And if you have a friend at the same longitude, you can even measure its circumference.
This year’s equinox, on March 19/20 of 2020 (longitude-dependent), is Earth’s earliest in 124 years.
At the moment of equinox, the Sun’s rays will strike the Earth perfectly perpendicular to its axis.
It’s also the perfect time to do an experiment that reveals the curvature of the Earth.
Wherever you are on Earth, find a perfectly vertical object and measure its height.
When the Sun reaches its highest point in the sky on the equinox, measure the length of that object’s shadow.
With a little math, you should be able to calculate the angle that the Sun makes with your vertical object.
On the equinox — and only on the equinox — that angle will be exactly equal to your latitude.
Someone who experiences the Sun’s highest point at the same exact moment will have an identical longitude.
The measured difference in angle will tell you your latitude difference, demonstrating the Earth’s curvature.
If you measure (or know) the angular and physical distance between both observers, a 360° extrapolation reveals the Earth’s circumference.
This experiment was first performed ~2300 years ago: by Eratosthenes in Egypt.
This first proved Earth’s roundness, centuries before the space age, Biruni, or even Columbus.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.