The Stonehenge Eclipse Calculator

Bill Kramer

During a visit to Stonehenge in 2001 the tour guide said that Stonehenge might have been used as an astronomical calculator.  He mentioned that it was said to predict eclipses however he did not know the specifics when I questioned him.  This led me to begin researching the subject deeper in order to learn more about Stonehenge and how it could have been used as an observatory.  The gift shop at the Stonehenge monument site contained several small booklets that assisted greatly in this quest.  I've supplied a list of good references at the end of the article.

The fact that many of the large stones line up to mark the solstice and equinox as well as other astronomical timings related to the seasons is well known.  However, this knowledge alone cannot help in eclipse prediction unless it is done to a much higher degree of precision than can be accomplished with large rocks and wooden posts.  And the degree of sophistication needed to conduct a long term experiment into the repeating cycles of the Sun, Moon, and Earth would have resulted in more than just a ring a well placed stones.  Archeological studies have found evidence of holes that once held wood posts that mark the positions of the moonrise relative to other stones.  This leads one to wonder just how much the ancients who built Stonehenge were researching the cycles of the heavens. 

Several clever explanations for the stone configurations related to solar system objects have been proposed along with the sun and moon tracking.  Other stone circles from the same archeological time period do not have the same configurations thus making Stonehenge somewhat unique in that regard. A lack of written records from the time period in question means that we can only guess.  Some of the guesses have been interesting while others can only be considered preposterous given what is known these days about the civilization and times through archeological study.  But then, archeology is a science of based on the scientific method with little opportunity for definitive proof.  This means that sometimes there are breakthroughs that prove previous conceptions incorrect.  Perhaps the builders of Stonehenge were astronomical geniuses and their work is only preserved in our spirit to understand.

Stonehenge Eclipse Calculator Origins

So where did this story that Stonehenge could be used as an eclipse calculator come from?  Turns out that the stones themselves are not the source of the story.  The story stems from a ring of holes around the outside of the circle. 

What were the original purposes of the pits?  They do not have the same characteristics as pits dug to hold posts nor those dug to support the stones.  Perhaps they were instrumental during the construction or they were built for other symbolic purposes.  Without any form of written records it is guesswork.

Set Up

The elements required are simple.

                56 holes

                4 markers

Start Up

On the day of the summer solstice, place the Sun marker in hole #56.

On the evening of a full moon, place the Moon marker in hole #56.

Begin advancing the Sun and Moon marker every day.

• If you observed a solar eclipse, the Moon and a Node marker are in the same hole as the Sun. 
• If you observed a lunar eclipse, the Moon marker is opposite the Sun marker and each have one of the Node markers included with them.

You can now begin the regular ritual of advancing the markers.

(Note, the solar eclipse of June 21st, 2001 was a unique opportunity to set up a Stonehenge style calculator base.  Summer Solstice plus the Solar eclipse on the same day.)

Predicting an Eclipse

Once the markers have been placed you can use the model to predict the next eclipse.  The markers are moved from one hole to the next according to the following schedule.

Moon marker - move twice a day, once in the morning and again in the evening.

Sun marker - move every six and one half days.  Alternate mornings and evenings.

Node markers - move every four months (moved in opposite direction)

The Moon and Sun markers are moved in the plus direction (hole 1 to 2, 2 to 3, and so on) while the nodes are moved in the minus direction (hole 3 to 2, 2 to 1, 1 to 56, and so on).

The Moon and Sun markers can be "corrected" at standard alignment times.  The Moon during the Full Moon each month and the Sun during the Solstice.  The nodes would be recalibrated each time an actual eclipse was observed.

The method is not 100% accurate but it is close and can be refined even further.  We can refine the model by understanding more about why it works.

Refining the method

The orbits of the Sun, Earth, and Moon are cyclical.  When counting days the cycles do not divide into nice neat integer numbers.  But that does not mean integer numbers (also known as whole numbers) cannot be used.  The key to using integer numbers is to use a common multiplier result.  It just so happens that the number 56 works pretty well.

The Sun: Multiply 6.5 by 56 and the answer is 364.  This is just 1.25 days short of a proper year.  Thus moving the Sun marker every six and one half days is "pretty close".  Every six months the Sun marker can be corrected during a Solstice (Northern most or Southern most travel of the Sun during a year).  The maximum error would be just over half a day with regularly applied corrections.  When doing a simulation of the calculator sequence you must take into account the extra day and add yet another every four years just like a leap year.

The Moon: Divide 56 by 2 and you have 28, just about the number of days required for the Moon to orbit the Earth and return to the same position relative to the background stars.  The actual value is 27.322 days thus requiring regular corrections to be applied based on actual observations.  To apply the corrections in a simulation of the calculator for future eclipse prediction skip one hole each orbit about the circle.  This would allow the Moon marker to complete a circuit in 27.5 days.  If another hole is skipped every third circuit then the average cycle over three lunar orbits is 27.33 days.  The result of (27.5+27.5+27)/3 is 27.333 and that will keep the simulated future position of the Moon very close to reality.

The Nodes: The nodes movement, three times a year, means than 18.66 years are needed for a complete circuit.  The actual orbit of the nodes is 18.61 years.  Every 18 years, the node values will be off by about one month.  Thus four cycles (4 x 18 years) are needed before the nodes are off by one position in the circle.  Corrections applied by direct observation will allow the Stonehenge calculator to keep an accurate eye on upcoming eclipse events.  As for simulations, 72 years before the nodes are off one position is not too bad.

Refined Method Description:

• The Sun advances one position every 6-1/2 days.  Twice a year, the interval will be 7 days (adding a half day before the advance of the marker).
• The Moon marker advances twice a day.  It skips one hole on each orbit of the circle.  Every third orbit an additional hole is skipped.
• The Nodes are advanced in the opposite direction three times a year.

Using the refined method a simulation was able correctly predict eclipse event dates up to ten years before and after a known eclipse date.

Such knowledge may have been useful to the builders of Stonehenge.  We can never know however it is fun to speculate.

Reference material

Written works -

• The Astronomical Significance of Stonehenge - C.A.Newham  (Coates & Parker LTD (2000 edition), first published 1972)
• Stonehenge Sun, Moon, Wandering Stars - M.W.Postins (Malthouse lane, Kenilworth, Warwickshire)
• Stonehenge Decoded - G.S. Hawkins (Souvenir Press 1965)
• Beyond Stonehenge - G.S. Hawkins (Hutchinson 1973)
• Sun, Moon, and Stonehenge - Robin Heath (Bluestone Press 1998)
• Megalithic Lunar Observatories - A. Thom (Oxford University Press 1971) 

Web Sites -