Samrat Yantra 
Written by Yogesh Soman 
Sunday, 26 March 2006 00:06 
The Samrat yantra or the 'Supreme Instrument' is Jai Singh's most important creation. The instrument is basically an equinoctial sundial, which had been in use in one form or the other for hundreds of years in different parts of the world. In India, Brahmagupta (AD 598) describes Kartari yantra, an equinoctial sundial, which operates on the same principle as the Samrat. Jai Singh however, turned the simple equinoctial sundial into a tool of great precision for measuring time and the coordinates of a celestial object. Figure 1 illustrates the principle of a Samrat yantra. The instrument consists of a meridian wall ABC, in the shape of a right triangle, with its hypotenuse or the gnomon CA pointing toward the north celestial pole and its base BC horizontal along a northsouth line. The angle ACB between the hypotenuse and the base equals the latitude lambda of the place. Projecting upward from a point S near the base of the triangle, are two quadrants SQ1 and SQ2 of radius DS. These quadrants are in a plane parallel to the equatorial plane. The center of the two 'quadrant arcs' lies at point D on the hyptenuse. The lengh and radius of the quadrants are such that, if put together, they would form a semicircle in the plane of the equator.
The tangential scale over Samrat gnomon. The scale indicates angle of declination. The quadrants are graduated into equallength divisions of time measuring units, such as ghatikas and palas, according to the Hindu system, or hours, minutes and seconds, according to the Western system. The upper two ends Q1 and Q2 of the quadrants indicate either the 15ghatika marks for the Hindu system, or the 6 A.M. and the 6 P.M. marks according the Western system. The bottommost point of both quadrants, on the other hand, indicates the zero ghatika or 12 noon. The hypotenuse of the gnomon edge AC is graduated to read the angle of declination. The declination scale is a tangential scale in which the division lenghts gradually increase according to the tangent of the declination angle as illustrated in figure 2. The zero marking of this scale is at point D. Further, the gnomon scale AC is divided into two sections, such that the section DA reads angle of declination to the north of the celestial equator, and the section DC reads the declination to the south, as illustrated in the figure 2. Samrat Yantra perspectives. The primary objective of a Samrat is to indicate the apparent solar time or local time of a place. On a clear day, as the sun journeys from east to west, the shadow of the Samrat gnomon sweeps the quadrant scales below from one end to the other. At a given moment, the time is indicated by the shadow's edge on a quadrant scale. The right ascension (RA) of an object is determined by simultaneously measuring the hour angle of the object and the hour angle of a reference star. From the measurements, the difference between the right ascension of the two is calculated. By adding or subtracting this difference to the right ascension of the reference star, the RA of the object is determined. To measure the declination of the sun with a Samrat, the observer moves a rod over the gnomon surface AC up or down until the rod's shadow falls on a quadrant scale below. The location of the rod on the gnomon scale then gives the declination of the sun. Declination measurements of a star or a planet require the collaboration of two observers. One observer stays near the quadrants below and, sighting the star through a sighting device, guides the assistant, who moves a rod up or down along the gnomon scale. The assistant does this until the vantage point V on a quadrant edge below, the gnomon edge above where the rod is placed, and the star  all three  are in one line. The location of the rod on the gnomon scale then indicates the declination of the star.

Last Updated on Sunday, 26 March 2006 00:37 