Every school kid learns that Italian explorer Christopher Columbus discovered the Americas in 1492. That Norsemen had colonized parts of North America in the 10th century apparently was unknown in Europe at that time.
Columbus successfully lobbied Spanish monarchs Queen Isabella and King Ferdinand II to finance his voyage west to the East Indies. The purpose was to facilitate trade by greatly shortening the travel distance to the Far East rather than sailing south around Africa, and then northeast.
On 3 August 1492, Columbus departed Palos de la Frontera on the southwestern corner of Spain on the carrack Santa Maria, accompanied by caravels Nina and Pinta. His first sailed southwest to the Canary Islands where his ships were restocked for the western voyage, leaving the port of San Sebastian de la Gomera on 6 September.
Columbus estimated his voyage would take about a month. This was based on his estimation that the circumference of Earth was about one half to three quarters of its actual size. By some accounts Columbus had convinced himself that Japan was only 3000 miles west of the Canaries.
But what is the actual circumference of Earth? Amazingly, its true size had been calculated fifteen centuries earlier, so to that fascinating story I now turn.
Eratosthenes of Cyrene (c.276-194 BCE) was chief librarian of the great Library of Alexandria in Egypt. He happened to read in one of the books (scrolls?) that on the day of the summer solstice (the longest day of the year in the northern hemisphere), June 21, the sun shone directly down a deep well, and columns cast no shadows, in Syene (modern Aswan), 5000 stadia (approx. 500 miles) to the south.
This gave Eratosthenes an idea. Assuming the Earth was spherical, he could do an experiment by which he could calculate its circumference.
Knowing that Alexandria was 5000 stadia in north of Syene, and that the sun was distant enough that its rays striking Earth were parallel all over, he figured that on the day of summer solstice, columns (or obelisks) in Alexandria should cast shadows.
So, on June 21 he measured the shadow of an obelisk at Alexandria and found it to be about seven degrees. See diagram:
From there it was a straightforward calculation. Since seven degrees was about 1/50th of the circumference of a circle, 5000 stadia should be 1/50th of Earth’s circumference. Fifty times 5000 then gave 250,000 stadia for Earth’s circumference. Eratosthenes published a figure of 252,000 stadia because that number is divisible by all whole numbers from 1 to 10 and would therefore make geographic calculations easier.
It turns out that Eratosthenes’ value of Earth’s meridian circumference is about one percent in error.
Back to Columbus.
In the early morning of 12 October, after a journey lasting five weeks and one day, a lookout on the Pinta spotted land, an island which he named San Salvador (in the Bahamas). The distance he had traveled from la Gomera to San Salvador was 7544 kilometers, or 4,073 nautical miles.
Columbus must have felt pride that his estimated distance to Japan was not far off.
But Columbus was lucky. What if the Americas had not been in his way? On his average course and speed, it looks like he would have landed, after a voyage of eighteen weeks, in the Philippines, 26,400 kilometers (14,254 nautical miles) from la Gomera.
If his destination had been five times more distant would Columbus and his intrepid sailors have become discouraged and turned back? Did they have provisions for four-and-a-half months? Fortunately for Columbus, his men (and us), that issue did not arise.
There’s an old saying “It’s better to be lucky than good.” Christopher Columbus was both.
Bob Moores retired from Black & Decker/DeWalt in 1999 after 36 years. He was the Director of Cordless Product Development at the time. He holds a mechanical engineering degree from Johns Hopkins University.