The earth

4.1.1 Is the earth round?

The earth from the spacecraft Apollo

Modern satellite pictures clearly show that the earth is round. To the ancient astronomers the shape of the earth was, however, not at all obvious, but they had both philosophical and experimental reasons for believing that it is round.

Greek philosophers often argued that it should be possible to reconstruct Nature, and its laws, on the basis of the premise that things exist and behave in the way that is 6most perfect\ The case for a round earth was made by Aristotle (340 BC), on a number of such 'philosophical' Earth. Courtesy of NASA. grounds.

4.1.2 Philosophical reasons why the earth should be round

Symmetry. A sphere is the most perfect geometrical figure. Nowadays, this might sound like a rather weak argument. One must remember, however, that symmetry in Nature and in its laws is still sought as a foundation to physical theories, often with considerable success.

Gravity. Aristotle postulated that the component pieces of the earth should seek their natural home at the centre. They are attracted towards the centre and therefore tend to compress into a round form. This reasoning seems to be a combination of both philosophical and physical. About 2000 years later Isaac Newton formulated his universal law of gravity in a precise mathematical form, but the fact that things fall obviously had to be justified since time immemorial.

General intuition — Zhang-Heng (Chinese astronomer, 78-139) The sky is like a hen's egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying in the centre.'

There seems to be no record of the reasoning behind this statement, either physical or philosophical. Nevertheless, it shows artistic expression, typical of the ancient Chinese civilization.

4.1.3 Experimental evidence that the earth is round Terrestrial measurement

Eratosthenes (~ 330 BC) knew that at noon on the first day of summer the sun was directly overhead at Syene (near today's Aswan Dam). Sunbeams shone directly down a deep vertical well, and were reflected straight up by the water at the bottom. At exactly the same time, at his home near Alexandria, he measured the sun's rays to be about 7.5 degrees away from the vertical. Eratosthenes knew the distance from Syene to Alexandria, which in modern terms is about 500 miles, almost directly due north. Hence he calculated the circumference of the earth, as described in Figure 4.1.

Astronomical measurement

Of course, the ancients were not in a position to travel into space in order to observe the earth 'as others see it'. However, they parallel light from Sun 360o subtends a complete circle parallel light from Sun 360o subtends a complete circle

Figure 4.1 The method Eratosthenes used to show that the earth is round.

could examine the shadow cast by the earth on the moon. In this way the moon could serve as a 'mirror' in which we could view ourselves — or at least examine our shadow! Let us first look at some properties of the moon and then come back to study the earth's shadow on its surface.

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