Beginner’s Astronomy: Precession

By Duncan Lunan

Earlier in this series, I’ve talked about the Precession of the Equinoxes, the wobble in the Earth’s axis produced by the pull of the Sun and the Moon on the Earth’s equatorial bulge.  In the co-ordinate system of right ascension and declination, produced by projecting terrestrial longitude and latitude on to the celestial sphere, it causes the plane of the equator to move around the Zodiac with a period of 26,000 years.  The Vernal Equinox, where the Sun crosses the equator on its way north in spring, moves steadily along the Ecliptic  (the plane of the Earth’s orbit, projected on to the sky)  from year to year  (Fig. 1). 

When Greek astronomers mapped the sky it was in Aries, and by convention it is still referred to as  ‘The First Point of Aries’,  although it has been in Libra for the whole of the Christian era, which is why one of the earliest Christian symbols was the fish.  It’s no coincidence that it was in Aries at the time of the Golden Fleece legend, in Taurus during the bull-cult of ancient Crete, and in Leo at the time of the Sphinx.  And as I mentioned, on that basis we have not entered the new Age of Aquarius – the Vernal Equinox won’t actually get there for another 400 years.  If you go with the idea that the Zodiac should instead be divided into twelve arcs of equal size, then  (depending on where you place the start point), it might not arrive for nearly 4000 years, by which time the celestial pole will be nearing Errai in the constellation Cepheus  (Fig. 2).

As the equator precesses around the Ecliptic, the north celestial pole  (directly above the geographical pole)  moves in a circle among the stars with the same period of 26,000 years.  One of the great Victorian astronomers remarked, “I thought the precession of the equinoxes marked the slowest and most impressive process accessible to the intellect of man, but that was before I saw the workings of a printer’s office”.  At the time of building Stonehenge and the Pyramids, the star nearest the Pole was Thuban in the constellation Draco, which had held the celestial pole for 5000 years.  Draco also surrounds the Ecliptic pole;  in classical times, the Vernal and Autumnal equinoxes were known as the Head and the Tail of the Dragon, and it possibly explains why the long dragon in the sky features in the earliest Mesopotamian art, and may belatedly have inspired the building of the Great Wall of China, when the emperor Ch’in rediscovered the ancient knowledge of nearly 3000 years before.

13,000 years ago, half-way back around the precessional cycle, the pole star was Vega in the constellation Lyra, the bright blue star which now dominates the summer sky at the highest apex of the Summer Triangle.  One of the things which impressed me about the late Charles Chilton’s Journey into Space, which riveted the nation on the Light Programme in the 1950s, was the sequence in which the characters go through a time-warp and make a forced landing on a planet like the Earth.  Trying to find out if it is the Earth, they watch through the night until it’s confirmed by the waning Moon, rising shortly before dawn.  But in the meantime they’ve realised that the night sky is turning, not around Polaris, but around Vega.  They’re at least 13,000 years in the past, it seems – but there’s no way for them to know how many times Vega will be the pole star, before our own era.

At the time when Prof. Alexander Thom was conducting his great survey of Neolithic sites in Britain, it was thought that the major era of building stone rings and erecting monoliths had been between 2000 and 1500 BC.  His famous histogram of the alignments he found, illustrating which of them are most frequent, was drawn for that time  (Fig. 3).  

In 2009, Dr. Euan MacKie made the important point that Thom’s histogram was drawn up before the scale of radiocarbon dating was revised  (Euan W. MacKie., ‘The Prehistoric Solar Calendar:  an Out-of-Fashion Idea Revisited with New Evidence’, Time and Mind:  the Journal of Archaeology, Consciousness and Culture, 2, 1, 9-46, (March 2009).  By inspection, extending the inclined lines showing the changing declinations of the stars, due to precession of the equinoxes, gives improved fits for Rigel, Sirius, Antares, Betelgeuse, Procyon, Altair, Spica, Pollux, Castor, Vega and Arcturus, and earlier ones for Pollux, Castor and Capella.  The highest peak is for Capella in Auriga, 2000-1600 BC  (even higher than the ones for the winter and summer solstices!), but there is an explanation for that:  between those dates Capella was becoming circumpolar in the British Isles for the first time, clearing the northern horizon instead of setting, and Thom suggested that the Hill o’ Many Stanes at Mid Clyth recorded the observations made as it did so.  On the island of Unst, at the north tip of Shetland, the Moon is circumpolar at its furthest north, every 18.6 years, and taking the two discoveries together, Thom believed that the Neolithic astronomers must have realised that the world is round. 

Euan MacKie sent me his paper when I was writing my book The Stones and the Stars, in 2011-2012, and there was no time to examine those star alignments more accurately.  (As it was I had to request an extension of the deadline, for the first time in my career, because I was moving house.)  As with the 1979 calculation of the alignments for the Sighthill stone circle, where I had to cut corners under pressure of the upcoming helicopter operation, there simply wasn’t time to extend all those star-lines back using spherical trigonometry, with several pages of calculation for each alignment and again for each date.  In 2017 I was able to reconcile the Sighthill observations and recalculate the alignments for the new site using the Interactive Star Chart of the excellent website, but that only goes back to 0 BC, and the other two astronomical programmes I had on my computer didn’t go back far enough, either.  But I now have one that does, and when time allows, I intend to produce a version of Fig. 3 going back far enough to clear the peaks on Thom’s histogram, just to see what it shows. 

When I do, readers of Orkney News will be the first to know about it!

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