Because the Earth is slightly flattened at the poles, its radius increases as one moves from the poles towards the:

The Earth is not a perfect sphere. It is slightly flattened at the poles and bulges at the Equator, so the distance from the centre to the surface (the radius) is greatest at the Equator.

Shape of the Earth:

• The Earth's shape is described as an oblate spheroid, or more precisely a geoid.
• The word "geoid" means "Earth-shaped", since the actual shape is irregular and unique.

Key measurements:

• Equatorial radius: about 6,378 km.
• Polar radius: about 6,357 km.
• The difference is about 21 km.
• The equatorial circumference is about 40,075 km, while the circumference through the poles is about 40,008 km.

Why the Earth bulges at the Equator:

• The Earth rotates on its axis once a day.
• Rotation produces a centrifugal effect that is strongest at the Equator, where the surface moves fastest, and zero at the poles.
• Over billions of years, this caused the Earth's material to bulge outward at the Equator and flatten at the poles.
• Isaac Newton predicted this flattening in the 17th century, and French survey expeditions in the 1730s confirmed it.

Effects of the Earth's shape:

• Gravity is slightly weaker at the Equator than at the poles, because the surface is farther from the centre and the centrifugal effect is greater.
• So, an object weighs slightly less at the Equator than at the poles.

Evidence that the Earth is round:

• Ships disappear hull-first over the horizon.
• The Earth casts a curved shadow on the Moon during a lunar eclipse.
• Photographs from space show the Earth as a sphere.

Why not the other options?

• ✗The radius does not stop increasing at the Tropic of Cancer; it continues to increase till the Equator.
• ✗The Arctic Circle is closer to the North Pole, where the radius is smaller.
• ✗The Prime Meridian is a line of longitude running from pole to pole, so the radius changes along it.