Glossary X

Horizon Astronomy

Positional astronomy

The astronomy of the Megalith Builders in Britain was not a quest to discover what a celestial body was nor how far away it might be, nor what it is made of, but rather what it's position is, was or will be. This is termed positional astronomy and it is still the bread-and-butter work of modern astronomy. Positional astronomy is largely concerned with maintaining strict time- keeping of the movements of Sun and Moon.


If we imagine a strong light at the centre of the Earth then lines of declination would be the shadows of lines of latitude cast upon the screen of the sky. These are the tracks along which the Sun and Moon travel. From the declination of the Sun the time of year can be read. From the declination of the Moon the current condition of it's orbit may be deduced. Until Alexander Thom's work it was thought that meridional astronomy was the only positional form developed to any degree of practical use in the ancient world.

Meridional astronomy

The Gnomon
Even with a gnomon 1 mile tall the daily difference in the length of it's shadow, at a solstice, would be less than 12 inches,(305 mm).
The meridional system depended on observing where the tip of the shadow of a very tall pillar lay at local noon when the Sun reached the zenith, its highest point in the sky, due south of the pillar. The position of the shadow tip was marked and the procedure repeated each day. Such a pillar or 'needle' is termed a gnomon.

The Gnomon

The Gnomon of Alexandria was used by Eratosthenes, the Greek astronomer, to calculate the circumference of the Earth in 240 B.C. However meridional astronomy using the shadow cast by a pillar, does not give the necessary resolution for accurate measurement of the solstices nor limiting positions of the Moon. Only by employing long distance alignments observing to a distant hill horizon could the early astronomers hope to differentiate among the very small movements at the extremes of the orbits of the Sun and Moon.

Megalithic horizon astronomy

Instead of marking the length of the shadow from a vertical pillar the british bronze age astronomers choose to use marks on the great circle of the horizon to define the positions of Sun and Moon as they rose or set. The rise/set points move along the horizon daily- faster near the equator at the equinoxes, slower close to the extreme points north and south which are termed the solstices (of the Sun), and standstills, (of the Moon).

Required declinations for megalithic astronomy in Britain

Here, observing south, are the required declinations given by alexander thom for the Bronze Age 16 interval calendar and
Even with a gnomon 1 mile tall the daily difference in the length of it's shadow, at a solstice, would be less than 12 inches,(305 mm).
the lunar standstill positions both Major and Minor. The yellow lines are solar and the blue lunar. The double yellow lines are the declinations of the Sun at summer solstice, (in the North), and winter solstice, (in the South), when the position of the Sun appears to 'stick' for three days before slowly winding back to it's other extreme in six months time.
The treble blue declinations are the Lunar extremes at the Standstill positions. The three close set lines denote the extremes of the cyclic perturbation a small wobble in the path of the Moon with a period of about 7 months. Cyclic perturbation could only be observed by a horizon astronomer at a lunar standstill once every 9.3 years. Horizon astronomy is a system of positional astronomy used to time the extremes of solar and lunar orbital cycles which the early British astronomers seem to have brought to an unparalleled standard of perfection.

Megalithic astronomical alignments

Horizon marks of sufficient impressiveness were erected on high ground at or near the point where a required declination would intersect with the horizon as viewed from lower ground. A suitable place, at lower levels, would be found from which the Sun or Moon,
Calendar lunar standstill declinations 1800 BC
Even with a gnomon 1 mile tall the daily difference in the length of it's shadow, at a solstice, would be less than 12 inches,(305 mm).
(on the correct day), would sit cleanly on the horizon mark This stance, probably located by patient trial and error over many months or years, would be astronomically unique and permanently marked by a stone of sufficient weight to avoid accidental removal. This astronomically selected place, where the astronomer must stand to make an observation, is termed the backsight, whilst the horizon markers which he looks to are termed foresights


Tumuli are Bronze Age round barrows often, but not always, containing a burial, with and without a stone box, (cist). They are often carefully constructed of many alternate layers of turf and clay.

Cairns are large piles of boulders and pebbles. Again they may often be built in separate layers of graded size stones or even different geological material. Some may have contained burials but they are of a different period to the similar looking chambered cairns of the Early neolithic Period, circa 3000 BC, though often the much older monuments are incorporated into astronomical alignments dating from after 2000 B.C.

The British Middle Bronze Age.

The period from about 2000 BC to 1600 BC saw an unprecedented
Foresights - these are usually earth mounds, or large piles of stones.
development in ritual building activity in Britain, predominantly in the high ground of the west. Over 40,000 cairns and tumuli are thought to have been erected
Backsights - these may be stone circles stone rows or single standing stones and often an arrangement of more than one of these types. Tall, slender, flat stones are termed cleits, (quills or feathers), in northern Gaelic. Large, broad, flat faced stones are called lech-wedd, (stone hands), in Welsh.
in this period. Most measured upwards of 2/3 metres high and 10/15 metres diameter, but many, especially on high hill tops, are considerably larger. See Drygarnfawr.


Stone circles and rings vary in diameter between a few metres and nearly 180 metres. Stone rows are three or more stones set in straight lines. The stones may be a series of cleits, lechwedd or blocks. Single blocks may have steps or seats cut in them. There is some evidence that the megalith builders recognised the basic solids. Cubes, rectangular blocks, (rhombs), and tetrahedrons seem to have been favoured in Scotland and Wales