The upper planes of the two walls on the sides of the steps in the middle of the instrument are parallel to the axis of the earth. In the direction of the the planes the pole star is visible. To the east and the west of the wall the quarter of the circle is formed in the plane of the celestial equator on which hours, minutes and a third part of a minute are engraved.
When the Sun shines in the sky, the shadow of the edge of the wall falls on some mark indicating local time of the Ujjain by calculating the hours and minutes. By adding minutes of this clear time table given on the east and the west side of the instrument one can know the Indian standard time.
This instrument is mainly used to find out the declination of any celestial body from the celestial equator towards the north or the south. At first we find out the particular point on the edge of the quadrant from where the centre of the celestial body could be observed to coincide with the edge of the wall. The reading at this point of the wall gives the declination.
THE NARIVALAYA YANTRA:
This instrument built in the plane of the celestial equator has two parts the north and the south parts. When the Sun in the northern hemisphere for six months, the northern disc is illuminated. But when the Sun is in the southern hemisphere for the remaining six months, the southern disc is illuminated. The exact time of Ujjain is known by the shadow nail fixed parallel to the Earth's axis in between these two parts.
This instrument is used to ascertain whether a celestial body is in the northern or the southern half. Observe a desired planet straight from a suitable point on the round egde of the northern part. If it is visible, then deem it to be in the northern hemisphere otherwise it is in the southern part Likewise information could be had from the southern part.
THE BHITTI YANTRA(TRANSITE DEVICE):
This Instrument built in the plane of the meridian circle (i.e. the circle joining the south-north and the zenith point) is used for observing the zenith distance of the celestial body (corresponding to its mid-day).
There are two nails at the top of the instrument fixed with string in the centre of graduated quadrants. When the object is in the south of the prime vertical (the circle joining the east, the west and the zenith point) the southern nail is o be used. The northern nail is to be used. The northern nail is used likewise if the object is found in the north.
At the time of the transit of the celestial body, the observer has to keep his eye on the string and move it forward or backqward to determine the particular position of the string t which the centre of the heavenly body could be seen, through the point of the intersection of the nail and the wall.
The reading of the quadrant at this position of the string gives the zenith distance.
THE DIGANSHA YANTRA:
This Instrument is used to find out the the Altitude (distance from the horizon) and the Azimuth (angular distance from the east or the west point measured along the horizon) of any celestial body. For this purpose sextant type device called Turiya Yantra is fitted on the at the centre of the circular platform.
We arrange the position of Turiya Yantra in such a way that the two holes of the Yantra are in one line joining the celestial body so that it may be visible through both the holes.The pointer of the Turiya Yantra moving along the round graduated disc at top of the poles gives Azimuth. The suspending thread of the Yantra gives the Altitude on the graduated seal of the quadrant.
YANTRA (THE GNOMON DEVICE):
A vertical Gnomon (Shanku)is fixed at the circular of the circular platform having ahorizontal shape .The seven lines drawn according to the shadow of the Gnomon indicate the twelve zodiac signs. Among these lines, 22nd December makes the shortest day, the 21st March and the 21st September makes the days and nights equal, and the 22nd June makes the longest day of the year.
With the help of the shadow of the Gnomon the angle elevation and zenith distance of the Sun can be determined. The Altitude of Ujjain is determined by the mid-day shadow of Shanku Yantra when the day and the nights have equal length.