What is the Earth's diameter?

• A. 12,700 km
• B. 1,270 km
• C. 127,000 km
• D. 7,900 km

Answer: (A)

The diameter is the distance across Earth through its center. For Earth, this is about 12,742 km on average. Using the commonly cited mean radius of roughly 6,371 km, doubling gives about 12,742 km, which is why a rounded figure of 12,700 km fits well. In reality, the equatorial diameter is about 12,756 km and the polar diameter about 12,714 km, so 12,700 km is a reasonable approximation.

The other numbers don’t match Earth's size: 1,270 km is far too small for a planet; 7,900 km is also too small to be the planet’s diameter; 127,000 km is far too large (it would be more like several times the diameter of Earth and far exceed its circumference). The relationship between circumference and diameter, C ≈ πD, also supports D around 12,700 km since Earth’s equatorial circumference is about 40,000 km.