Situated on a gravel plateau near Thornborough in Yorkshire, northern England, is a remarkable monument consisting of not just one, but an alignment of three large, round Neolithic earthwork enclosures with high outer banks, known as henge monuments. The three henges are almost identical in size and form, each being some two hundred meters across with two entrances on opposite sides, and they are more or less equally spaced some five hundred meters apart, forming an alignment over 1.5 kilometers (1 mile) long. The entrances are oriented along the alignment. Surrounding the henges are a number of other prehistoric features, including two large pits, which appear to have held sizeable wooden posts, aligned upon a double ring-ditch that represents the remains of a ploughed-out barrow.

What makes this monument fascinating from an astronomical point of view is the possibility that it was a pilgrimage center whose astronomical associations tell us not only the time of year when the major ceremonies took place here, but also reveal how people from far afield may have known when to set out in order to arrive in time for those ceremonies.

A number of factors support the pilgrimage center idea. First, the henges were constructed along what is known from other archaeological evidence to have been an important routeway. Second, the formal design of the monuments was evidently important, and it developed over time. The henges are embellishments of earlier, smaller earthwork enclosures, and the central one was built over the central part of an earlier linear earthwork known as a cursus monument, itself over two kilometers (1.5 miles) long. Third, the high outer banks would have obscured the view of the distant skyline (something that is still evident from what remains of the banks today), and this suggests that the henge interiors were exclusive places, cut off from the outside world. Fourth, archaeological field-walking has revealed an almost complete absence of surface scatters of flints from the area immediately around the henges, strongly implying that this was not a place where people routinely lived and worked. Finally, and most extraordinarily, about one kilometer (half a mile) to the east is a large concentration of surface flint, but a good deal of it is non-local and from scattered sources, and lightly worked. It appears that people came here from some distance and brought the stone with them, which implies in turn that this may have been a visitors’ encampment, rather than a permanent settlement.

There are a number of locations at the site where prominent features of the monument seem to frame parts of the horizon. Intriguingly, several of these align upon the same asterism, namely the three stars of Orion’s belt. It is worryingly easy to identify potential astronomical, and particularly stellar, alignments at almost any site, whether or not they were intentional. But this consistency argues in favor of the Thornborough alignments being deliberate. Viewing along the cursus to the west, the three stars would have been framed by the western terminal as they set. Viewing along the line of the two large timber posts, toward the barrow, an observer would have seen them rising in the alignment. And by the time the bright star Sirius rose, in roughly the same alignment, Orion’s belt would have moved up and to the right, to be in line with the henge alignment itself and visible from the henge interiors over their southern entrances. A final, more speculative suggestion is that the three henges themselves formed a representation on the ground of Orion’s belt. Because of precession, the rising and setting positions of any star or group of stars varies significantly over a time scale of centuries. Though it is highly inadvisable to try to date sites astronomically by fitting stars to alignments, precession serves at Thornborough to reinforce the Orion’s belt alignments, in that the “best fit” dates are wholly compatible with the archaeological estimates of the dates of different features at the monument.

If Thornborough really was a pilgrimage center, then two questions arise. Were there particular times when people congregated here, perhaps for large ceremonies? And if so, and if people had to come from some distance, how did they know when to leave in order to arrive on time?

The astronomical associations of the site may provide answers to both questions. The annual first appearance of Orion’s belt in the eastern sky before dawn—its heliacal rise—would have occurred in the late summer. The annual reappearance of Sirius—the brightest star in the sky—would have occurred two or three weeks later. Once Orion’s belt had appeared in the predawn sky, it could have provided the necessary reminder for surrounding peoples that a ceremony was imminent. The actual ceremony could then have taken place somewhat later, at the time of the dawn reappearance of Sirius, with Orion’s belt now prominently visible above the southern entrance of each henge just before dawn. Such a ceremony would have taken place in early autumn, when the harvests were in and the seasonal workload had eased up for a while.

This interpretation of the Thornborough henges was made possible using a combination of sky visualization software and virtual reality software. It shows the potential of this type of approach in contrast to the more traditional procedure, which is to survey horizon alignments and then calculate declinations.

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