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On a central raised platform, in the middle of the Observatory Complex, are the twin hemispherical bowls of the Jai Prakas.

These twin hemispherical bowls, are each a reflection of the sky above. The bowls are marked in sectors and gaps. Observers move inside the gap regions and make observations using the markings on the sectors. The instruments are complimentary, in the sense that where there is a gap in one of the bowl, is a sector placed in the other bowl and vice versa. Spliced together, they make a whole bowl that is a complete reflection of the sky above.

Cross wires are stretched in the North-South and East-West direction on the surface of the instrument bowls. Shadow of the centre of this cross wire, on the surface of the bowl, shows the position of the Sun in the sky.

The sectors on the surface of the hemisphere have markings on them that would facilitate measurement of the local as well as global (equatorial) co-ordinates of a celestial object. There are also markings for the measurement of time and the approach of a sign of the Zodiac, on the Meridian.

The Jai Prakas, is a complete instrument, with an ability to make many of the measurements that are done individually, by the other observatory instruments.

Let us look at the details of the usage of the instrument for the many different celestial quantities that the instrument is capable of measuring.

The instrument can measure the local co-ordinates of a celestial object - the Altitude and Azimuth. The Altitude is 90 degrees at the centre of the bowl and is zero at the rim of the instrument. There are circles of equal Azimuth, spaced 6 degrees apart, that spread out from the centre and reach the rim of the instrument.

The gaps that have been cut into the bowl of the Jai Prakas, are in the North South Direction. This means that the spread of the sectors and gaps is in the East West direction, that is, in the direction of the Diurnal movement of the Sun.

Notice that there are six sectors and six gaps in each of the bowl? This is significant, as each of these divisions is equal to one hour. The shadow of the cross wire, moves across a sector in an East-West direction, in one hour. There are hour circles drawn in an East-West direction, all across the North South Length of the bowls.

By noting the position of the shadow of the crosswire, parallel to these hour circles, one can obtain the local time. This is then to be converted to the clock time, by adding the correction factor for the day, that is displayed in the obsrevatory. Time measurments with the Jai Prakas are a little cumbersome and much better accuracies can be obtained by using either of the Samrat Yantra instruments.

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Page last modified on June 17, 2008, at 02:20 AM EST