Tag Archives: Alexandrian Prime Meridian

Zero degrees in Alexandria

Eratosthanes figured out how to mark places on a map using latitude and longitude.

I know what you’re thinking: “Eratosthanes marked Benghazi 9 degrees west on a map. Nine degrees west from where?”

That’s a terrific question and I’m glad you asked it. When you measure longitude, you have to start at zero degrees. Everything else on that 360° circle is either east or west of 0°. Eratosthanes was the head librarian of the Great Library in Alexandria, so he used his home town as that starting point. It’s the Alexandrian Prime Meridian. The Prime Meridian is an imaginary line that extends from the North Pole to the South Pole. The other meridians—lines of longitude—are east or west of it.

Longitude doesn’t represent distance—there, I said it

In the last post I said I’d tell you how Eratosthanes knew how many degrees of longitude Benghazi is from Alexandria.

We know that lines or parallels of latitude are about 69 miles apart. Latitude measures degrees of two 90° quarter-circles, each starting at the Equator and ending at the poles. The Equator is 0° and the North & South poles are 90°.

Longitude measures time. Or to be more precise, longitude converts time into degrees of a 360° circle. “WHAT?” I hear you holler as you spring from your comfy chair. “Have you finally lost your marbles, Manders?”

Al-Biruni (973–1048), another one of those amazingly-accomplished scholars

Ptolemy’s geocentric vision of the universe reckoned that Earth doesn’t move—she is stationary while the heavens whirl around her. But a Persian scholar, Al-Biruni, thought that the Earth spins on her axis—just like the globe in your classroom. If that were true (and of course it is), you’ll realize that the Earth spins all the way around every day, every 24 hours.

I drew 24 sections on this sphere—for the 24 hours it takes for Earth to spin around on her axis. Each line is a meridian, a longitude line.

THAT CHANGES EVERYTHING! Mapmakers could divide the Earth’s surface into 24 units. Each unit would represent one hour. Each unit would also represent 15 degrees. Why? Because there are 360 degrees in a circle. 360 divided by 24 equals 15. Now I ask you, who was it who divided the day into two 12-hour halves? Who was it who came up with Base Sixty counting, which makes it so easy to divide 360 by 24? Who? WHO?

Thanks, Sumerians!

The Sumerians, that’s who! I love those guys!

Getting back to Eratosthanes’ experiment: if that lunar eclipse began at 12:36 am Alexandria time and midnight Benghazi time, Eratosthanes knew that Benghazi is 36 minutes west of Alexandria. The Earth rotates on her axis 360° every day, 15° every hour, and 1° every 4 minutes. So, 36 minutes difference in time from Alexandria to Benghazi ÷ 4 = 9 degrees of longitude. Thinking of a chunk of time as a chunk of a circle, Eratosthanes could confidently mark Benghazi’s longitude on a map.

https://www.britannica.com/biography/al-Biruni

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