Fix the mistake: Structure of the Earth

The Shape and Size of Earth

The Earth on which we live is not a perfect sphere; it is shaped more like an oblate spheroid. Accurately understanding Earth's size and shape is the foundation of surveying, navigation, and all artificial-satellite orbit calculations.

Basic Knowledge

Because Earth rotates, centrifugal force makes it an oblate spheroid that bulges slightly toward the equator.
Major values:
・Equatorial radius: approximately 6378 km
・Polar radius: approximately 6357 km
・Mean radius: approximately 6371 km
・Flattening: approximately 1/298（the difference between the equator and poles is approximately 21 km）
・Surface area: approximately 5.1×10⁸ km²
・Volume: approximately 1.08×10¹² km³
・Mass: approximately 6.0×10²⁴ kg
・Mean density: approximately 5.5 g/cm³（because surface rocks are 2.7〜3.0 g/cm³, we can see that Earth's interior consists of much denser materials）
The first person to measure Earth's size was Eratosthenes of ancient Greece. In the 3rd century BCE, he estimated Earth's circumference from the difference in the Sun's altitude and the distance between Syene and Alexandria, achieving an accuracy within a few percent of the modern value.

Important Terms
Oblate spheroid（Earth's actual shape, bulging toward the equator）
Flattening（(equatorial radius−polar radius)/equatorial radius ≈ 1/298）
Equatorial radius（approximately 6378 km）
Mean density（approximately 5.5 g/cm³, about twice that of surface rocks）
Geoid（a surface extending mean sea level beneath the land; an equipotential surface perpendicular to the direction of gravity）

Further Exploration

There are several pieces of evidence that Earth is round. ① Earth's shadow is always circular during a lunar eclipse, ② ships disappear below the horizon, ③ the altitude of Polaris changes when moving north or south, and ④ photographs taken from artificial satellites.
Today, geodetic measurements using GPS satellites precisely determine Earth's shape, which is published as the International Terrestrial Reference Frame (ITRF).
Flattening depends on a planet's rotation speed and density distribution. Planets that rotate faster, such as Jupiter (1/15), have greater flattening.

Key Points
  • Earth is not a perfect sphere but an oblate spheroid
  • Equatorial radius (6378 km) > polar radius (6357 km)
  • Mean density 5.5 g/cm³ > surface rocks 2.7〜3.0 g/cm³
  • Flattening ≈ 1/298
  • Eratosthenes estimated Earth's size in the 3rd century BCE
  • Geoid = gravitational equipotential surface

Points to Note

① Flattening is very small (1/298), so on an everyday scale Earth can be treated as nearly spherical. ② A geoid and an oblate spheroid are different. The geoid is uneven and is used as the reference surface for elevation. ③ Do not mistakenly interpret the mean density of 5.5 g/cm³ as the density of surface rocks.

Practice

  1. What is Earth's equatorial radius approximately, in km?
  2. What is Earth's mean density approximately, in g/cm³? What can be learned by comparing it with surface rocks (approximately 2.7 g/cm³)?
  3. Choose the correct term describing Earth's shape. (① perfect sphere ② oblate spheroid ③ cylinder)

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