3.
The Earth’s diameter measured across the equatorial plane is about 40 km more than that
measured between the poles. This is because of:
a. excess ocean water at the equator
b. the rotation of the Earth
c. freezing of water at the Earth’s poles
d. the Moon’s gravitational pull
Earth’s Equatorial Bulge: Why Rotation Makes Equatorial Diameter Larger Than Polar
Earth’s equatorial diameter exceeds its polar diameter by about 43 km due to the planet’s rotation, which creates a centrifugal force causing an equatorial bulge. This makes Earth an oblate spheroid rather than a perfect sphere.
Correct Answer
The right choice is b. the rotation of the Earth. Rotation generates centrifugal force at the equator, pushing mass outward and flattening the poles over billions of years.
Option Analysis
-
a. excess ocean water at the equator: Ocean distribution follows Earth’s shape but does not cause the bulge; the solid Earth beneath is already oblate.
-
b. the rotation of the Earth: Centrifugal force from spinning (about 1670 km/h at equator) counteracts gravity, bulging the equator by ~21 km radius difference.
-
c. freezing of water at the Earth’s poles: Ice formation affects sea level slightly but has negligible impact on planetary diameter compared to rotational forces.
-
d. the Moon’s gravitational pull: Tidal forces cause minor deformations (meters), far smaller than the 43 km rotational effect.
Why Rotation Causes the Bulge
Earth rotates once daily on its axis, producing centrifugal force strongest at the equator where speed peaks. This force equals gravity there, displacing mass outward while poles remain unaffected. Precise measurements confirm equatorial diameter at 12,756 km and polar at 12,714 km. The effect aligns with Newton’s predictions, verified by 18th-century expeditions.
