asks for the actual numbers, so learn the rounded values. Because of the equatorial bulge, the distance from the centre to the equator is greater than the distance to the poles.

The equatorial radius is about 21 km longer than the polar radius. That difference may sound small against a 6,000 km radius, but it is exactly the equatorial bulge that proves the Earth is oblate. About 71% of the surface is water and roughly 29% is land.

Proofs That the Earth Is Round

Several everyday observations show the Earth is curved rather than flat. These proofs are a popular question type, so learn at least four.

1. Ships on the horizon: As a ship sails away, its hull disappears before its mast, and an approaching ship's mast is seen first. On a flat Earth the whole ship would simply shrink evenly.
2. Circular horizon: From a height — a hill or an aircraft — the horizon always appears as a curved circle, and the circle widens as you go higher.
3. Lunar eclipse: The Earth's shadow on the Moon is always circular. Only a sphere casts a circular shadow from every angle.
4. Circumnavigation: A traveller moving steadily in one direction eventually returns to the starting point, as Magellan's expedition first proved.
5. Satellite and astronaut images: Modern photographs from space show the Earth directly as a round, blue planet.

How Rotation Shapes the Planet

The link between spin and shape is worth understanding, not just memorising. A point on the equator travels a full circle of about 40,000 km in 24 hours, so it moves at over 1,600 km/h. A point near the poles travels almost no distance in the same time. This big difference in speed means the centrifugal effect is strongest at the equator and almost zero at the poles.

The result is that surface material is pushed outwards most at the equator, creating the bulge, while the poles stay closer to the centre. If the Earth spun much faster, the bulge would be larger; if it did not spin at all, gravity alone would pull it into a near-perfect sphere. This is why fast-spinning planets like Jupiter and Saturn are even more visibly flattened than the Earth.

This rotation effect also has a small but real impact on gravity. Because a person standing at the equator is both farther from the Earth's centre and partly thrown outward by the spin, the value of acceleration due to gravity (g) is slightly less at the equator than at the poles. The same object therefore weighs a tiny bit more at the poles than at the equator. NDA questions sometimes connect Earth's shape to this difference in g, so it is worth remembering the cause-and-effect chain: rotation → equatorial bulge → greater distance from centre → weaker gravity at the equator.

Worked Example

This single calculation explains the equatorial bulge in a number you can state in the exam. Notice the diameter difference (about 42 km) is exactly twice the radius difference (about 21 km).

Common Mistakes to Avoid

Quick Fact Sheet

Lock these one-liners into memory; each one has appeared in some form in NDA papers.

Previous-Year Style Question

Quick Revision

Read this recap a couple of times before the exam and Earth-shape questions become certain marks.