Measuring the Earth - Physics Demonstrations - Measuring the Earth | Physics Undergraduate Labs

Purpose

Recreates Eratosthenes' method for measuring Earth's circumference using a globe, a light source, and two magnets. Students use shadow lengths and proportional reasoning to calculate the globe's circumference, then verify by direct measurement.

Close-up of a dark globe with white grid lines. Two small red-and-black magnets stand on a meridian, separated by a white measuring tape. The upper magnet casts a visible shadow.

Hand wrapping a flexible measuring tape around the equator of a large dark globe with white grid lines

Figure 1: Two magnets on the globe along a meridian, separated by 10 cm. The lower magnet casts no shadow (simulating Syene); the upper magnet casts a measurable shadow (simulating Alexandria).

Figure 2: Verification: wrap a measuring tape around the globe to check the calculated circumference against direct measurement.

Surveyor's Measuring Reel

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