Tag Archives: Alexandria

Good news for canaries

Ptolemy published a book titled Geography which gave the latitude and longitude for over 8,000 places.

Ptolemy’s map* shows all of Earth he knew back then: Europe, the north coast of Africa, India and most of Asia. The Prime Meridian of Ptolemy’s map runs through the Canary Islands just west of Africa. Everything else is east.

As mapmaking took off as a business in the Age of Exploration, mapmakers in every little country, state and principality used a different Prime Meridian. You can see how this might get confusing.



* Maybe Ptolemy drew this map, maybe he didn’t. The locations on the map, though, are based on their coordinates given in Ptolemy’s book.

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

Eratosthanes and longitude

Old-time tv newsrooms had clocks on the wall set to local times of the big cities.

How did Eratosthanes or Ptolemy determine where the longitude lines should go? I got this from the History Stack Exchange site:

Longitude is calculated by comparing the elevation of an astronomical object to the pre-calculated (or observed) elevation of the same object at a reference location at the precisely simultaneous moment in time. Everything in the sky rotates once around that vast celestial sphere every 24 hours, so the more precisely one can establish simultaneity the more precise one’s measurement of longitude will be.

Whew! In other words: 2 people standing in 2 different places can measure the height in the sky of the moon, or the Sun, or the North Star to figure out how far east or west they are from each other. BUT—the measurement must be taken at exactly the same moment. Eratosthanes figured a way to find longitude without the measuring. Eratosthanes (in Alexandria) and an assistant (in someplace to the west—maybe Benghazi?) watched a lunar eclipse. They agreed to mark the exact local time the eclipse began. The local times won’t be the same, right? When it’s midnight in Benghazi, it will be 12:36 am in Alexandria. The difference in their times told Eratosthanes how many degrees apart from each other they were.

I’ll tell you how in the next post.

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


We don’t have Ptolemy’s original map. It’s one of those documents lost to history. Lucky for us there’s a copy made by monks centuries after Ptolemy shuffled off the ol’ mortal coil. I drew this version of Ptolemy’s map showing only the longitude lines. He drew them curved to give you the idea that the world is round.

So Eratosthanes and Ptolemy knew that the Earth was round. And it was relatively easy to locate the big latitudes like the Equator and Tropics. To find each degree of latitude was a little more difficult—you need to accurately sight the Sun or moon or a star. Eratosthanes located his latitude in Alexandria by measuring the angle of a shadow at noon of the solstice. After the astrolabe was invented mapmakers used that device to find their latitude.

Latitude lines are parallel, so each degree of latitude is about 69 miles from the next one. On the other hand, longitude lines travel from north to south poles. They join at the poles and are farthest apart from each other at the Equator. There are 360 degrees in a circle; the Earth is round; so there are 360 lines of longitude. If you’ve been following this history from a year ago, you remember that our pals the Sumerians came up with Base 60 method of counting—that’s why there are 360 degrees in a circle.

One of those longitude lines needs to be zero degrees. Which one? There’s no natural starting point for longitude (like the Equator is Point Zero for latitude). Eratosthanes started his system of longitude in Alexandria, where he lived. Alexandria—and every place due north or south of it—was zero degrees: the Alexandrian Prime Meridian. It was also 360 degrees, because it represented the beginning and end of a full circle.

So far, so good. Here’s the kicker, though: how do you measure longitude? How do you know where, say, 87° is on Earth in real life so you can put it on a map?


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

Hypatia, the lady philosopher/mathematician/astronomer

Sometimes writing about history is hard. At the beginning of this project, I’d envisioned a fun and slightly wacky tour through Western Civilization presented by a fun and slightly wacky uncle. The story of human beings is often violent and cruel, though. Human beings are flawed creatures. There are parts of civilization’s history that make me heartsick.

I want to tell you about a glorious and brilliant woman who lived in Alexandria, Egypt in the 4th century ad. Her name is Hypatia. She was the daughter of a philosopher and mathematician, and she became a philosopher and mathematician herself. Hypatia was lovely to look at, but her intellect was what made her irresistible. I became interested in her when I read that if she hadn’t outright invented the astrolabe, at the least she played an big part in developing it.

Hypatia helped her dad expand Ptolemy’s work on astronomy. She eventually did her own scholarly work on the stars and the geocentric model of the universe. It might have been she who came up with the idea of ‘flattening’ the spheres to create the star-map that is central to the astrolabe’s design (this is my own conjecture).

Hypatia was also a philosopher. Her philosophy was Neo-Platonism, which adapts Plato’s ideas about what makes the world tick. Hypatia was a gifted teacher. She often gave lectures in the agora (Greek for public square) about philosophy, astronomy, and her many intellectual interests. At that time Alexandria was boiling with religious trouble. Christians and Jews battled with each other and with philosophers who rejected a belief in God. The Roman Empire was in its last days, about to split into East and West. Politics absolutely played a part in the violent clashes between religion and philosophy.

I’m sorry and ashamed to tell you that a mob of Christians ambushed Hypatia, and murdered her. It’s nearly impossible for me to believe that these were followers of Jesus. But it happened. They behaved as a mob—the very opposite of civilization—and murdered a woman for promoting a philosophy that rejects Christianity.

You can read more about Hypatia here and here. There’s a movie about her, too.  Here’s a review—caution: some violence.  (I have a quibble with History Buff’s conclusion that, because of Christianity, scholarship was discouraged in the Dark Ages. Beginning with Charlemagne, the Holy Roman Empire fostered a culture of learning that was encouraged in the monasteries. Monks copied books by hand—the books from the Greek philosophers that had been translated into Arabic, which the monks then translated into Latin. See How The Irish Saved Civilization by Thomas Cahill.)

The well at Syene

So how did Eratosthenes figure out how big around the Earth is? Here’s how:

First, he assumed the Earth is round like a ball.

Well at Syene.

Second, he knew several things about a little town called Syene. It is 5,000 stadia (575 miles or 925 kilometers) directly south of Alexandria. There was a well in the center of town, dug deep and plumb—straight toward the Earth’s center. At noon on June 21st, the Summer solstice, you could see the Sun reflected on the water way down in that well, which means the Sun is directly overhead.

Bird’s-eye view of the Sun’s reflection in the well at Syene.

That’s because Syene is located on the Tropic of Cancer. If you happen to be standing on the Tropic of Cancer and it’s noon where you are on June 21 you can draw a straight line from the center of the Earth, through the Tropic of Cancer, to the Sun.

A stadion is an ancient Greek unit of measurement—it’s 600 feet (an eighth of a mile). Stadion is singular (nominative, singular, second declension); stadia is plural (nominative, plural, second declension). 5,000 stadia = 575 miles or 925 kilometers. I hope I declenched in all the right spots. Many thanks to my Greek-scholar pals Jackie J., Michele J. & Joann W!