The circumference of the Earth and the Route towards the West

In the 3rd century BCE, Eratosthenes calculated the circumference of the Earth with remarkable precision.

He observed that, at the summer solstice, the sun shone directly on the bottom of a well in Syene and was therefore perpendicular to the Earth while, at exactly the same time, an obelisk cast a shadow in Alexandria.

By measuring the angle of the sun at Alexandria and then the distance between the two towns, Eratosthenes deduced that the circumference of the Earth was 250,000 stadia, or 39,375 km, which was a fairly accurate calculation for the time.  

In later centuries, other Greek geographers, including the most famous of them all Ptolemy, suggested a much lower figure for the circumference for our planet.

This under-estimation was adopted by 15th century map-makers, whose charts, based on information provided by Marco Polo, also exaggerated the size of Asia.

These two errors were reproduced in a map showing the route to the Indies, which had been drawn up by a Florentine astronomer and cartographer, Toscanelli. He sent this map to the King of Portugal, Alphonse V in 1474.

On the basis of this map, which places the coasts of Europe and Asia face to face, Toscanelli thought that the distance that navigators would have to travel to cross the Atlantic was approximately 10,000 km.

A legendary island, called Antilla, was shown at the same latitude as the Canaries and Cipangu, and was thought to offer a good staging place in the middle of the Atlantic Ocean.

This false vision of the world led Christopher Columbus to believe that he had reached islands close to Asia, whereas, in fact, he had discovered a new continent.

Cipangu: name given to modern Japan by Marco Polo