Sundial, calendar and Khmer temples
The Indian Circle
An ancient Vedic method for determining
an east-west alignment
This web-page illustrates by
photos the implementation of the Indian Circle (or Vedic Circle) for the construction
of a true east-west line, an alignment regarded as auspicious for Vedic
Altars in India - and eventually for later Khmer Hindu temples as well.
Orientation of monumental
structures has a history far beyond the invention of the compass. The
ancient sages and master-builders were capable of constructing the
Pyramids in Egypt and Meso-America very close to the cardinal
directions. Harappan cities from northern India used E-W orientations in
the second millennium B.C. The lay-out of later Chinese cities also
followed a cardinal grid, but with celestial north as auspicious. The
construction of the ancient Khmer city, Shrestrapura, was initiated in
the first part of the first millennium with a centre line oriented
straight E-W, a preference implemented by later Khmer master-builders in
most royal palaces.
The oldest description of the Indian Circle Method is found in the
(400-300 BC) giving
instructions for construction of Vedic fire altars: Yano (1986, p. 18)
translates the Sanskrit text as follows: ‘‘Driving the gnomon into the
leveled (ground), and drawing a circle with the rope whose length is
equal to the gnomon (length), one drives two pegs at (the intersections
of) the two lines where the shadow of the tip of the gnomon falls . This
is the east (-west) line.’’
Re-written as: Fasten a stick (gnomon) on a water
leveled surface and draw a circle with radius identical to the height of
the gnomon. In the morning and in the afternoon and mark where the
shadow of the sun crosses the circle: These two marks are orientated
Ancient Indian literature
on astronomy always refers to a vertical gnomon raised on
a water levelled surface.
The ancient Chinese astronomers used gnomons
oriented parallel to the axis of the Earth, thus pointing
towards the celestial north - close to the present location of
the Polar Star.
Between the tropics
the Sun passes zenith twice a 'year, and the shadow of a gnomon
is a point below it. The use of a vertical gnomon results in
some practical problems determining the exact location of the
solar noon shadow.
The 'Chinese gnomon' will therefore be
implemented in my next sundial. When the sun is at zenith a
plump line hanging from the tip of the gnomon will show the exact
location of the shadow. The noon shadow on zenith-day is significant as it
marks the centre of the Vedic Circle.
Sunrise and gnomon, 24
('shadow pointer') is an ancient astronomical tool and probably
the first device used for celestial observations.
The shadow of the
sun will on any given day of the year follow a curved path from
west towards east. From spring equinox to autumn equinox the
path will curve towards south. From autumn equinox to spring
equinox (yellow area above) the curving is northerly. On the two
equinox days the shadow will follow the nearly straighy line between the yellow
and white areas.
On a coincidental
day in late October (right after the rainy season) a circle was
drawn around the gnomon with a radius = the length of the
The curving path of the shadow was marked throughout
the day. On the photos above the shadow has just passed the
circle for the second time. The Indian textbook on astronomy,
the Surya Siddhanta, informs us that the two
intersections between the curve and the circle are on an
east-west line. The rope passing the two intersections is
thereby oriented straight east-west (or rather very close to).
To the ends of the
rope there are attached two plumps, which hang down below the
terrace and at the same time keep the rope above the terrace
Aligning the two
plump lines one can extend the east-west orientation of the the
two inter-sections on the Vedic Circle (not described in Indian
texts on astronomy).
In the first millennium the
Indian astronomers became aware of that the E-W line described above was
only approximate, because the declination of the sun is slightly
differing in the morning and the afternoon. Corrections to the method
was done in the end of the first millennium and versified in the 11th
An over-looked aspect is the
error-margin in reading the shadow of the gnomon. Within the circle on the photos above the shadow of the
gnomon is easily readable. Outside - with a longer shadow - the centre
of the top of the gnomon is difficult to determine exactly.
Around the solstices the
declination of the sun is practically the same for 3-5 days, which the
observer will perceive as the sun follows the same daily path. The
solstices therefore seem to be the best days to perform the Vedic Circle
with a high accuracy.
Of the two solstices the summer solstice (in the rainy
season!) is the best candidate! On the photo of the solstice floor above
the red circle (exact reading) intersects with the summer solstice path
where the white area curves the most. Winter solstice is where the
yellow part of the sundial floor curves the most. The distance between
the summer solstice intersections is clearly much longer than the winter
solstice intersections. The longer distance of the summer solstice
results in a longer sightline, which gives higher accuracy.
A new series of pictures were taken the 11th of
Determining celestial north - A method for
determining cardinal directions. Dr. Kate Spence. NATURE
Surya-Siddhanta. A Textbook of Hindu Astronomy.
(From: Journal of the American Oriental Society, Vol. 6,
1959-60. Page 141-498)
Bose, D.M., Sen S.N.
& Subbaratapra, B.V.
Concise History of Science in India,
New Delhi, 1971
Lyle, Emily (ed.)
J. Mckim Malville et. al.
Sacred Architecture in the Traditions
of India, China, Judaism and Islam, Edinburgh University
Press, 1992. COSMOS, The yearbook of the Traditional Cosmology
|Malville, John McKim
and the Elements...
The Intuition of
Cosmos in Science and Myth.
|Malville, John McKim
Sacred Skies, Cosmic Geometries and City Planning in Ancient
India. New Delhi, Aryan for Indira Gandhi
National Centre for the Arts, 2000, ISBN
Knowledge of Astronomy in Sanskrit Texts of Architecture.
In Indo-Iranian Journal 29, pp. 17-29. 1986.
|Planet Worship in
Ancient India, Studies in the
History of the Exact Sciences in Honour of David Pingree, Leiden/Boston,
2004, pp. 331-348.
12 November 2007
© Asger Mollerup