Gazing at the horizon.

Anybody who devotes themselves to the history of astronomy, as I now done for many years, at some point starts to study the history of humanity’s attempts to measure, mark and record the flow of time, as these activities were, since the their beginnings in the mists of prehistory, the province of the astronomers. In my capacity as a historian of the recording of time I held a public lecture on the history of the calendar, the origins of Easter and the Gregorian calendar reform and especially Christoph Clavius’ role in this reform in the Nürnberger Planetarium on Wednesday, it being of course the 29th of February a fitting date for the subject. The lecture went very well, which is not guaranteed because although I put an incredible amount of preparation into my lectures I actually hold them completely free without notes and only use slides as illustrations without texts thereby risking a total disaster. It was good that it went well as the lecture was comparatively well attended, which is always pleasing when one has gone to a lot of effort.

I managed to field all of the questions that followed my talk without any real problems, always a period of uncertainty on such occasions. Am I going to make a fool of myself? Although I have no problems admitting that I don’t know the answer to a question, asking if anybody in the audience does or simply saying I’m afraid I don’t know that I’d have to look it up. Funnily enough some of the questioned surprised me because I simply hadn’t given enough thought to the questions that a lecture on the history of calendars might provoke. Somebody, for example, asked if other cultures had other lengths to their weeks or whether the seven-day week was universal. Another asked about the Mayan Long Count and 2012, which I wasn’t expecting but should have been if I had given the subject enough thought. As I said, I had no problems with the answers but had not been expecting the questions.

This having been the case I’m currently skim rereading my small library of books on the history of calendars and measuring time in order to refresh my background knowledge on the subject as I shall be holding the same lecture twice in Berlin shortly before Easter. Now several of the books I own on the subject are well written and entertaining but are unfortunately full of historical errors. The authors suffer from the common problem of letting a good story get in the way of the facts. I was reading one of these books on the bus this morning when I stumbled over the following:

Egyptians were not alone in their early turning to the sun. Far beyond the great Nile valley and even the Mediterranean, on the distant edge of the Eurasian continent, a little-understood people also figured out a close approximation of the solar year a few centuries after the Egyptians. We know this only because they left behind what appears to be an enormous calendar constructed out of immense slabs of bluestone, standing upright to form megaliths, some of them topped by lintels called henges. Standing on the barren Salisbury plain, this structure, Stonehenge, was used for over two thousand years by ancient Britons, who aligned the stones so that at the precise moment of the summer solstice a ray of sun shines down the main avenue and into the centre. […]

the layout leaves no doubt that the people who built it were astronomically sophisticated enough to build a device to accurately measure the solar year. (The bold emphases are mine)

The ‘at the precise moment’ statement is of course a hopelessly exaggerated claim. The solstice alignment is vague enough that there is an academic discussion as to whether it’s the summer or winter solstice that is intended. However that is not the reason I have chosen to criticise the passage here. The thing that disturbs me is that the author obviously thinks that one can only determine the solstice alignment if one has first accurately determined the length of the solar year. This is not the case.

Somewhere in Neolithic Europe around two or three thousand BCE, you are a herdsman, a night watchman or just an insomniac who likes looking at the stars. It is spring shortly before dawn and you are facing east as the sun comes up over the horizon and you unconsciously note that it has risen to the left of a prominent tree on the horizon. A few days later you realise that the sun has now risen directly behind the same tree and another few days later clearly to the right of the tree. Each day it continues its slow journey northwards too slow to really follow from one day to the next but over the period of three or four days the movement is clearly perceptible. You continue to observe as the summer comes along and at some point after the sun has appeared to stand still for two or three days apparently rising at the same point you realise it is now moving to the left travelling back the way it come. You make a mental note of the position on the horizon where it stopped its journey northwards and started its journey back to the south a jagged outcrop, a prominent tree or maybe a notch in the skyline. You continue to follow the sun at regular intervals along the horizon till in the winter it stops again and turns back towards the north. The two points on the horizon that you have mentally registered give you the alignment of the sun at summer and winter solstice but you don’t actually know how long the year is. Of course the lengthening and shortening of the days as the sun journeys back and fourth strengthen your awareness of the cyclical nature of this process and that what you are experiencing is the solar year but how long it is, is still not very clear. You could try counting the days from one summer solstice to the next but which day exactly is the solstice day. The sun appears to hover around the solstice point for two or three days. You might strike out lucky and choose exactly the right day giving you a year length of 365 days but you might equally well, in fact more probably, arrive at a total two to four days too long or too short; however your solstice alignments are pretty accurate and more than accurate enough to align a monument such as Stonehenge.

This type of horizon astronomy that I have just described is considered by the archaeoastronomers to have been the origins of astronomy in prehistoric times and to have been the method actually used to align prehistoric monuments. It is all too easy to make the mistake that my calendar book author made and attribute more advanced knowledge to earlier culture than they actually probably possessed by projecting our way of thinking about the problem on to them. We know the length of the solar year and the lengths of the four quarters between the solstices and the equinoxes so it seems logical to us that if somebody can fairly accurately align an architectural structure to one of those points then they must have possessed the same knowledge that we do. I hope I have shown that this is far from being the case and one should as a historian be more than somewhat circumspect when making such claims. You don’t have to know the length of the year to record the summer solstice.


Filed under Archeoastronomy, History of Astronomy, Myths of Science

11 responses to “Gazing at the horizon.

  1. Pingback: Gazing at the horizon. | Whewell's Ghost

  2. I can almost think of this as an example of early calculus: looking for an extreme value. It’s local information (position of the sun against the horizon each day) put to good use.

  3. hairyape68

    In the spring the sun might rise to the right of a tree and then a few days later to the left of that same tree if we are in the northern hemisphere. In the southern hemisphere it would work the other way around. Looking for the sun to rise in the same place in the spring in one year and again in the next would give a pretty accurate measure of the length of the year; the sun moves north fast in the spring. One would of course need to count the days from one year to the next. One could locate the direction of the summer solstice sunrise by noting the direction of sunrise a few days before the solstice and noting the number of days until the sun again rose in that direction; the solstice direction would have been on the day mid-way between these two days. I might do this to find the direction of the solstice sunset. I’m not getting up early enough for sunrise, 5:44 AM Central Daylight Time here in mid-Missouri.

  4. You say: “the author obviously thinks that one can only determine the solstice alignment if one has first accurately determined the length of the solar year”. This is not how I read it. It’s the other way round. If one knows the day on which the solstice occurs in two successive years then one has a good approximation to the solar year, better than on other seasonal observations. Of course this assumes several things: that the henge-builders could count, and had a reliable calendar based on other observations (moon, stars, seasons, etc). Thought I should post here rather than trying to communicate in a series of tweets.

  5. The builders of Stonehenge did not determine the solstice to better than somewhere within two to three days. To quote Clive Ruggles, “Stonehenge appears to be roughly aligned on the midsummer sunrise and midwinter sunset…” (my emphasis) Determining the actual day of the solstice needs a many year series of observations and then the necessary mathematical knowledge to calculate an arithmetical mean.

  6. Interesting post. I love public lectures for exactly the reason you give, Thony. You never know what you will get with questions, and they usually inspire me to expand my field of vision of the material I’m working on.

    • I probably spend ten times as long researching material that I don’t use in my lecture than that which I do use just to be certain that I really master the subject for the questions and discussion after I’ve finished talking.

      For this reason I always try to chose subjects for my public lectures that I want, for what ever reason, to deepen my knowledge of anyway. Preparing for a lecture is a deep learning process.

  7. Richard Heath

    The established evidence for Fifth Millenium astronomy of very high accuracy, using the horizon, cannot be doubted, at Carnac for example, but this has simply been ignored by academics. It is not a true or useful picture that “Somewhere in Neolithic Europe around two or three thousand BCE, you are a herdsman, a night watchman or just an insomniac who likes looking at the stars”. Megalithic astronomy indicates that an entire culture sought an accurate description of celestial events and built monuments of high accuracy in the process..

  8. Will your Berlin lectures be in German or in English? In the latter case, we might grab a bus and come up.

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