PLANETARY DIMENSIONS — Mean radius of Earth (order-of-magnitude recall). Approximately what is Earth’s mean radius rounded to the nearest hundred kilometres?

Introduction / Context:
Estimating Earth’s size is a common general-knowledge and science item. The mean radius is a simple benchmark that supports quick mental checks for circumference and surface area in many problems.

Given Data / Assumptions:

• We want the approximate mean radius (not diameter).
• Acceptable rounded value for exams is about 6,400 km.
• No precision beyond the hundred-kilometre scale is required.

Concept / Approach:
Earth is an oblate spheroid with equatorial radius ~6,378 km and polar radius ~6,357 km. The mean radius is ~6,371 km. Rounding to a memorable figure gives ~6,400 km, which is simple to use for mental calculations.

Step-by-Step Solution:
Recall: mean radius ≈ 6,371 km.Round to the nearest hundred → ≈ 6,400 km.Compare options; select the closest appropriate rounded value.

Verification / Alternative check:
Using circumference formula C = 2 * pi * R with R ≈ 6,371 km gives C ≈ 40,030 km, consistent with standard Earth metrics.

Why Other Options Are Wrong:
32,000 km — closer to Earth’s circumference, not radius.9,600 km — too large by ~50%.12,800 km — roughly Earth’s diameter, not radius.1,740 km — approximate radius of the Moon, not Earth.

Common Pitfalls:
Confusing radius and diameter or mixing Earth and lunar values. Always check whether the question asks for radius.

Final Answer:
6,400 km