Ages

Introduction

Summary

To estimate the Earth’s circumference, Eratosthenes calculated the length of a meridian arc by measuring the shadow cast by a vertical axis at noon on the summer solstice. In Syene (northern tropic), no shadow is cast whereas 800 kilometres further north, in Alexandria, the shadow is cast at an angle of 7.2° from the vertical, that is to say 1/50 of 360°. The circumference is therefore equal to 50 times the distance between the two cities.

Goals

• To illustrate the Eratosthenes's experiment: The measurement of Earth's circumference.

Features

Click and drag the obelisk along its terrestrial meridian.

Student sheet

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Tags

Eratosthenes radius earth astronomy shadow tropic meridian latitud longitud parallel