sundial introduction

Astro-archaeology = archaeo-astronomy = Astroarchaeology = archaeoastronomy
Sundial, calendar and Khmer temples

Introduction to the gnomon

The gnomon is probably mankind's oldest astronomical device. The gnomon as a vertical pole has been used for calendric purposes throughout the year determining annual religious ceremonies and agricultural cycles. The shadow of the gnomon has been used for time-keeping throughout the day. The rising sun tells when the day starts and the gnomon tells midday, or solar noon when the shadow points towards north (in the tropics seasonally towards south as well).

The gnomon has also been used to determine the cardinal directions and has for millenniums been used as a device for constructing Vedic alters oriented straight east-west. Ancient cultures with cities constructed with a cardinal layout would also have known the gnomon as the compass is a relative new invention dating back to the end of the first millennium.
On the pictures below the shadow points towards celestial north. The shadow follows the straight line at equinox and will reach the curved line at winter solstice.

The author was born and brought up in Denmark and has since 1998 conducted astronomical experiments at a test site in in the Phu Phan Mountain Chain in NE-Thailand, Isan. As a keen photographer the initial purpose was to get acquainted to the movements of the sun throughout the year.
The empiric knowledge from the sundial has later on been used in field research about orientation of ancient Khmer temples in SE-Asia and eventual relationship to archaeoastronomy.

From vernal equinox to autumnal equinox

Analemma and declination lines on the terrace floor


Above: The red line marks the position of the shadow of the sun as it moves on a given day. Here called the 'day path'. Astronomically it depicts the declination of the sun.		Above: The red line marks the position of the sun's shadow at 12:00:00 mean time on various days from November to February. The astronomical term is an analemma.


February the 9th. at 12:07	February the 9th. at 10:30

Explanation to the horizontal sundial

The declination lines (the daily paths of the sun) are very easy to make. One only needs a gnomon (pointer) and a levelled surface. After making declination lines throughout a year it will be evident for the observer when it is equinox day: The day when the shadow moves (nearly) straight east-west. The equinox day is easy to determine and easily perceivable.
Conclusion 1: Constructing cardinal grids in a larger scale in rather simple.
Ancient cardinally orientated layouts in the size of Angkor Wat (Cambodia), Shestrapura (Vat Phou, southern Laos) or Harappa (NW-India) do not indicate sophisticated astronomy.
The solstices are more problematic to determine. On the solstice days the declination lines curve northernmost and southernmost, but the exact day of solstice is difficult to determine, because the change in declination around the solstices is very small. For 3-5 days the sun apparently follows the same daily path.
Conclusion 2: Determination of the solstices is more problematic.
Conclusion 3: The ease of determining equinox day makes this a prime New Year candidate.

Analemma
The analemmas are a little more sophisticated, because they can only be made by using a modern watch telling modern mean-time. The analemmas are irrelevant for determining the cardinal directions and therefore irrelevant in an astro-archaeological context.
The position of the shadow is marked for every 30 minutes from 9:00 to 15:00 occasionally throughout the year.
Full drawn lines are marked lines. Dotted lines are extrapolated lines.
On an horizontal analemmic sundial one can read mean time time and date throughout the year.

Gnomon and archaeoastronomy

The gnomon chosen by random was a iron rod nailed to a wooden pole casting the shadow on a wooden terrace. This has turned out to have some disadvantages. First of all: Wood is of limited durability. After five years termites caused the construction to tilt. Adjusting the dial took 3 years. And now in year 8 the platform is falling apart...
In 2007 the emphasis will be put on painting the platform in colours depicting the year. The purpose is to take photos visually demonstrating the use of a gnomon. Analemmas will not be depicted as they seem to irrelevant in relation to archaeoastronomy.
In 2008 the dial will be left to the termites - and a new permanent dial will be constructed.
The new dial will not be a a vertical gnomon, because this has shown out to have limitations in determining the two days when the sun passes zenith. The new lay-out will have the gnomon pointing towards the Northern Star and thereby parallel to the axis of the Earth. This equinoctial type of a sundial has been used in China for millenniums, even China is located north of the tropics.

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macsida@thai-isan-lao.com