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The Cosmic Distance Ladder

Rung 1: Earth

Rung 1: Size of the Earth

Eratosthenes (~240 BC)

Historical Context

Eratosthenes heard that in the city of Syene, the sun shone directly down a well at noon on the summer solstice, casting no shadow. In his own city of Alexandria, a vertical rod cast a shadow.

The Mathematics

$$ C = \frac{360^\circ}{\theta} \times s $$
  • $C$: Circumference of the Earth
  • $\theta$: Shadow angle difference ($^\circ$)
  • $s$: Distance between cities (km)

Raw Data & Controls

Adjust the variables based on the Observatory simulation to match reality and calculate the result.

Your Calculation ($C$): 0 km

The Observatory

Click the glowing markers on the globe to inspect the locations