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How Was Earth's Size Measured?

Published in Earth Measurement 2 mins read

Early estimations of Earth's size were remarkably accurate, considering the limited technology available. The most famous calculation was performed by Eratosthenes, a Greek scholar living in Egypt around 240 BC. He cleverly used basic geometry and readily observable phenomena to determine the Earth's circumference.

Eratosthenes' Method: A Stroke of Genius

Eratosthenes knew that at noon on the summer solstice, the sun shone directly down a well in Syene (modern-day Aswan), Egypt. This meant the sun was directly overhead. However, at the same time in Alexandria, a city north of Syene, the sun cast a shadow. By measuring the angle of this shadow and knowing the distance between Syene and Alexandria, Eratosthenes could calculate the Earth's circumference. He used the following logic:

  • The angle of the shadow in Alexandria represented a fraction of a circle. This fraction corresponded to the angular distance between Alexandria and Syene.
  • The distance between Alexandria and Syene represented the arc length of that fraction of the circle.
  • By applying simple proportions, he calculated the Earth's total circumference.

While the exact distance between the cities was estimated (many scholars believe Eratosthenes measured the distance by pacing and counting paces from Syene to Alexandria), his calculated circumference was remarkably close to the modern-day measurement.

Modern Methods

Today, measuring Earth's size is far more precise. Techniques include:

  • Satellite measurements: Satellites orbiting Earth provide extremely accurate data on Earth's shape and dimensions.
  • Global Positioning System (GPS): GPS uses a network of satellites to pinpoint locations on Earth, indirectly contributing to our understanding of Earth's size and shape.
  • Geodetic surveys: These ground-based surveys utilize advanced instruments to precisely measure distances and elevations across large areas.

Many different methods have been used throughout history to determine the Earth’s size, from early estimations using simple geometry and observation by Eratosthenes to modern, highly sophisticated techniques using satellites and GPS technology. The accuracy has increased dramatically over time, but the fundamental principles remain the same—measuring angles, distances, and applying geometrical principles.