The Tropics

Here’s something interesting: the Tropic of Cancer and the Tropic of Capricorn don’t stay put. They move around a little bit. The Tropic of Cancer marks where the Sun will shine the longest in the northern hemisphere for this year—that is, the summer solstice. It’s at around 23-24 degrees north. On the summer solstice, June 21st-ish, the Sun will shine directly over the Tropic of Cancer.

The Tropic of Capricorn marks where the Sun will shine the longest in the southern hemisphere—that is, the winter solstice. It’s at around 23-24 degrees south. On the winter solstice, December 21st-ish, the Sun will shine directly over the Tropic of Capricorn.

Here’s kind of what they look like on a flat map.

The reason the Sun shines directly on the Tropic of Cancer in summer and the Tropic of Capricorn in winter is that our Earth is tilted as she revolves around the Sun. We learned that way back when we read about Eratosthanes.

It takes one year for Earth to orbit the Sun. When the Northern Hemisphere is tilted toward the Sun it’s Summer there. When the Southern Hemisphere is tilted toward the Sun it’s Summer there.

Latitude and Longitude

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

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!