Observations at Jantar Mantar
Why did the possibility of making observations related to the Transit of Venus (in particular, the Venus elongation measurements), at the Jantar Mantar catch our imagination so much?
Jantar Mantar were imposing masonry observatories designed and constructed by Raja Swai Jaisingh, during the early 18th century, at various locations in India. The Delhi Observatory is thought to have been completed in the year 1724. While Jai Singh wished to use the instruments to make accurate celestial co-ordinate measurements and hence improve ephemeris tables for Astronomical calculations, one of his aims in the construction of these massive instruments had also been that anyone (not necessarily an Astronomer) interested in making celestial co-ordinate measurements should find it easy to do so, using these instruments.
It was precisely this purpose which seemed to have been achieved when it was possible to train students and visitors to the Delhi Jantar Mantar, on the 29th of March 2004, to make celestial co-ordinate measurements using the Jaya Prakas and Ram Yantra instrumens.
Some background on the Jantar Mantar
Most visitors to the Jantar Mantar, do have some idea that the instruments they first encounter - the Misra Yantra and the Samrat Yantra have something to do with time measurement :-)
When they venture beyond the Samrat Yantra, they are usually befuddled by the twin hemispherical bowls of the Jayaprakas and the imposing cylindrical buildings of the twin Ram Yantra.
We started the public interaction with an explanation of local co-ordinates for celestial objects - the Altitude and Azimuth (the measurements on the 29th were confined only to obtaining these co-ordinates for the Sun, Moon, Planets and some bright stars at fixed times). Later analysis will then convert some of these observations into a determination of maximum elongation for Venus.
Here is how we asked visitors to visualise these co-ordinates
It was then easy to explain the usage of the two instruments - Jayaprakas and Ram Yantra for obtaining these co-ordinates of the Sun in the day-time and the Planets and stars in the night time.
The observations started with calibration of the western bowl of the Jayaprakas for the measurement of the Altitude and Azimuth of the Sun before noon, on the 29th of March. The markings on the Jayaprakas are visible close to the centre of the hemispherical bowl and are more obliterated at the outer sections. It was, therefore decided to use this instrument only close to the noon when the shadow of the crosswires would fall closer to the centre of the bowl.
Plastic wire was used for making a makeshift cross-wire for the Jayaprakas. Two of the pegs meant for tying the crosswires - were still intact and the wire was tied to these pegs in the East-West direction. In the North-South direction a makeshift arrangement was made for holding the crosswires in place, as the pegs were not completely intact.
Once the crosswires were in place, one had to observe for the shadow of the intersection of the two wires to fall on the bowl of the instrument. As mentioned in the notes on these instruments above, both the Ram Yantra and Jayaprakas have been designed as twn complemenatry instruments - so that - is the shadow of the cross wires or the gnomon falls in the gap regions in one of the instruments, it will fall on the sector in the complimentary instrument. When we started the public interaction the shadow was still in the gap, for the western bowl where we were situated. The time was used to explain the working of the Jayaprakas instrument to the people gathered there. By the time the shadow fell on one of the sectors, the students of Sanskriti school were very enthusiastic in marking the region on the sector where the shadow fell, as well as in measuring the angles involved.
We then moved to the Ram Yantra, for more measurements of the Altitude and Azimuth of the Sun, throughout the day.
