Samrat Yantra

This is an equatorial sundial. The central wall is aligned with the rotation axis. As the Sun moves from east to west the shadow of the wall reads the time on the dial. The time marked here is specific to Bengaluru; to get IST, 19 minutes should be added. Over and above this another seasonal correction called equation of time is to be added.

Maharaja Jai Singh's Observatories in Delhi, Jaipur, Varanasi and Ujjain have very large Samrat Yantras.

The Samrat Yantra is basically a sundial- a time measuring device. Though the literal meaning of Samrat Yantra is "The King of instruments", it is a very simple device in terms of its design.

The essential parts of a Samrat Yantra are the gnomon, a right triangular wall with its hypotenuse parallel to the Earth’s rotation axis and a pair of quadrants on the either side lying exactly in the plane of the equator.

A scale is fixed on to this semi circular region. The graduations on the scale are made such that 1800 corresponds to 12hours and further divisions are made accordingly. As the Earth rotates from west to east and the sun appears to move from east to west. The shadow of the gnomon on the scale is therefore a measure of this movement. Looking at the edge of the shadow, one can read the local time.

To obtain the standard time, one should apply corrections corresponding to the earth’s elliptical orbit and the longitude difference between the instrument’s location (Bengaluru 77.60E) and its time zone(IST 82.50E)

A larger sundial offers scope for improving the precision in the measurement of the local time by adding subdivisions for every unit on the scale.

For example, the huge Samrat yantra at Jaipur is capable of measuring time to an accuracy of two seconds. That means there are fine markings on the scale that indicate two second time intervals.

The large size of the instrument poses a problem too. As the distance between the edge of the gnomon and the scale is lerge, the shadow will have a diffused edge. That is the edge of the shadow is not sharp but instead will spread across a finite area, though small, rendering it difficult to fix the tip of the shadow accurately.