In 1666, the meridian arc measurements indicated the length was smaller towards the poles, implying the Earth is which shape?

• A. prolate
• B. oblate
• C. spherical
• D. ellipsoidal

Answer: (A)

The main idea is how the surface shape affects the distance covered along a meridian as you move from the equator toward the poles. If measurements show that the meridian arc becomes shorter toward the poles, that means the radius in the north–south direction is smaller at higher latitudes. An Earth that is flattened at the poles and bulges at the equator—a oblate spheroid—has a larger equatorial radius than polar radius. In such a shape, a degree of latitude near the equator spans a longer arc than a degree near the poles, so meridian arcs shrink as you approach the poles.

If the Earth were prolate (elongated at the poles), the polar radius would be larger and meridian arcs toward the poles would be longer, which isn’t what was observed. A spherical Earth would imply equal arc lengths per degree of latitude everywhere, which also doesn’t match the measurements. While an ellipsoid can describe the Earth, the specific pattern of shrinking meridian arcs toward the poles identifies flattening at the poles, i.e., an oblate spheroid.