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.