Daily Archives: April 30, 2019

Plumb

A plumb line or plumb bob.

Last post I rattled on about how the well at Syene was plumb.

‘Plumb’ means straight up-and-down. Builders used to use a plumb line—a string with a weight tied onto it—to check that their walls were straight up-and-down. A plumb line will always point to the center of the Earth. Nowadays they use a spirit level. Builders who dug wells made sure that the hole they dug was plumb—pointing directly toward the Earth’s center.

Why was it so important to Eratosthenes that the well at Syene (a town located on the Tropic of Cancer) was pointing directly at the Earth’s center at noon on June 21st—the Summer solstice?

Back to the beginning of The Western Civ User’s Guide to Time & Space