Astronomy as Relevant to Astrology
The Vedic Sun-Dial
This article explores the ingenuity of ancient Vedic
scholars in extracting the vast astronomical information with
the help of very simple instruments.
Here we propose to explain the method of erecting
a Vedic Sun-dial as described in the Narada Purana
(II.50.125-131), believed to be one of the ancient classics
compiled around 1700 BC, and the Surya Siddhanta, one of the most
important Indian astronomical classics.
The erection of the Vedic Sun-dial
Select a flat smooth stone or prepare a cemented
surface, and level it with water. Draw an even circle having a
radius of 12 units of any measurement. In the centre of this circle
fix a cylindrical shaft, called Shanku. The Shanku
is divided in twelve equal parts and has a height equal to the
radius of the circle.
The classics refer to the unit of measurement as
angula or a fingers-breadth. The Shanku should
be two angulas in diameter, uniformly circular (cylindrical),
twelve angulas in height and made of strong wood. If according
to this measurement a Shanku of twelve angula is formed, it will
have a height of about nine inches.
Knowing the directions
Mark the two points on the circle where the extremity
of the Shankus shadow touches the circle, once in the forenoon
and again in the afternoon. Please refer to the diagram below.
Points A and B represent the forenoon and the afternoon points
Considering these two points as the centre, draw
two bisecting arcs a and b, forming the shape of a fish.
Draw a line passing through the mouth and tail
of this fish. This line is the north-south direction line.
The line will pass through the centre of the circle O and touch
the circle at two points N and S, indicating the north and
south points. This line also indicates the meridian
of the place.
Again form a fish-figure on the north-south
line by drawing two bisecting arcs n and s keeping the north and
south points as their centres. A line formed by joining the mouth
and tail will be perpendicular to the north-south
line and is known as the east-west line. The east-west
line passes through the centre of the circle and cuts the circle
at two points E and W indicating the east and west points.
The east-west line, is also called the prime vertical.
The point O, the point of intersection of the north-south and
east-west lines, which is also the centre point of the circle
where the base of the Shanku lies, is the zenith point.
Thus, after knowing the four directions, by forming
similar fish-figures (bisecting arcs) between the
two points of the directions, find out the four intermediate directions
and mark them appropriately.
Draw a circumscribing square, having its sides equal
to the diameter of the circle and the four corners of the square
touching the four cardinal direction lines NE, SE, SW, NW. The
east and west sides of this square are each divided into twenty
four parts to form a linear scale. Its use is to aid in ascertaining
the length of any given shadow.
Find the position of the Sun
The Indian system of astronomy primarily does the
calculations in the Sayana (tropical) system. To convert the planetary
longitudes to Nirayana (sidereal), subtract ayanamsha for the
given moment from the Sayana position.
Mid-day shadow: When the shadow of the Shanku
coincides with the north-south line, the Sun is exactly on the
meridian of the place. At that time it is local noon or mid-day
of that place. The shadow thus formed is referred to as the mid-day
Equinoctial shadow: When the Sun is either
on the vernal equinox or on the autumnal equinox, the shadow of
the Shanku thus formed on the mid-day is termed as Palabha or
Equinoctial line: Mark the extremity of the
mid-day shadow on the day when the Sun is at the equinox. Draw
a line parallel to the east-west line, touching this point. The
line is called the equinoctial line. Refer to the diagram on the
North declination line: On the day of the
summer solstice, when the Sun is at its maximum declination north,
mark the extremity of the midday shadow and draw a line touching
this point, parallel to the east-west line and mark it summer
solstice line or north declination line.
South declination line: Again on the day
of the winter solstice, when the Sun is at its maximum declination
south, mark the extremity of the midday shadow. Draw a line parallel
to the east-west line passing through this point and mark the
line as the winter solstice line or south declination line.
In the above paragraphs we have referred to summer
and winter solstice as applicable to persons in the earths
northern hemisphere. For people in the southern hemisphere, for
example Australia, Suns maximum northern declination is
referred to as winter solstice and Suns maximum southern
declination as summer solstice.
Now we have got three lines: north declination line,
equinoctial line and south declination line. The equinoctial line
will be in between the two maximum declination lines of the Sun.
On the north-south line, divide the distance between the north
declination line and the equinoctial line in three parts, and
the distance between the equinoctial line and south declination
line also in three parts. On the left side of these six divisions
mark vertically from top to bottom 9, 8, 7, 6, 5 and 4. On the
right side of these six divisions again mark from top to bottom
10, 11, 12, 1, 2 and 3. These numbers represent the rashis. If
the person is located near the equator, these divisions will be
uniformally spaced out. But if the person is located far north
of the equator, the divisions for signs Libra to Pisces would
be more spaced out compared to the divisions for Aries to Virgo.
Reverse would be the case for a person located far south from
the equator. For persons located at higher latitudes, north or
south, it is best to mark the extremity of shadows on the days
of Sayana Sankranti (entrance of Sun in Sayana rashis) either
of the six rashis of Uttarayana (Capricorn to Gemini) or Dakshinayana
(Cancer to Sagittarius).
Observe the extremity of the midday shadow of the
Shanku on any day of the year on the scale. When the Sun is in
its southward course, the shadow point on the scale indicates
the position of the Sun among six rashis from Karka (Cancer) to
Dhanu (Sagittarius). When the Sun is in its northward course,
the midday shadow point on the scale indicates the position of
the Sun among six rashis from Makara (Capricorn) to Mithuna (Gemini).