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G’day, Dr Karl here.
Last time, I introduced you to the Antikythera Mechanism. It’s really old (about 2,200 years), and it is also the first known computer. The Antikythera Mechanism is a small wooden box the size of a shoebox, with some 30 bronze gears inside. The gears are connected to three dials on the outside. The dials predicted the movements of the Sun, Moon, and planets through the skies, as well as seasons, festivals and even eclipses of the Moon and Sun. That’s pretty astonishing! So today I’ll keep it simple, and just start with eclipses of the Sun.
During a solar eclipse, the face of the Sun gets covered (either totally or partially) by the disc of the Moon,
Imagine that there is a line joining the Sun and the Earth. An eclipse of the Sun happens whenever the moon shifts exactly onto that line. Sure, the Moon goes around the Earth roughly once every “moonth”, or month. Most times, the Moon is either slightly above the line joining the Sun and the Earth, or slightly below that line. But every now and then, the Moon exactly crosses that line. So now the moon blocks out some, or all, of the Sun. Down here on Earth, we see a partial or total solar eclipse.
Nowadays, how do we predict a solar eclipse?
Well, imagine that the Earth is a globe. Now, wind an imaginary string a couple of dozen times around that globe, starting at the North Pole, crossing the Equator and finishing at the South Pole.
The path of this string is called the Saros Cycle. A Saros Cycle describes the pattern of successive solar eclipses. One Saros Cycle takes between 1,200 and 1,500 years to spiral from the top to the bottom of the Earth. In each Saros Cycle, there are about 70-or-so eclipses, with each eclipse about 18 years, 10 days and 8 hours apart.
The first eclipse is at the top end of the earth, somewhere up around the North Pole. The next eclipse happens a bit further South, and about 120° further around the earth. And it just keeps on going.
At any given time, there are about 42 separate Saros Cycles winding their way around the globe, from the North Pole down to the South Pole.
But here’s the problem.
You cannot mathematically work out these Saros cycles, until you’ve accurately observed both where, and when, solar eclipses happen.
So how did scientists do that a few thousand years ago?
Sure, a total solar eclipse, where the sky goes black in the middle of the day, that’s pretty obvious – but they’re not that common.
Far more common are the partial solar eclipses, where the Moon covers only part of the face of the Sun. In most cases, you would completely miss a partial solar eclipse. Your eyes would simply adjust to the lower light level, and you would probably think that a cloud was covering the sun.
And even if you looked directly at the Sun, you couldn’t tell that a partial solar eclipse was happening. The glare of the Sun is so great that you wouldn’t see that any of the Sun was blocked by the Moon.
About the only way you could eyeball a partial solar eclipse was by looking at blobs of dappled sunlight, for example coming through the gaps between the leaves of a tree or bush. Normally, you would see little circles of sunlight. But in a partial solar eclipse, suddenly you would see little crescent shapes of, like your classic fingernail moon.
So a few thousand years ago, there had to be somebody on duty looking at the dappled sunlight – all day, every day.
Another problem is being in the right place at the right time. Remember, in a Saros Cycle, each successive solar eclipse happens 120°, or one third of the way around the earth. So that means the ancient scientists had to gather information all the way from Spain to Greece, the Middle East, across to India and to the Pacific Coast of China, and probably Africa. And then, over a few centuries, those scientists would have to consistently share this information with other scientists across the known world.
It seems almost impossible, but somehow the scientists two-and-a-half thousand years ago did it. They worked out the Saros cycle. Back then, the Greek thinker Thales from the city of Miletus accurately predicted an upcoming solar eclipse.
We have absolutely no written records of how the scientists worked this out. But remember, while the motto of the philosopher Descartes is, “I think, therefore I am”, the motto of the scientist is, “I think, therefore I get paid”.
Although we don’t know how scientists worked out the Saros Cycle, somebody built it into the bronze gears of the Antikythera Mechanism. Now, mechanical gears are a bit easier to get our heads around – they haven’t eclipsed our understanding yet - so I’ll talk more about that, next time.