Ancient Observatories and the Solstice

By Doug Criner

I am a student of ancient man-made structures that served as astronomical observatories aligned to sunrise at the summer solstice.   Maya structures, Stonehenge, and the Callanish Standing Stones are examples.  There is little doubt that these and other ancient structures served as celestial calendars, but they would have been imprecise in telling the exact day of the year. 

Callanish Standing Stones, Isle of Lewis, UK

 Television Documentaries

Most of us have been enchanted by television documentaries of Stonehenge, Chitzen Itza, or other ancient observatories—with the sun’s rays, at dawn on the solstice or equinox, magically aligned through a stone aperture or slit, now accompanied by the festivities of modern druids, mystics, and curious tourists in rapture.  I have not attended one of these extravaganzas, but it would appear from my studies that you could visit a few days before or after the solstice, when lodging rates are less expensive, and enjoy the same solar experience with perhaps a more intense and solitary rapture. 

At the summer solstice in the northern hemisphere, the sun’s azimuth reaches its most northerly position.  With proper instruments, one could theoretically detect when the rising sun's azimuth (measured in degrees, clockwise from north) is at its minimum.  However, I question whether such ancient observatories had the necessary precision to pinpoint the week, much less the date of the solstice. 

Sample Almanac Data 

For the Year 2004, I obtained the following early-morning almanac data for Chicago from the U.S. Naval Observatory’s website, which provides sun position at one-minute intervals:

Date	June 18	June 19	June 20	June 21
Time (CDT)	05:18	05:18	05:18	05:18
Sun Altitude	0.1°	0.1°	0.1°	0.1°
Sun Azimuth (east of north)	57.3°	57.3°	57.2°	57.2°

 2004 Chicago Almanac Data, U.S. Naval Observatory

Please examine the above data, and see if you can discern the date of the summer solstice.  You can’t with certainty, but you might guess either June 20 or 21.  (In fact, the summer solstice occurred at 19:57 CDT on June 20—a little earlier than usual because of 2004 being a leap year—and was viewable at sunrise only in locations where the sun rose at 19:57 CDT.)  The above data show the sun slowly dropping in its tracks as it stops its northerly travel, and reverses its movement southward. 

“Solstice,” you may remember, means “the sun stands still,” and there is very little difference in the sun’s azimuth at sunrise on the date of the solstice versus the days immediately before or after.  If the sun’s position varies less than 0.1° from one day to the next near the summer solstice, ancient astronomers detecting the solstice at Chicago's latitude would have needed an optical apparatus with a precision of better than 0.1°.  This would be difficult to achieve, especially considering that the sun itself subtends about 0.5° of arc in Earth’s sky.  Another problem is that the actual solstice may occur up to 12 hours earlier or later than the time of local sunrise.

My theory is that ancient observatories, relying on solstice observations, couldn’t detect, much less predict, the date of the solstice with an uncertainty of less than a few days.  That was sufficient accuracy if, as I suspect, the main interest was to schedule planting dates and annual rituals,  and to keep their calendar synchronized to the heavens within a few days of what we now would term “correct.” 

The ancients may have believed they were observing with high precision, but how could that belief have been confirmed?  A few days here or there would not result in a significant variation in seasonal climate changes and lunar calendars would provide limited if any insight into the solar calendar.  If a year comprised an integer number of days, it wouldn’t be necessary to continuously update the calendar through celestial observation; it would only be necessary to determine the number of days in a year, and then not lose count.  Perhaps, with multiyear records of the solstice, the ancient astronomers could begin to discern a trend toward what we now call leap years. 

Better Methods Than Observing the Solstice

Compared to observing the sun at solstice, there are other, more accurate, methods that the ancients could have used to synchronize their calendars.  In latitudes between the Tropics of Cancer and Capricorn, where the sun appears directly overhead twice a year, a vertical shaft built with the aid of a plumb bob could be used to accurately observe those events, especially in years when the sun’s altitude of 90° occurs near noon, local time. 

There are better times than the solstice to observe the rising sun’s azimuth to calibrate the calendar.  At the solstice, when the sun’s azimuth is barely moving from day to day, it is a very difficult proposition to discern the minimum azimuth day as shown by the Chicago data table above.  Around the time of the equinoxes, on the other hand, the azimuth of the rising sun in Chicago is changing by about 0.5°/day—perhaps 5-10 times faster than near the solstice. 

Although it is a unique event, there is nothing special about the solstice as far as indexing a time of year.  An observatory could be aligned in any arbitrary direction or azimuth, where the position of the rising sun is moving fast.  With a faster moving sun azimuth, a more useful data point, with better resolution, could be obtained; with this procedure, the time of year could be measured within a day or two.  Further, this approach, unlike a summer solstice observation, would provide two opportunities per year to update the calendar.  Relying solely upon a summer solstice observation, overcast weather could cause the calendar to go unadjusted for an interval of two years or more. 

Possibly, ancient astronomers were mistakenly leery of trying to record the sun’s passage past a particular azimuth when it was jumping quickly, say 0.5°/day:  in some years, the sun would rise exactly “on target,” and in other years, it would jump over the target azimuth while the earth was turned.  A stationary target is easier to hit with a projectile than a moving target—but that analogy fails because we are not trying to “hit” anything, we are trying to detect minima or transition times.

Another alternative is to observe the azimuth of a particular rising star.  Unlike the sun, a star is a pinpoint of light, and thus affords a more precise measurements of azimuth.  Also, it's obvious that a setting sun or star would be just as useful as a rising celestial body.  Perhaps ancient shamans considered a dawn demonstration of their intellectual prowess more spectacular than at dusk.

Better Observatory Designs

Majestic manmade structures with slits or peepholes aligned in a particular direction are not the most precise way of observing a rising celestial body.  A gun, for example, can be most precisely aimed when its front and rear sights are spaced far apart.  Likewise, accurate observation of the sun’s rising position would best be performed with a “rear sight,” perhaps a narrow notch in a rock or structure, and a distant “front sight,” a sharply defined terrestrial feature on the horizon, silhouetted in the dawn twilight.  Such a system could achieve much higher precision with less expense and folderol than a Stonehenge-like configuration of large stones arranged relatively close together. 

Conceivably, in their admiration of majestic and mysterious prehistoric structures, archeologists may have overlooked other ancient celestial observatories composed of such simple front and rear sights, separated by long distances.  Or, perhaps, the ancients' infatuation with the solar solstice, not unlike that of modern Man, distracted them from developing more precise and simpler methods for calibrating their calendars.

 © 2004 Doug Criner