Most people who lived in the tropics in the past would have been broadly aware of the two times of year when the sun passed more or less directly overhead at noon and they ceased to have shadows. If they wished to determine the precise dates when zenith passage occurred, though, they would have needed to construct an instrument. A vertical, straight-sided post would have sufficed—a gnomon whose shadow would have dwindled to nothing at the appropriate time. So runs a plausible argument; but to reason in this way is to approach the issue from a modern Western perspective. If people had wished instead to harness the sacred power of such a moment, for whatever ideological or political ends, they would surely have created a hierophany. One way to achieve this would have been to construct a long vertical tube opening into a dark chamber below. This would create a breathtaking effect when the light of the zenith sun suddenly passed directly down the tube and, for a few precious moments, brought a sudden transformation to the normally dark space.
Two of the most discussed examples of possible zenith tubes are found in Mesoamerica. One is at the Early Classic site of Monte Alban, a little under four hundred kilometers (250 miles) southeast of Mexico City, close to the modern city of Oaxaca. Monte Alban was the capital city of the Zapotec state, which formed in the valley of Oaxaca around 500 b.c.e. and grew steadily in power and influence until it reached its apex around c.e. 300-600. The massive main plaza, built on a flattened hilltop, was its ceremonial heart. Visitors cannot fail to be impressed by the sheer size of this plaza, which measures more than three hundred meters by two hundred meters (1,000 feet by 650 feet). Nor can they fail to notice something extremely strange about Building J, situated on the central axis towards one end: it is wildly skewed with respect to all the other buildings. Rather than being oriented along the plaza like all the others, it is aligned directly upon Building P, one of several stepped buildings along the eastern side. And halfway up the steep steps of Building P is a curious feature. Here, reached through a narrow entrance, is a small room built directly under the steps, and at the back of it is a bench. Above the bench is a narrow vertical tube about 1.5 meters (five feet) high leading up to an opening higher up the staircase. Some have suggested that this tube could be merely a chimney. But several other features in the alignments at Monte Alban suggest that it could have been an astronomical observing device.
The other Mesoamerican zenith tube, at Xochicalco, is on an altogether different scale from the one at Monte Alban. Located in the state of More-los, some seventy kilometers (forty-five miles) south of Mexico City and about fifteen kilometers southwest of the city of cuernavaca, Xochicalco’s situation—draped over a series of artificially flattened and terraced hilltops overlooking a deep valley over a hundred meters (330 feet) below—is nothing short of stunning. This city-state flowered early in the Epiclassic period, c. c.e. 700-900 following the collapse of Teotihuacan to the north and Monte Alban to the south. The tube here is over forty centimeters (sixteen inches) wide and descends for more than five meters (sixteen feet) before opening out into the roof in the far corner of a cave chamber almost twenty meters (sixty-five feet) long and twelve meters (forty feet) wide, dug out of the living rock. The chamber is supported by three rock pillars and reached by a network of subterranean passages.
The effect of the midday sun shining down the Xochicalco zenith tube is spectacular indeed, sending a vertical beam of bright sunlight down into the dark cave. But because of the width of the tube, this phenomenon is not confined to the days of solar zenith passage. In fact, the sun first shines directly down the tube, producing a brief spot of sunlight on the chamber floor, at local noon on April 30. As the sun passes closer to the true zenith at noon on successive days, the length of time when its light shines directly onto the floor increases, as does the proportion of its disc that is visible at maximum, until May 15, when it passes directly across the zenith at noon. After this, the length and proportion visible start to decrease again until the solstice is reached, when the noonday sunbeam has almost disappeared. The sequence is then repeated in reverse: zenith passage is reached on July 29, and the last spot of direct sunlight is seen on around August 12. A number of early descriptions dating back to the mid-nineteenth century reassure us that no dubious reconstructions have taken place and that these alignments are reliable.
Building J at Monte Alban, Mexico, showing its anomalous orientation, as viewed from Building P, upon which it is aligned.
Since it was first investigated astronomically, there have been many further suggestions about the Xochicalco tube. One is that this is just an approximate zenith device, but others argue that the dates of appearance and reappearance of noon sunlight are significant, since they fall exactly fifty-two days before and after the solstice respectively, and the fifty-two-day interval is calendrically significant. Furthermore, the length of the period of darkness—before the sun starts to appear at noon again—is (within a day or two of) 260 days, exactly the length of the sacred calendar cycle or tonalpo-hualli. Still others suggest that the moon might have been at least as important as the sun at Xochicalco, since at minor standstill it passes directly through the zenith at this latitude. The idea is given some credence by the discovery, in excavations during the 1980s, of a unique crescent-moon-shaped stele.
We cannot be certain that the Monte Alban and Xochicalco zenith tubes were really used for the purposes of astronomical observation, but a good deal of circumstantial evidence supports this view. Furthermore, the existence of solar zenith passage devices like this seems plausible enough in the context of Mesoamerican worldview, in which (with variations) the up-down axis is a fundamental means of orientation, together with the four cardinal directions. The importance of the zenith sun is also well attested in historical accounts, iconography, and ethnography. The effect of sunlight entering the underground chambers is certainly spectacular, and it is hardly surprising that the Xochicalco “observatory” has become a popular tourist attraction. If we are lucky, other examples still wait to be discovered in the future, in Mesoamerica and elsewhere.
