Solar eclipse: It took thousands of years to determine the phenomenon's cycle
More than 300 years ago, Isaac Newton came up with the revolutionary idea that objects have gravitational mass. He also developed the mathematics that could use the resulting gravitational force to yield equations defining how the planets orbit the Sun. It was a stunning achievement, explaining detailed observations made previously by astronomers such as Tycho Brahe in Denmark and Johannes Kepler in Germany.
But what causes mass? As we plumbed the depths of the quantum world in the 20th century, elementary particles exhibited mass, but how did they acquire it? The proposed answer lies in the Higgs boson — the so-called God particle — whose elusiveness is reminiscent of Lewis Carroll's long poem The Hunting of the Snark. This mysterious particle can only be found using extremely high energies, and the builders of the Large Hadron Collider at Cern believe they will either find it, or show that it doesn't exist.
Fine, but let us turn to an intriguing analogy from ancient Babylonia. For thousands of years it had been known that the Sun and the Moon were occasionally eclipsed, but it was in Babylon that people asked what caused this. Could eclipses be predicted?
Starting with the Sumerians in the third millennium BC, Mesopotamian scribes compiled masses of data, inscribing it all on clay tablets. They made lists of all sorts of natural and man-made things, and kept records of strange phenomena such as eclipses. With observations going back hundreds, if not thousands, of years they were later able to see patterns to the occurrence of eclipses, but it was not easy. Solar eclipses are notoriously hard to predict because they can be seen in one place and not another, but lunar eclipses are easier, being seen at any place on Earth where the Moon is visible. The Moon goes a dull red colour; the darker it is the more total the eclipse.
Observers in very ancient times would have noticed that a lunar eclipse occurs only at a full moon, and a solar eclipse only at a new moon. Helpful, but not enough to make predictions.
Part of the problem is that even in the case of lunar eclipses just half are visible at any fixed point on the Earth, because you can only see them between dusk and dawn, so in finding a pattern the Babylonians did a phenomenal job. After coming up with cycles that went out of phase after several repetitions, they eventually arrived at the remarkable cycle of 223 lunar months, which is about 18 years and 11 days. This is called a Saros cycle, a Greek term derived from the Sumerian word sar, referring to a large measurement.
Each lunar and solar eclipse reappears 18 years, 11 days, 7 hours and 43 minutes later, and the extra 7 hours 43 minutes means that a night-time eclipse of the Moon may recur in the daytime and be visible only at some far distant region of the planet. In the intervening 18-year period there are some 40 other lunar eclipses, all from different Saros cycles, so it's remarkable that the Babylonians managed to sort it all out.
Each cycle repeats itself for more than 1,000 years, and 5,000 years of data is now readily available on the Nasa website. For example, the next partial lunar eclipse occurs on June 4, 2012, covering a four-hour period around midday British Summer Time. It lies in a sequence starting in Elizabethan times and repeating itself for 1,370 years.
When did the Babylonians discover these remarkable cycles? No one quite knows. One expert thinks it was probably between 750 and 500 BC. Another suggested it might even have been back in the late Bronze Age. We don't know, but are on more certain ground about when the Babylonians came up with their explanation for this "Saros mechanism". As with any scientific discovery, explanations can be a long time coming — just ask Higgs.
Having discovered the Saros cycles, Babylonian astronomers — the Chaldeans of the Bible — continued making very careful observations, and they noticed a curious fact. The speed of the Moon's motion across the heavenly dome changes, and they measured its period from slow to fast to slow again, finding it was just under a month. This periodic anomaly — caused as we now know by the elliptic nature of the Moon's orbit — is called the anomalistic month, and they noticed that their Saros period of 223 lunar months was almost exactly equal to a whole number of anomalistic months. Very odd, but not much help because the speed of the Moon's motion turned out to be unrelated to the timing of eclipses. Science can be a frustrating business. One almost feels like giving up.
But the Babylonians didn't give up. They had been keeping astronomical records for 2,000 years, and what they needed were more measurements. Finally they found them. Although the Sun, Moon and planets all traverse the same path in the heavenly dome, there are slight differences. The Moon occasionally crosses the path of the Sun, and then crosses back again. The points of crossing are called nodes, and the time taken to return to the same node is called the nodical month. It was tough to measure without large equipment, but they eventually found that the Saros period was extremely close to a whole number of nodical months — the difference is less than one hour in over 18 years. And that was the solution to the Saros mechanism.
Lunar eclipses arise not only when the Moon is full, but when it's also at a node. Likewise for solar eclipses and new moons. Eureka! A solar eclipse arises when the Moon and Sun are at almost exactly the same point in the sky, and a lunar eclipse when they are almost exactly opposite. It's not enough for the Moon to be simply on the opposite side of the earth from the Sun. That yields a full moon, but to create an eclipse it needs to be dead opposite the Sun, and hence in the Earth's shadow. Conclusion: the Moon shines by reflected light.
Did the Babylonians draw this conclusion? If so, they don't appear to have written it down, but the Greeks wrote many things down, and were already visiting Babylon by 500 BC. After the Babylonians finally found the explanation for eclipses, the Greeks took it on board and in the third century BC, Aristarchos of Samos wrote down the conclusion about the Moon shining by reflected light.
Wonderful, but now like the Babylonians we do careful observation, and like the Greeks we speculate on causes. Will we find the Higgs boson? Possibly, but remember what happened to that Lewis Carroll character who thought he had finally found the Snark:
In the midst of the word he was trying to say,
In the midst of his laughter and glee,
He had softly and suddenly vanished away
For the Snark was a Boojum, you see.
Boojums aside, when we finally find the explanation for mass, I hope it throws up something a bit more, just as the Babylonian explanation for eclipses showed it is the sun's light that causes the moon to shine.