An Investigation of the Sundial at Abhayagiri Monastery in Anuradhapura, Sri Lanka

Abstract

The ancient text of vāstuvidyāśāstra states that lot of beliefs and rituals were performed before making a sundial. Thereafter, these rituals were performed daily at the beginning of the day beforetaking the first reading. Though we call it traditional knowledge, through this research we came to the conclusion that behind all these rituals and formalities there is a scientific theory. A question arises whether the expert ancient sundial users may have known that the earth circles around the sun while rotating on its geographical north-south axis. The Abhayagiri Sundial has been found in the refectory. This leads to the belief that this was used to calculate the noon meal time for the Buddhist monks. But in truth, this was also used to calculate the times of a broader range. For example; auspicious times, seasons of the year, the fall of Sinhala and Tamil New Year (the new year times change every year according to the calculations of the traditional calendar called (‘litha’). Sinhala and Tamil New Year, in other words is called as Sun Festival (Sūrya Mangalya). True meaning of this name comes to light through this research. All ancient sculptures, frontispieces and walls were plastered and painted. We believe that the sundials too were plastered with a thin white layer and have been painted with lines and motifs. It appears that the users of Abhayagiri Sundial did not possess a clear knowledge of the equation of time. This is the inequality of the sun’s motion which results in the true noon crossing the observer’s meridian before or after the mean noon, which can have a positive or a negative value through the solar year. This needs a correction chart or a curve to verify the true noon. But surely they knew how to mark out the shift of the sun across the celestial sphere between solstices and equinoxes, which will basically relate to the seasons of the year. This is in addition to keeping time. With available data on Abhayagiri Sundial, further mathematical resolutions are not possible. This being a unique design, when compared to standard horizontal sundials, how the markings on the sundial face collaborated with the sun-earth motion is immediately unpredictable.