How to Build a Yardstick from Celestial Observation

Some ideas arrive politely, present their evidence, and wait patiently for peer review. Others wander in from the horizon pointing at the sky and insisting that time itself is a measuring tool. The megalithic pendulum belongs firmly to the second category. Yet beneath the romance lies a genuinely intriguing proposal: that prehistoric builders may have derived a reliable unit of length not from stored objects, but from the predictable motion of the Earth and planets. 

At first glance, the pendulum seems almost insultingly simple; a weight on a string. But simplicity is often where robustness hides. Once you understand what a pendulum does reliably, and what it ignores entirely, its appeal as a measuring device becomes clear.

What a pendulum actually measures 

A pendulum swings under gravity. Crucially, for small swings, its period; the time it takes to swing from one side to the other and back again, depends only on two things: the length of the string and the local strength of gravity. It does not depend on how far the pendulum is pulled aside. Swing it gently or with more force; the timing remains the same. 
 
This property, known long before it was formalised mathematically, makes the pendulum a natural timekeeper. It is also why pendulums first appeared not in clocks, but as plumb lines. A hanging weight always seeks vertical, quietly obeying gravity without argument. 

Galileo and the move from observation to understanding 

Although pendulums had been used for millennia, it was Galileo Galilei in the seventeenth century who first studied their behaviour seriously. According to tradition, he noticed a swinging lamp in church and timed its motion against his pulse. Whether apocryphal or not, the conclusion was sound: pendulums keep time with impressive consistency. 
 
This insight eventually led to pendulum clocks, which regulated timekeeping across Europe for centuries. In essence, clocks are simply disciplined pendulums. 

Why time beats distance 

Distance is inconvenient. Wooden rods warp, knots loosen, and no two human feet are quite the same length, despite historical attempts to pretend otherwise. Time, by contrast, is stubbornly repeatable, especially when tied to astronomical motion. 
 
If a society can measure time accurately, and if that time can be converted into distance, then a physical measuring stick becomes unnecessary. The standard can be recreated anywhere, at any time, provided the sky behaves itself. 

The Megalithic Yard 

The Megalithic Yard, proposed by engineer Alexander Thom, is a unit of length measuring approximately 82.97 centimetres. Thom identified this length repeatedly across stone circles and monuments throughout Britain, from Orkney to Brittany. 
 
The controversial claim is not that prehistoric people could measure accurately, but that they could do so consistently over large distances and long periods of time, without metal tools or written standards. This is where the pendulum re-enters the story. 

Turning the sky into a ruler 

The method described in the book Who Built the Moon [Christopher Knight and Alan Butler - Watkins Publishing]? suggests that time can be converted into distance using celestial motion. The horizon is divided into 366 equal angular parts, achieved through simple stepping and alignment methods (An angle can be represented by a distance if the viewing point is fixed). A braced wooden frame marks one such division. 
 
Venus, observed as an evening star during the part of its cycle when it moves fastest against the background of stars, passes through this narrow slice of sky. As it does so, an observer swings a pendulum and counts the number of swings. 

From swings to yards 

If the pendulum completes exactly 366 swings while Venus crosses the frame, the pendulum’s length corresponds to half a Megalithic Yard (41.48 centimetres). Doubling that length produces the full yard. 
 
No stored ruler is required. The standard is recreated from scratch using gravity, planetary motion, and careful counting. 

Accuracy, tolerance, and gravity’s quiet variations 

Gravity varies slightly with latitude and altitude, meaning pendulums of identical length will swing marginally differently in different places. However, these variations are small. The resulting differences fall within the tolerances identified by Alexander Thom in ‘real’ monuments. 
 
In other words, the method is not perfectly precise, but it does not need to be. It only needs to be consistent enough to explain the archaeological evidence. 

Why this idea unsettles archaeologists 

There are two principal objections. First, transmission: how was this knowledge preserved and taught accurately over generations? Second, assumption: did prehistoric societies really maintain the observational discipline required for such measurements? 
 
Neither objection disproves the idea outright. They simply demand stronger evidence. As with much of archaeoastronomy, the debate is not about possibility, but plausibility. 

Why it matters, even if it’s wrong 

Even if the Megalithic Yard ultimately proves illusory, the exercise remains valuable. It demonstrates that sophisticated measurement does not require advanced technology; only careful observation and patience. 
 
More importantly, it challenges the reflex assumption that prehistoric people were imprecise or unsystematic. Sometimes the tools are simple because they are sufficient. 

A final thought 

Is the megalithic pendulum proven? No. Is it ingenious? Absolutely. For a method involving sticks, stars, and a swinging weight, it does a surprisingly convincing impression of science. 
 
Gravity keeps the beat. The sky sets the clock. All we have to do is count.