THE SIZE OF THE EARTH

by Knut Jørgen Røed Ødegaard

What is the size of our planet? In order to answer that question one must know that the Earth is a globe (almost spherical). During a total eclipse of the Moon this is evident. Before and after the total phase, the edge of the shadow of the Earth is seen - it is circular!

The size of the Earth was determined already more than 2000 years ago. This is quite impressive, but was due to a clever observation. The director of the famous library in Alexandria in Egypt, the geographer Eratosthenes (about 276-195 BC.) did the historic achievement.

Eratosthenes had learned a fascinating fact about the city of Syene in southern Egypt, not far from Aswan: When the Sun was at its highest in the sky in this city on the longest day of the year (today we call it summer solstice), the Sun did not cast any shadows. Actually, the Sun shined into deep wells. The story says that nobody dared to stare into the wells since they could become blinded by the intense light reflected by the water deep down.

In Syene the Sun therefore had to be in the zenith at this time. In Alexandria, further north in Egypt, Eratosthenes knew from his own experience that the shadows at the same time was equivalent to the Sun being one 50th of a circle away from zenith.

The distance from Syene to Alexandria had to be one 50th of the circumference of the Earth.

He could not measure the distance between the cities directly, but estimates of people travelling between the cities and the time they spent, gave a distance of about 5000 stadions. The circumference of the Earth therefore had to be 250 000 stadions, and the radius about one third of this.

Today we don't know the exact length one one stadion, but many sources claim that 250 000 stadions correspond to 39 900 kilometers. This would be astoundingly close to the correct value which is 40 074 kilometers!

Anyway, the method is correct and can be applied even today to estimate the size of our Earth.

This is how Eratosthenes measured the size of the Earth more than 2000 years ago. The difference in the solar altitude between the Egyptian cities Syene and Alexandria meant that the distance between them had to be one 50th of the circumference of a circle, or 360/50 degrees. When Eratosthenes got an estimate of the distance between the cities, he could easily calculate an astoundingly accurate value for the size of the Earth. Illustration: Astronomy.no

CONTACT INFORMATION - PRESS CONTACT