We may have a clue from Diodorus Siculus. He was a Greek living in Sicily who wrote a history in 40 books. He was alive about 30 BCE which means he was further away in time from the builders of the Cursus than we are from him. But he says he is quoting Hecateus, a classical Greek historical writer who lived in the 300s BCE and this is what Hecateus said:

Leto (Apollo's mother) was born in Hyberborea (Britain). Therefore Apollo was honoured there above all the Gods and there are men who serve as priests of Apollo. There is also a precinct sacred to Apollo and suitably imposing and a notable temple decorated with many offerings and looking like a globe.....It is said the god returns to the island every 19 years, the period when the stones complete their cycle.

Now here we may have some information to work with. Although Diodorus is often an unreliable teller of tales, there may be a kernel of truth in what he says. We now know that Stonehenge was so constructed from the earliest of its several phases as to show the midsummer and midwinter sunrise solstice (nearest and furthest point of the sunrise in the year) by sightings along certain major stones.

As you can see it still works very well at a sunrise solstice today.

Stonehenge also apparently had a circular series of holes, probably for wooden posts which marked the most northerly position of the moon at different winter solstices, a position which changes slowly over a cycle of 18.61 years. We're told these were installed during the first stage of Stonehenge, about 2,750 BCE. So here we have an astronomical explanation for the 19 year period.

A replica of Stonehenge was built in Maryland, USA, but of course it has no astronomical alignments because the Stonehenge co-ordinates would only work at the exact location of Stonehenge.

We also know that Apollo was referred to as a Sun god. Hecateus seemed to have heard that the temple was like a globe. Presumably Stonehenge must have had a half globe above the uprights. If so this would have been very similar to the shape of a present day astronomical observatory. A precinct, referred to by Hecateus, is a subdivision or district, which we've already drawn attention to, as groupings of the round barrows look rather like those of modern housing developments.. And we now have an astronomical explanation that fits his enigmatic phrase about the stones completing their cycle in 19 years.

There's more to it than this. The 19 year cycle is a good working arrangement, but far greater accuracy is obtained using a 56 year cycle. How many 'Aubrey holes' have been found in a circle at Stonehenge? Fifty six. To put this in perspective we should remember that ancient Egyptian mummies or remains of average ancient Egyptian citizens are said now to have had an average life span of 38 years. The builders of the remarkable prehistoric dwellings at Maes Howe on Mainland, Orkneys, are now said to have been little more than teenagers, as their life span was very short. This means that a 56 year cycle was probably well beyond the average life span of priests and other humans when Stonehenge was being built and used.