Technically, precession is quite a difficult concept, but it is an important one for anyone wishing to study perceptions of the skies in the past. Precession has the effect of shifting the overall positions of the stars in the sky over a time scale of centuries, and (from any given place) can make some stars disappear completely from view while others become visible for the first time (in thousands of years). For example, when prehistoric farmers first colonized the area of southern England where they would eventually build Stonehenge, the Southern Cross was prominently visible in the night sky at certain times of year. However, it has not been visible from this latitude for over 5,000 years. Taking account of precession is critically important for anyone investigating possible architectural alignments upon stars. But it also enters into archaeoastronomy in a different way, in that some have claimed that the effects of precession are described and documented in myths that have been perpetuated over many centuries.

In order to explain precession, it is best to adopt the astronomer's perspective, looking in on the solar system.

As it progresses on its annual orbit around the sun, the earth also spins once daily around its own axis. This axis maintains the same orientation in space, so there is one point in the earth's orbit where its north pole leans directly toward the sun while the south pole leans directly away from it. When the earth is at this point in its orbit, for people in the northern hemisphere, the sun is seen following its highest daily path through the sky, while for people in the southern hemisphere its path is the lowest. This is one of the solstices: the summer solstice in the northern hemisphere and the winter solstice in the southern. In the modern (Gregorian) calendar this solstice falls on or very close to June 21. The opposite point in the earth's orbit, when the north pole is leaning directly away from the sun and the south pole toward it, is the December solstice. A quarter of the way around the earth's orbit, halfway between the solstices, are the two equinoxes, where the axis joining the two poles leans neither toward nor away from the sun.

The earth's axis maintains a fixed direction with respect to the stars. This is because the stars are so staggeringly far away compared with the dimensions of the solar system that the earth's motion around the sun makes no difference. If the sun were scaled down to the size of a beach ball, then the earth would be the size of a ball bearing about thirty meters (a hundred feet) away from it, and the farthest planet, Pluto, would orbit the sun at a distance of a little over one kilometer (a little under a mile). On this scale, the distance of the nearest star from the sun would be about the distance of London from New York, with some of the other visible stars considerably farther away still. Having established these vast distance scales, it is conceptually far easier to pretend that the solar system is surrounded by an enormous sphere with all the stars attached to it. This means that the earth's axis, extended out through the north pole, is always pointing at a fixed position on the celestial sphere: this is called the north celestial pole. Similarly, the axis extended out through the south pole is always pointing to the south celestial pole.

Over a time scale of centuries, the direction of the earth's axis in space (and with respect to the distant stars) does change. This is because the earth not only spins about its axis, but the axis itself gradually turns in a way that resembles a spinning top. One effect of this is that the position in the earth's orbit where we find (say) the north pole tilted toward the sun is now different: in fact, it has "precessed" around the earth's orbit. So have the positions corresponding to the other solstice and to both the equinoxes. This is the reason that the term precession is used to describe this phenomenon: it is in fact short for precession of the equinoxes.

From the point of view of someone watching the skies from the surface of the earth, precession makes it seem that the celestial poles gradually change position. In fact, each of them traces out a wide circle on the celestial sphere, completing one circuit every 25,800 years. Occasionally during this period, one of the poles will come close to a bright star, making it easy to identify the pole; this is the case with Polaris at the present time, but it is more often not so.

The principal effect of precession is that the declination of every star in the sky changes over the centuries. One consequence of this is that the horizon rising and setting positions of any particular star as viewed from any particular place on earth will generally change significantly over a period of a few centuries (although how significant this effect is depends upon the precision of the alignment being considered). This means that there is a nasty trap waiting to ensnare the modern investigator looking for possible astronomical alignments—at least, stellar alignments—at prehistoric monuments such as stone circles. The danger is this: these monuments are rarely datable to within a few centuries (and very often the range of possible dates is far wider than that). If we notice that a particular structure is aligned upon a horizon point fairly close to the rising or setting position of some bright star in the past, then we may well be able to find a date within the permissible range that fits the alignment perfectly. While there may be exceptions (such as the prehistoric sanctuary at Son Mas in Mallorca), this sort of astronomical dating generally proves nothing because of the ease by which such alignments can arise fortuitously. To give an idea, suppose we take the fifteen brightest stars in the sky and a date range of five hundred years. Then by choosing an appropriate combination of star and date we can cover about a third of the horizon. In other words, if an alien giant were to come along and scatter fake prehistoric aligned structures randomly over the landscape, it would be possible to choose a star and a date within the permitted range that would fit one in three of them. The problem is further confounded by atmospheric extinction. What all this means is that the aspiring archaeoastronomer must pay particular attention to problems of methodology. Two case studies that illustrate the dangers particularly well are the stone row at Ballochroy in Scotland and the stone configuration at Namoratung'a in Kenya.

See also:

Astronomical Dating; Equinoxes; Methodology.

Ballochroy; Gregorian Calendar; Namoratung'a; Son Mas; Stone Circles.

Celestial Sphere; Declination; Extinction; Obliquity of the Ecliptic; Solstices.

References and further reading

Aveni, Anthony F. Skywatchers, 100-103. Austin: University of Texas Press, 2001.

Krupp, Edwin C. Echoes of the Ancient Skies, 10-11. Oxford: Oxford University Press, 1983.

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