Rama Yantra

Hunter states that Jai Singh himself devised the Samrat Yantra, the Jaya Prakasa, and the Rama Yantra. These three instruments are indeed peculiar to Jai Singh’s observatories, and must be to some extent attributed to Jai Singh’s personal ingenuity.G.R. Kaye, The Astronomical Observatories of Jai Singh
  • Introduction

    The Rama Yantra consists of a pair of cylindrical structures, open to the sky, each with a pillar or pole at the center. The pillar/post and walls are of equal height, which is also equal to the radius of the structure. The floor and interior surface of the walls are inscribed with scales indicating angles of altitude and azimuth. Rama Yantras were constructed at the Jaipur and Delhi observatories only.

    Rama Yantra perspective
    The Rama Yantra pairs and the Digamsa Yantra at the Jaipur observatory.
  • How it works

    The Rama Yantra is used to observe the position of any celestial object by aligning an object in the sky with both the top of the central pillar, and the point on the floor or wall that completes the alignment. In the daytime, the sun’s position is directly observed at the point where the shadow of the top of the pillar falls on the floor or wall. At night, an observer aligns the star or planet with the top of the pillar and interpolates the point on floor or wall that completes the alignment through the use of a sighting guide.
    The floor is constructed as a raised platform at chest height, and is arranged in multiple sectors with open spaces between them. This provides a space for the observer to move about and comfortably sight upwards from the inscribed surface. The instrument is most accurate near the intersection of floor and wall, corresponding to an altitude of 45 degrees. Here, the markings are at their widest spacing, and give an accuracy of +/- 1’ of arc. For altitude readings greater than 45 degrees, the accuracy diminishes, and diminishes to +/- 1 degree near the base of the pillar.

    Watch this 30 second animation of the Rama Yantra at the Delhi Observatory to see how it is used to locate a star at night.
  • Non-identical Twins!

    Among Jai Singh’s many contributions to sky observation, perhaps the greatest was the design of paired instruments such as the Rama Yantra and Jai Prakash at the Jaipur and Delhi observatories. These instruments incorporate inscribed surfaces at regular intervals, with an equal space between them for an observer to stand to take readings. The instruments were exact complements (or opposites) of one another - where one had an inscribed surface, the other would have an empty space for an observer to stand. If you could lift one and superimpose it over the other, the surface would be continuous, since where one had a void, the other would have a solid, inscribed surface. In the Rama Yantra, the floor was constructed with either 30 (Delhi) or 12 (Jaipur) sectors. As a celestial object’s position changes, its position on the floor or wall also changes, and when the object’s position no longer aligns with an indexed surface (when it moves past the edge) the observer simply has to walk to the other instrument to continue the observation.

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