How ancient scholars calculated the size of our planet using only shadows and math

Countries & Cities · Published 29 September 2026 Swipe more like this →

In short: Long before satellites, Eratosthenes determined the Earth's circumference with remarkable precision using nothing but a gnomon, a well, and geometry. His work, alongside later efforts by medieval astronomers and French scientists, shaped our modern understanding of the globe and even defined the very units of measurement we use today.

Around 240 BC, Eratosthenes performed the first known scientific calculation of Earth's size in Ptolemaic Egypt. By comparing the shadow cast by a gnomon in Alexandria with the sun's position directly overhead in Syene at the summer solstice, he estimated the Earth's circumference to be 250,000 stadia. While the exact length of a stadion remains debated, his method—relying on the assumption that the two cities sat on the same meridian—demonstrated a sophisticated grasp of spherical geometry.

Centuries later, the quest for precision continued. In the 9th century, astronomers under Caliph Al-Ma'mun measured the distance between Tadmur and Raqqa to estimate the globe's size, while in 1037, Al-Biruni pioneered the use of trigonometry and dip angles to calculate the circumference from a single location. These efforts were vital for navigation, eventually leading to the 17th-century proposal of the nautical mile as one minute of latitude. By the 18th century, the French defined the metre as one ten-millionth of the distance from the pole to the equator.

Despite these advancements, historical errors persisted. Christopher Columbus famously relied on a lower estimate by Ptolemy, which led him to underestimate the distance to Asia and mistakenly believe he had reached the New World. Today, we know the equatorial circumference is 40,075 km and the polar circumference is 40,008 km. The slight discrepancy between the original metre definition and the actual polar measurement—a result of early miscalculations regarding Earth's flattening—serves as a reminder of the evolving nature of scientific precision.

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