The sundial is one of the oldest and simplest time reckoning devices. It is based on the fact that shadow of an object changes with the position of the sun and hence time. Here we have a plate with a slit in it. The slit is aligned exactly in the geographic north – south direction and inclined at an angle equal to that of the latitude of the place. One can read the local time by the position of sunlight on the dial. In order to obtain the Indian Standard Time (IST), we have to add 19 minutes and 36 seconds to the local time. Another correction from the graph known as 'Equation of Time' has to be applied to arrive at the precise time.
This is a vertical sundial. In any sundial, there will be basically two parts :
Gnomon is generally a rod or a triangular wall pointing north pole of Earth’s rotation axis. Depending on whether the dial is in the plane of the equator or on the horizontal plane of the Earth, they are named equatorial dial ant the horizontal dial respectively. In both the types of sundials, the shift in the shadow position is a measure of Sun's apparent motion and in turn is an indication of time.
In an equatorial dial a sector of 15corresponds to 1 hour and the graduations are all equally spaced where as that is not the case in a horizontal dial.
The sundial here has special design.
There is no rod or triangular wall here to serve as a gnomon. Instead there is a big plate in a plane perpendicular to the earth’s equator. The plate has a long slit in the direction of earth’s rotation axis. As a result, it is not the shadow position which indicates the time but the beam of light from the slit itself serves as the indicator.
The dial too is special because it is neither in the plane of the equator nor in the horizontal plane of the earth surface. Rather it is freely suspended so that it is exactly vertical. That means it is inclined at 130(latitude of Bengaluru) with respect to the equatorial plane.
The plate with slit is not perpendicular to this dial. It makes 1030 with the northern face of the dial and 770 with the southern face of the dial.
Therefore the graduations are not equally spaced as in an equatorial dial. At the same time it is not similar to the graduations of a horizontal dial. The graduations are actually like the projection of the equatorial dial on to a vertical plane.,
The local time can be read by looking at the light beam on the dial.
IST can be obtained by applying corrections corresponding to the Earth’s elliptical orbit and the difference in longitude between the instrument’s location( Bengaluru 77.60E) and the time zone(IST 82.50E)