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As the Earth turns  @psychohistorian.org

As the Earth turns

posted: 3542 days ago, on Saturday, 2009 Mar 07 at 07:07
tags: astronomy.

During the day, you don't really notice it, and at night only the slow shifting of the stars gives a subtle clue that our planet is rotating on its own axis. But in the twilight its more obvious just how fast things are happening.

At dawn, for example, a host of stars can still be seen. But just 15 minutes later, almost all the stars are gone, swamped by the Sun's light. Similarly, at the beginning of dusk, only a single star can be made out, but within minutes, the heaven's lights are visible.

In the olden days it wasn't at all obvious that the Earth was turning, or even that it was a sphere. In the Bible, for example, the Earth is described as being a flat disc, in line with the author's understanding of matters astronomical. This is a reasonable deduction, because on human scales, the Earth is indeed shaped like a pancake.

That our world is not flat is slightly more difficult to see. Once, on the naval vessel SAS Tafelberg headed half-way to the south pole, I spent many evenings star gazing from the catwalk outside the bridge, surrounded by black. And during the day, I'd be surrounded by blue – the wide ocean all-around, the sky above, with the subtle curve of the horizon joining the two.

One of the young crew members (coincidentally dressed in bright blue overalls) speculated that the sky was blue because it reflected the colour of the ocean. In that case, I said, I suppose the sky above Sutherland would be butterscotch.

The sky is blue because blue light, coming from the Sun, is scattered by nitrogen and oxygen molecules more strongly than other colours. Red, orange, yellow and green light from the Sun penetrates the atmosphere without much scatter. But a blue light beam behaves much like the shiny ball in one of those old pinball machines – it bounces around repeatedly before coming down to Earth. If you look up in the direction of the Sun you'll see precious little blue light coming directly from there – most of it is scattered elsewhere. Thus, if you look elsewhere, you'll see blue light. Ergo, a blue sky.

On the other hand, the sea is blue because water absorbs the other colours (longer wavelengths) a lot better, and its mostly the blue (shorter wavelength) photons that make it out.

Another clue that the Earth is a sphere, and not a pancake, is that the sky is red at dawn and at dusk. The atmosphere follows the curvature of the Earth, and when the Sun is near the horizon its seen through a lot more atmosphere. Light from the Sun thus has to struggle through a great deal more atmosphere to get to us. Not only blue light, but also green, yellow and orange light is scattered away, leaving only the flimsy red light to penetrate and reach our eyes. In fact, if the Earth wasn't spherical, there would be no dramatic sunsets.

In the traditional cosmography of many African peoples, the sky is a solid dome (some say made of blue rock) that rests on the Earth and upon which the Sun moves. In Zulu tales, the Sun shines through tiny holes in the rock, causing the stars. This sky dome is typically not seen to be very high, with the Sun and Moon perhaps as big as a large dish.

Compare this with the traditional Greek view, that tells of the massive Atlas holding the heavens up on his shoulders. At one time, the Greeks thought the Earth ended just on the other side of the Mediterranean ocean, where the land of Atlantis could be found. Other Greek thinkers held that the Earth was a wooden disk adrift in a large ocean.

By the time of Aristotle (b. 384 BCE) most Greeks accepted that the Earth was neither flat nor round, but spherical. He had pointed out that as one moves northward or southward, the positions of the stars changed. From this simple observation one can conclude that the Earth has to be a sphere.

Aristotle got it wrong, however, when he thought that the Earth stood still, and the Sun, Moon and stars moved around it.

Two years after his death, Aristarchus of Samos was born, and he would argue that the Earth was in no way stationary: it turns on its own axis and moves around the Sun.

For the next two thousand years, his idea was opposed. It took the hard work of Copernicus (b. 1473), Galileo (b. 1564) and Kepler (b. 1571) to show that the Earth isn't round but spherical, and moves around the Sun. And that it turns – at a speed that always impresses me – on its own axis.

nothing more to see. please move along.

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