N. Rathnasree, Nehru Planetarium, Nehru Memorial Museum and Library, New Delhi
(Page under construction)
The instruments which enable measurements of the positions of celestial objects in the Equatorial co-ordinate system, are the Jayaprakas present in the Delhi and Jaipur observatories, the Chakra Yantra of the Jaipur and Varanasi observatories and the Samrat Yantra present in all the Jantar Mantar observatories.
The Jaiprakas Yantra at the Jaipur Jantar Mantar observatory
Jai Prakas are twin hemispherical bowl instruments, each a reflection of the sky above and marked in sectors and gap regions. The center of the bowl is a reflection of the Zenith. Starting from the center, lines are marked along the bowl to indicate the Azimuth. Altitude circles are marked along the length of the bowl. Reflection of a cross wire stretched North-South and East-West over the surface of the bowl, shows the position of the Sun in the sky. From these markings, local co-ordinate measurements can be made with this instrument, although, more interestingly, there are also markings on the instrument which will allow measurements in equatorial co-ordinates.
The bowls are complementary, in the sense that, the gap region in one bowl is the sector region in the other and vice versa. The idea being that, the observer needs to be inside the bowl, to take readings - which means that readings would not be possible in the regions where the observer would be able to walk - and hence the complimentary bowl.
How does the instrument work? At the surface level on the depressed bowl - There are pegs in the North-South and East-West direction to hold cross wires. One has to view the shadow of the junction of the cross wires on the bowl of the instrument to determine the co-ordinates of the Sun in the daytime sky. The sectors on the surface of the hemisphere are marked with altitude and azimuth circles, diurnal circles, the tropics and intermediate circles and also circles of the signs of Zodiac.
In the Delhi instrument most of these markings are rubbed off. The Altitude and Azimuth circles towards the center of the bowl are still visible, so that Sun measurements in local co-ordinates, close to the Noon time are still relatively easy. For measurements at other times and for Equatorial co-ordinate or elliptical co-ordinate measurements more rigorous calibration will be needed.
The twin Jai Prakas Yantra of the Delhi Jantar Mantar Observatory
The Samrat Yantra is another instrument which has capabilities of measurements in equatorial co-ordinates, for celestial objects. In addition to its function as a sundial for the determination of time, the Samrat Yantra also has the capability of measuring the Declination of the Sun and other celestial objects as they transit the Meridian. One method for this measurement is with the Sasthamsa inside the walls of the Samrat Yantra.
The gnomon and the quadrants of the Samrat Yantra could also be used to determine the declination.
(To be completed)