The Earth. Size. Shape.

Task. Scan the text and compare the early measurements of the Earth’s circumference made by Aristotle, Archimedes and Eratosthenes. Explain what gives the Earth a spherical shape.

Size

The ancient Egyptians saw the universe as a great box, with Egypt in the centre of its long, narrow floor. The top of the box was the sky, from which lamps were suspended by means of ropes. These were the stars. Other lamps, which were carried in heavenly boats, travelled about the sky and appeared as planets. They thought the Milky Way was the equivalent of the Nile, and the regions through which it flowed were where dead Egyptians lived.

As time went on, people began to put together a remarkably accurate picture of the earth and the solar system. As early as the fifth century B.C., Parmenides declared that the earth was a sphere. It is probable that he realized this from listening to travellers. These discovered that, when they went north, a greater number of stars remained above the horizon all night. They also realized that, when they went south, they could see other stars (for instance, Canopus, which can not be seen from Greece). The early travellers also reported that the length of the day changed with what we now call latitude. This was rather difficult to explain in terms of a flat earth.

In time, when the ancient Greeks accepted that the earth was round, attempts were made to estimate its size. Aristotle quotes 400,000 stadia for the circumference. This is much too big. He does not say where he got this figure from. Probably he took it from the earlier work of Eudoxus, a mathematician and astronomer. Archimedes later gave the circumference as 300,000 stadia. This is better, though still 20 percent in error.

Eratosthenes made the best of the early measurements of the earth’s circumference. He worked in the great library at Alexandria. He knew that, at Syene, which was due south of Alexandria, the sun was directly overhead at midday on the first day of summer. On the first day of summer in 250 B.C., he carefully measured the extent to which the sun’s rays slanted away from the vertical at midday in Alexandria. He found that this angle was 1/50 of a complete circle, of a little over 7º. Since the distance from Syene to Alexandria was 5,000 stadia, the circumference of the earth, corresponding to a full circle of 360º, must be 50 times 5,000, or 250,000 stadia.

How long is a stadium? There were several different stadia in use in the ancient world. Eratosthenes probably used the stadium of 517 ft. This means a circumference of 24,500 mi. This is not far from more recent calculations of 24,860 mi. In round numbers, the earth's radius is 4,000 mi, the same as 6,400 km.

Shape

Why is the earth round? Why does it not look like an egg or a pyramid?

We can understand the shape of the earth by considering the pressures beneath the earth's surface. Pressure in water is familiar enough: a dam must be much thicker at the bottom than at the top in order to withstand the greater pressure. A submarine can descend only a few hundred feet under the surface of the sea. Below that its hull will collapse. These forces are due to the weight of the overlying liquid. This in turn is a consequence of the earth’s gravitational pull. With increasing distance below the surface of the earth’s pressures quickly become enormous. No material can resist these pressures without flowing in response. This means that one part of the earth cannot project outward very much farther than other parts; if it did, the pressure under it would be greater than under surrounding regions. The rock beneath the bump would then flow out to the sides until the pressures were equalized.

Thus gravity is what gives the earth a spherical shape and keeps all parts of its surface the same distance from the centre. Such minor irregularities as mountains and ocean basins do not greatly disturb the pressure balance, but no large protuberance can exist.

The Earth

Task. Scan the text. Choose the one best alternative to each question following it. Answer all the questions on the basis of what is stated or implied in the text.