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Ram Yantra Usage

(Photographs by Guntupalli Karunakar)

Ram Yantra are twin cylindrical instruments to measure the astronomical quantities Altitude and Azimuth, of any object in the sky.

Each of the cylindrical Ram Yantra consists of a circular wall, floor sectors and a cylindrical vertical pillar acting as a gnomon, at the center.

The central gnomon, floor sectors and gaps in the Northern Ram Yantra

The wall sectors and gaps in the Northern Ram Yantra

In this instrument every point in the sky is a point on the inner surface of a cylinder. Gaps, however, are cut into the cylinder so that observers can walk through the instrument. The complimentary cylinder is built in such a way that it has segments where gaps exist in the other instruments. Spliced together, the two instruments will have a complete inner cylindrical surface which is a reflection of the sky.

Ram Yantra seems to be constructed precisely to make it very easy for any one to make local co-ordinate measurements of objects in the sky - the Altitude and Azimuth.

(Just for completeness, here is a working definition of the quantities Altitude and Azimuth. The angular height of an object, from the Horizon, is the Altitude.

Here is an image adapted from that illustrates the concepts of Altitude and Azimuth :-

One can drop a perpendicular from the object, onto the Horizon, and mark this location P. Then, starting from the Direction North and moving Eastwards, one measures the angle when this point P is reached. This angle is the Azimuth.)

Each of the cylindrical instruments consists of a circular wall and a gnomon at the center. The height of the walls and the gnomon, has been designed to be exactly equal to the inside radius of the building measured from the outer circumference of the thick gnomon - that is, the height of the gnomon is exactly equal to the length of the floor of the instrument measured from the outer circumference of the gnomon to the inner circumference of the wall.

Vidur Prakash, Haran, Sneh Kesari and Priyanka Sharma helping with the measurement of the lengths of various floor sectors, for calibration purposes. In 2005, we measured the (somewhat varying) floor sector lengths for all the sectors of the Northern Ram Yantra along with a few other calibration measurments and submitted a partial report to the Archeological Survey of India.

Etched at about five feet above the raised floor of the instrument - on the gnomon itself, is a circular scale of Azimuth markings. The edge of one of the sectors is aligned to the North. This is marked as 360 degrees (in Hindi) on the Gnomon markings.

The shadow of the Sun is falling in the gap region of this Ram Yantra and it is time to take observations from the other unit

The walls and floor are graduated for reading Azimuth and Altitude angles - with the Azimuth markings being a linear scale in degrees, while the Altitude markings are in tangents of degrees and therefore not linearly marked. What is needed is to observe the shadow of the gnomon - determine its center and mark it on the floor or the walls of the instrument - wherever it falls.

Azimuth angles are marked on the floor and walls of the Ram Yantra as sectors. The circular ring near the (non-existent) roof of the building has markings for the Azimuth. The smallest division is one fifth of a degree. Azimuth is zero towards the North and increases eastwards.

Once the center of the gnomon shadow (this is not so straightforward and is discussed later) is determined and marked with pencil - one can just read off the Azimuth - by starting with the edge marked as 360 degrees and counting the number of sectors and gaps and individual degree markings up to the center of the shadow. A little trick here, though - the shadow marks a position exactly 180 degrees away from the actual position of Sun in the sky - and therefore 180 degrees would need to be added to the Azimuth reading obtained by counting the sectors and degree lines. Thus angles of 1 degree each can just be read off from the Azimuth markings - finer graduations seem to be missing from the Delhi instrument, in its present condition, but, temporary calibrations for finer accuracy can always be achieved using tape measure placed parallel to Altitude circles.

It is the Altitude markings in tangents of scale that form the beautiful simplicity of usage of this instrument. When the shadow falls at the top of the wall of the instrument - the Altitude of Sun is zero. When the shadow is at the junction between the wall and the floor, the Altitude of the Sun is 45 degrees. Another 45 degrees are marked inward from the wall towards the circumference of the Gnomon - so that, altitudes between 45 to 90 degrees can be read off on the floor of the instrument. The scale is not linear any more (unlike for the Azimuth) - the length of the shadow and the height of the gnomon (equal to the length of the floor sector) give the required Altitude.

Tan (Altitude) = Gnomon Height / Shadow Length

The southern Ram Yantra has certain peculiarities in its construction – some observing windows are closed up, some segments are open in a way different from the regular pattern of the instrument and so on. These peculiarities are yet to be completely understood and documented.

It is interesting to think of the accuracies possible with this instrument - a first time observer, initially, feels rather disheartened due to the uncertainties in estimating the center of the Gnomon Shadow, the blurring between the Umbra and the Penumbra and so on. But, simply because the instrument is built on such a massive scale - the errors induced by these uncertainties are minimal and wonderful accuracies (for educational purposes) can be achieved with this instrument. The instrument also gives a user friendly introduction to the concept of Altitude and Azimuth, to a beginner.

To observe any object other than the Sun – night time objects or the Moon or Venus in the day time, one needs to place one’s eye on the scales of the instrument in such a way that the celestial object is visible exactly against the centre of the gnomon. The corresponding point on the floor or the wall of the instrument gives the Altitude and Azimuth of this celestial object.

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Page last modified on February 06, 2009, at 09:14 AM EST