So, How Big Is It?

Almost 2,500 years ago in ancient Greece, early scientists thought the shape of the world was-well, there were several ideas. One early thinker believed that Earth was a squat cylinder, like a drum, and that people lived on its disk-shaped top. Another conceived of Earth as a rectangle held up by compressed air. But it wasn't long before the Greeks figured out the world had to be round.

Why? When a ship approached over the horizon, they would see its sail first. Similarly, when people traveled north or south, new stars appeared above the horizon ahead and visible ones disappeared below the horizon behind. During an eclipse of the Moon, they could see the shadow of the Earth against the Moon-and its edge was always curved. Besides, the Sun and the Moon were round, so why not Earth?

Thus, educated people, at least, knew that Earth was round like a ball. But how big a ball? No one knew that. How could they? You couldn't measure the world with a ruler. And no one could travel around the world to see how big it was. (After all, there were monsters out there, weren't there? And didn't people say that the sea boiled near the equator? Nothing could live there.)

So no one knew how big Earth was, until Eratosthenes measured it.

Eratosthenes was a Greek living in Alexandria, Egypt. In 240 BC, he was appointed head of the city's great library, which was the center of learning at the time. Because he had many interests, such as mathematics, poetry, philosophy, and geography, Eratosthenes was nicknamed Pentathalos, which referred to athletes who competed in five events. It's possible he was named this from jealousy, however, to suggest that he knew a little about a lot but didn't have true ability in any one area.

But Eratosthenes knew enough to be interested when he heard a tale from travelers that on the 21st of June, the longest day of the year, at precisely midday, the Sun shone down a deep well in the town of Syene in upper Egypt, and that it cast no shadows. Eratosthenes had never seen such a shadowless noon in Alexandria. Why were there shadows in one place and not the other? That was easy to explain. It was because the Earth was round. When the Sun was directly overhead in Syene, of course its rays would strike objects at an angle and cast shadows at other places around the globe, such as farther north in Alexandria.

Maybe, Eratosthenes thought, those shadows could enable him to calculate the size of the Earth. He found out that a camel caravan needed 50 days to make the journey from Alexandria to Syene, and that camels usually traveled about 10 miles a day. Royal surveyors, who were trained to walk with equal steps, may also have paced off the distance for Eratosthenes.

So it was about 500 miles to Syene. But what part, or fraction, of the Earth's circumference was that? To find out, Eratosthenes had only to step outside his library in Alexandria at noon on the 21st of June, when he knew the Sun was at its highest and shining vertically down the well in Syene, and measure the angle of a shadow cast by the Sun.

Eratosthenes already knew the height of a stone obelisk in his courtyard, and by measuring the length of its shadow he learned that the Sun was shining down at an angle of 7.2 degrees. A circle has 360 degrees, so the angle was equal to about one-fiftieth of a circle. This meant that the circumference of the Earth was 50 times the distance from Alexandria to Syene, or about 25,000 miles. Which is about right.

Unfortunately, another Greek geographer, Ptolemy, came along around AD 150, and his geography was the one the world came to know. Ptolemy invented the idea of scale in mapping. (You know, one inch equals 200 miles. That sort of thing.) He was the first to locate north at the top of a map. And he showed how to use lines of latitude and longitude to mark a location on a map. But he thought the Earth was only 18,000 miles around. (After all, what did Pentathalos know?) Ptolemy sort of left out the Pacific Ocean and the Americas.

A bad mistake, but it might have had good consequences. Much later, in 1492, Christopher Columbus, influenced by Ptolemy's beliefs about the size of the Earth, decided it wouldn't be too hard to reach Asia by sailing west from Spain. The way Columbus figured it, he would reach India or Japan after little more than a month's journey. Instead, he hit America.

A few years later, Europeans crossed the Isthmus of Panama and saw the Pacific Ocean for the first time. But they still didn't think this ocean could be very big. It wasn't until Ferdinand Magellan sailed around the world, spending miserable months crossing the Pacific, that they found out. Magellan left Spain in 1519 with a crew of 265; three years later only 18 men came back. Although Magellan himself was killed in the Philippines, his voyage around the world was a success.

Finally, everyone got the idea. The world is round. And it's bigger than you think.