The Earth spins on its axis (relative to any star) just over 366 times in one orbit of the Sun, one sidereal year, as against the solar year, which is shorter. Therefore, each rotation of the Earth also represents one degree of the great circle of the heavens that is the planet’s solar orbit. From an earthbound perspective it appears that the Sun moves just under one degree along the plane of the ecliptic each day.
Most people understand that there are 365.2564 days in a year, but many do not appreciate there are 366.2564 turns of the Earth on its own axis during the same period. This apparent contradiction comes about because a full rotation of the Earth does not take the same period of time as that between one sunrise and the next. Most of the angular movement of the Sun that we see as it passes across the sky is indeed caused by the spinning of the Earth on its axis but a small part is caused by the Earth’s orbit about the Sun. If the Earth was turning on its axis but not revolving about the Sun, the Sun would appear to remain in a fixed place relative to the background stars and a day on Earth would be exactly the same as one spin of the planet.
Conversely, if the Earth was not revolving on its axis, but simply travelling round the Sun, while facing the same direction in space, the Sun would then appear to go round the sky exactly once a year – and we would have one day per year. As this apparent movement goes in the opposite direction to the spin of the Earth it takes exactly one day off the ‘real’ year of 366¼ spins of our planet giving us the familiar 365¼-day year.
The earth turns once on its axis for each Megalithic degree of solar orbit.
In summary, according to our earthbound view of the Sun, there appear to be just over 365 solar days of 86,400 seconds but, according to the stars, there are just over 366 sidereal days of 86,164 seconds each. It follows that a 366-degree circle is a very logical invention for an early culture that is interested in astronomy, as the Neolithic people of western Europe are known to have been.
When viewed from Earth, the movements of the Venus within the zodiac are extremely complex yet, with an appropriate technique, an accurate unit of linear measurement can be achieved by appropriate observation of this planet’s movements.
Night 1 A star aligns with a fixed point before moving west
Night 2 The same star appears from the east
The Earth has revolved once on its axis and travelled through one Megalithic degree of its solar orbit
One turn of the Earth can be gauged by marking the position of a star.
Venus, after passing across the face of the Sun (known as the inferior conjunction or transit) rises ahead of it by anything up to two hours or more, and precedes it across the sky. So bright is Venus (known in this form as ‘the morning star’) in the reflected light of the Sun that it can be seen during the lightest part of the day if one knows where to look. Eventually, after approximately 72 days, it reaches its maximum elongation as the morning star (the greatest apparent distance from the Sun when viewed from Earth). It then drops back towards the Sun and crosses in a superior conjunction, after which it emerges as what is known as ‘the evening star’. In a repeat of its daytime movement it gradually moves away from the Sun, eventually setting after it. Ultimately, it reaches maximum elongation and then falls back towards to the Sun to begin its cycle again.
During these movements (which are directly related to the fact that the Earth is also travelling around the Sun) Venus takes a peculiar path through the zodiac. For periods of about two weeks at a time (and sometimes more), Venus moves quickly through the zodiac, bettering the Sun’s 59 minutes of arc per day by up to 17 minutes of arc. At other times, because the Earth is catching up to Venus as it too travels around the Sun, Venus can appear to stand still or even fall back within the zodiac. At such times it is referred to as being ‘retrograde’.
It is during its most rapid movement within the zodiac that Venus presents itself as the perfect ‘clock’ against which to check the half Megalithic Yard pendulum. At these times a Venus day can exceed the sidereal day by 303 seconds of time. (This Venus day being an Earth day that can be measured between Venus appearing at a specific point relative to a point on the horizon and it doing so again the next day.) This makes such a Venus day 86,467 seconds in length, as opposed to 186,164 seconds for the sidereal day.
When using Venus passing between the suggested angled framework in order to check the pendulum across one 366th of the horizon or sky, it will be observed that this planet behaves slightly differently to a star. Because Venus is also travelling in the opposite direction to the turning sky, it will take longer to pass across the one Megalithic-degree gap than would a star. Let us look at an example set for Orkney, Scotland, where we estimate such calculations must have regularly been made by our Megalithic ancestors.
One Venus day (with the planet travelling at maximum speed within the zodiac) is equal to 86,467 seconds
This means that in order to complete 1 Megalithic Degree, Venus would take 236.2486388 seconds. One 366th of this figure is 0.64548807071 seconds and this should be the period of one beat of the half Megalithic Yard pendulum if Venus was to reliably do the job we expect of it.
Meanwhile, we need to discover the time taken for one beat of a half Megalithic Yard pendulum of 41.48328 centimetres at Orkney. The acceleration due to gravity at this latitude is 981.924 centimetres/second2. A quick calculation tells us that one beat of such a pendulum would take 0.64572263956 seconds.
The difference between the theoretical timing for a Venus pendulum and the true half Megalithic Yard pendulum in this case is 0.00023456885 seconds, which equates to a difference in the size of the full Megalithic Yard of 0.05 millimetres. Alexander Thom found that the Megalithic Yard was 82.96656 centimetres to within a tolerance of + or – 0.06 millimetres. Therefore Venus proves to be an ideal pendulum-setting clock in this case.
We are suggesting that the Megalithic Yard could be checked and set at any latitude between 60 degrees north at the uppermost extreme, down to around 48 degrees north in its southern ranges. Although acceleration due to gravity alters slightly at differing latitudes, we found that the Venus-derived half and therefore full Megalithic Yard defined at any latitude from Orkney down to Brittany remained within Professor Thom’s findings.
It would be incredible to believe that the involvement of Venus, being so perfectly tuned to this experiment, is nothing more than a peculiar coincidence – particularly since the ability of the planet to act as a clock only occurs when it is travelling as fast as it is capable of doing within the zodiac. It is not possible to obtain a Megalithic Yard by this method that is ‘longer’ than that found by Alexander Thom. It could therefore be suggested that if our Megalithic ancestors had carried out their experiment during every day across the whole of a Venus cycle, the ‘longest’ half Megalithic Yard pendulum they could achieve would be the correct one. In reality this would not have been necessary because we are certain they knew exactly when to take their readings (see Appendix 5).
Remarkable as these findings are, the truly amazing fact is that those using the method managed to remain so incredibly accurate, since the deviations found by Professor Thom are so very small. This is indeed a tribute to our Megalithic ancestors who were not only great naked-eye astronomers but also very careful engineers.
The full procedure is itemized below.
1. Create a pendulum by taking a round pebble and make a hole in the centre to pass a piece of twine through (used as a plumb bob to find verticals by Megalithic builders).
2. Draw a large circle on the ground, in an area with a good view of the horizon and sky. Divide the perimeter of the circle into 366 equal parts. This is quite simply done by trial and error but it is almost certain that the Megalithic astronomers knew that a circle with a diameter of 233 units would have a circumference of 732 of the same units (732 being twice 366). They could therefore arrange a diameter of 233 units (any units will do) and then mark off two units on the circumference to identify one 366th of the horizon.
3. Build a braced framework across one 366th division of the circumference of the circle, which can be angled so that it is at 90 degrees to the angle of the path of the rising (or setting) Venus at that latitude.
4. Observe the framework from the centre of the circle. When Venus passes into the braced framework, begin swinging the pendulum. Some initial trial and error is called for, but when exactly 366 beats of the pendulum take place while Venus is traversing the 
gap, the pendulum must be a half Megalithic Yard in length.
5. Repeat the experiment on successive nights if necessary, to account for the differing speed of Venus within the zodiac. The longest pendulum achieved during the full cycle of Venus will be exactly half of the most accurate geodetic Megalithic Yard.
Note: This technique represents one way in which the Megalithic builders could have reproduced the half Megalithic Yard. Time and study might provide others. This horizon method could be subject to very slight inaccuracy as a result of ‘refraction’ of the rising or setting Venus when it is close to the horizon. (Refraction is the distortion of the size or position of an object caused by atmospheric conditions and proximity to the horizon.) It is most likely that Venus was tracked when it was above approximately 15 degrees above the horizon, in order to obviate distortion due to refraction.
Our astronomical associate, Peter Harwood, considers that on balance the setting Venus may have been used, rather than Venus rising as a morning star, though his consideration here has more to do with ease of observation than as a result of any technical considerations.