Three days from today. The moon will pass before the sun and then all will be in darkness.
Astronomy is the oldest science.The remains left behind by ancient civilisations show that they paid close attention to the celestial motions of stars and planets – and with good reason. The celestial sphere is a precise instrument for time-keeping and direction-finding, more constant and reliable than any earthly mechanism. They may have lacked telescopes, but the peoples of ancient Babylon, Egypt, China, Greece and Mesoamerica kept careful records of precise naked-eye observations that gave them a reliable dataset that stretched back centuries.
With so much detailed information on the rising and setting of the Sun and stars, of the phases of the Moon and its motion against the stellar background, of the peculiar wandering stars called “planets”, early astronomers could establish calendars, predict agricultural seasons, devise systems for navigation at sea. Astronomy today may seem remote from practical concerns, but in those days it was a matter of life and death for rulers, their subjects, and whole kingdoms.
Into this serene heavenly clockwork, disruptive influences would suddenly protrude. Meteors, that seemed like falling stars and occasionally brought chunks of heavenly iron to Earth. Comets, signs of ghostly foreboding that presaged great turmoil. But none were more closely studied, and none more feared, than eclipses.
To understand why, we first have to look at the two different kinds of eclipse: lunar and solar. This diagram shows the basic idea – not to scale, of course.
The Sun is in yellow, and the blue Earth orbits around it. The Moon, shown in white, orbits round the Earth. When the Sun, the Earth and the Moon are all lined up so that the Moon passes through the Earth’s shadow (shown in grey), we have a lunar eclipse. If you were sitting on the near side of the Moon, you would see the Earth pass in front of the Sun, blotting it out for a while. If you’re on Earth, looking up at the Moon, you see something more remarkable: the Moon turning a dark, ominous red.
Why red? Well, as the Sun’s rays pass through the Earth’s atmosphere, they bend a bit. This is diffraction – the same effect that makes a stick seem to bend when you put it in water. These light rays illuminate the Moon so it doesn’t completely disappear even though it is cut off from the Sun’s direct light. But these light rays aren’t just bent: they’re reddened. As the light passes through the atmosphere, it scatters off the molecules of nitrogen, oxygen and other gases, and blue light scatters much more that red light. So the blue light tends to scatter away, while the red light keeps going in more or less the same direction. This is why sunsets are red, and it’s why the light striking the Moon during a lunar eclipse is also red. Indeed, the Moon is being bathed in the light from every sunset on Earth.
If we now switch the alignment round, so that the Earth is in the Moon’s shadow as shown in the next diagram, we have a solar eclipse.
From a suitable vantage point on Earth, the Moon blocks out the light of the Sun. In a curious cosmic coincidence, the apparent size of the Moon as seen from the Earth is almost exactly the same as the apparent size of the Sun. This means the Moon can exactly block out the whole solar disc. When it does so the hot outer atmosphere of the Sun – the corona – that we normally can’t see due to the Sun’s glare suddenly appears in the darkened sky. It’s quite a sight. Meanwhile the Earth appears to darken and cool, until the Moon moves past the Sun and daylight is restored.
But notice that the Moon’s shadow is much smaller than the Earth’s. In the case of a lunar eclipse, the whole Moon could easily fit inside the shadow of the Earth, while in a solar eclipse only a small part of the Earth is covered by the Moon’s shadow. This fact was crucially important for the ancient astronomers, as we shall see in a moment.
Now these rather noddy diagrams I’ve drawn are rather too simplistic. They show the Sun, Moon and Earth tracing out perfectly circular orbits in a single plane. If that were the case, eclipse prediction would be easy: every Full Moon would be a lunar eclipse, and every New Moon would be a solar eclipse. One of each, every month. Of course, it’s more complicated than that.
The most important factor is that the orbit of the Moon around the Earth is not in quite the same plane as the orbit of the Earth around the Sun: there’s an angle of about five degrees between them. That may not sound like much, but it’s enough that perfect alignments of all three bodies are rare.
Nowadays we can predict eclipses accurately, thanks to Newton’s laws of motion and the theory of celestial mechanics that is built on them. But the ancient astronomers didn’t have that kind of understanding. If they were going to predict eclipses, they would have to do it by detailed, long-term observation of the motions of the Sun and Moon, analysing these long sequences of data to find any regularities that would hold a clue as to when these special alignments would take place.
And that’s just what they did.
The most important of these regularities is the Saros Cycle. This is a period of 223 months: about 18 years, 11 days and 8 hours, and it is the time it takes the Earth, Moon and Sun to orbit around and come back into approximately the same alignment relative to each other. So, if there’s an eclipse today, then there will be another in just over 18 years. Now, eclipses aren’t simply 18 years apart – there are all kinds of other cycles going on as well, which mean more eclipses within that period, but if you observe enough eclipses and use this 18-year trick for each one you can start to build up a reasonable set of predictions.
A big problem with this technique is that the Saros is not a whole number of days. For a lunar eclipse that’s not such a big deal – it may happen eight hours late, but you should still get it on the right night. For solar eclipses, though, it’s more of an issue. Remember that the Moon’s shadow on the Earth is relatively small, only about 100km across, and the eclipse happening eight hours later also means it happens in some distant part of the Earth that happens to be in the Moon’s shadow at the time – as far as you can tell, there has been no eclipse at all. It’s not all hopeless, though: three times eight is 24, and so three Saros cycles add up to a whole number of days, bringing the solar eclipse back to roughly your neighbourhood.
In the Western world, it was the Babylonians who first discovered this 223 month cycle, and we have records of their meticulous astronomical observations that survive from the 17th century BCE. By the time we get to the classical Greeks, eclipse prediction has become much more sophisticated. The 223 month cycle is built into the clockwork computer known as the Antikythera Mechanism, and the most famous Greek astronomer, Ptolemy, had a sophisticated method for predicting both lunar and solar eclipses. The Chinese, developing in parallel, also figured out how to predict eclipses and by the third century CE knew how to predict solar eclipses by analysing the motion of the Moon.
It seems a lot of trouble to go to for something that may be interesting, but isn’t obviously useful. Predicting eclipses won’t tell you when to sow your seeds or when to harvest your crop. So why bother?
In the West and in China, it comes down to this idea of omens: that heavenly occurrences foretell earthly events. Which is a load of rubbish, of course, but they didn’t know that. If the regular motions of the stars and planets predict the regular cycles of the seasons, then doesn’t it seem reasonable that irregular celestial events like eclipses foretell irregular events of Earth – sudden calamities and the like? Ancient rulers in particular took this possibility very seriously. This was good news for astronomers seeking funding, but not so great if they didn’t produce the goods, as two ancient Chinese court astrologers discovered when they were beheaded following an unexpected solar eclipse.
But if we want to see sheer cosmic terror in action we have to leave behind the Chinese, set aside the Greeks and go even further west, to Central America – and the Aztecs.
Like all ancient peoples, the Aztecs had a complex cosmology, explaining in mythic terms how the world came to be, how the gods set the Sun and Moon in the sky, the divine purpose behind creation, and so on. What really distinguishes the Aztec cosmology is the sheer amount of blood involved. Blood made the Sun rise in the heavens. Blood made the crops grow in the fields. Blood was the very fuel of the engine of creation, and without an endless cycle of blood sacrifices the Universe would grind to a halt and catastrophe would come to all humanity. It was the place of humans to play their part in this natural cycle, and the ritual killing of the appointed victims was recognised as a supreme moral duty.
Again, like the other great civilisations of the ancient world, the Aztecs were dedicated astronomers, with a calendar based on celestial observations that ordered their society. In fact, they had two. The first was a solar calendar spanning the familiar 365 days, divided into 18 months of 20 days, each with its own set of rituals – bloodletting, sacrifice, flaying of prisoners and so on – plus a special five-day period at the end of the year. Running alongside this was the ritual calendar of 260 days, comprising 20 periods of 13 days, each dedicated to a different god.
These two calendars would march along out of step with each other for the most part, but every 52 years they would coincide and then the cycle would start up again. The Aztecs believed that, at the end of each 52-year period, the gods might decide to end the world. To stave off this disaster, they performed the New Fire ceremony, in which all fires throughout the Aztec real were extinguished, a man was sacrificed atop the extinct volcano of Huixachtlan, and new fire kindled on his chest and passed out to all the people. This ceremony was always successful.
Every bit as dangerous were the solar eclipses, which the Aztecs understood as the Moon – depicted in their art as a monstrous deity – attacking the Sun. If this attack were not repelled by suitable rituals of bloody sacrifice, the Sun could disappear for ever and the world come to an end. A solar eclipse at the time of the New Fire ceremony would be particularly terrifying, and even a New Moon around this time, with its potential to turn out to be an eclipse, would be a source of great anxiety.
Whether the Aztecs could have predicted these calamitous events with any reliability is not known. Alas, much of the intellectual material of their empire was destroyed in the Spanish conquest, when the Aztecs were overthrown by a more technologically advanced bunch of blood-soaked religious fanatics. What we do have, from detailed records in surviving codices to precise astronomical alignments of key buildings, suggests a remarkably precise degree of astronomical measurement and clever ways of using alternating whole numbers to express fractions of a day in orbital motions. They certainly had some knowledge of the cycles underlying lunar eclipses, and it is entirely possible that they could have matched or even surpassed the Greeks and Chinese in eclipse prediction. It is unlikely that we shall ever know.