Astronomy as Relevant to Astrology

The Vedic Sun-Dial

Rajeev Jhanji

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 finger’s-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 Shanku’s 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 respectively.

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 shadow.

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 shadow.

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 right.

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 earth’s northern hemisphere. For people in the southern hemisphere, for example Australia, Sun’s maximum northern declination is referred to as winter solstice and Sun’s 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).