Eratosthenes is a giant for me.  Let me explain why.  Just yesterday we received a generous direct mail offer of a trip around the world if we would subscribe to a new credit card. I am sure that’s what they meant, because they promised 25,000 miles on our favorite airline.

I immediately opened our atlas to plan a possible itinerary with our new found gift. That’s when I realized the brilliance of this scheme. You see, leaving O’Hare Field and traveling in an easterly direction for 25,000 miles would mean I would be landing at, yes, O’Hare Field.  No need to leave at all!

Which brings me to the real point: 25,000 is a darn nice number to describe the earth’s circumference. It’s easy to remember, and best of all, it’s miles, not kilometers. Or hectares, or microns or centipedes.

Of course, the ancient Greeks measured the earth’s circumference in “stadia”. Without getting wordy, let’s just say that the measure was related to athletic competitions and stadiums, and was around 500 feet. For instance, “I bet that minotaur will eat the slave before he’s run 500 feet around the stadium.”

Anyway. What is fascinating is that Eratosthenes calculated the earth’s circumference back in 240BC.

One summer solstice, at high noon, he was staring down a well in Aswan, Egypt.

For you geography buffs, Aswan is located on the Tropic of Cancer. The sun was directly overhead, and he noted that there was no shadow on the walls of the well.

The germ of an idea came to him.   He already knew the earth was round, and concluded that a well several thousand “stadia” away would have a shadow, due to the curvature of the earth. So a year later, at noon on summer solstice, he told an associate to get to the well in Alexandria which was way to the north.

Sure enough, the well cast a prominent shadow, and in fact, it was 7.2 degrees off perpendicular.

Now stick with me on this. Alexandria was 500 miles to the north, or a lot of stadia for you classicists. Because that distance created a 7.2 degree tilt, a full circle of 360 degrees would be 50 times as much (50 x 7.2), or 25,000 miles for Eratosthenes. Turns out he was only off by 2%!

And 2,300 years later– we still get lost going to the post office with GPS.

The icing on the cake here is that Eratosthenes knew there were exactly 5,000 “stadia” between the two cities. How? Because he had measured this countless times by riding a camel between them.

You don’t see that kind of persistence any more. Nor such compassion for his ride. Writing his memoirs, he confessed “I needed to give Falafel a break after these journeys, so I would dismount, and go on foot outside the city gates.”

It turns out Eratosthenes preferred to speak in miles, too, explaining: “I’d walk a mile for a camel.”

So: I am back to my mailbox looking for a new offer. I have not given up on the 25,000-mile pitch. But will I need a saddle?