How Delhi’s Jantar Mantar read the cosmos

The observatory in Delhi includes three large instruments set up by Sawai Jai Singh for reading the time
and observing celestial coordinates, and a multifaceted structure that performs various functions,
believed to have been added by his son. Ahead of a planned restoration, a look at all the
instruments.

— Kabir Firaque

Delhi’s Jantar Mantar, currently
undergoing the first steps of a planned restoration, has four instruments that are a mix of science and
mystery. Three of them were set up by the Rajput ruler Sawai Jai Singh who built five observatories in
northern India 300 years ago. The fourth, which has become this Jantar Mantar’s signature, is the Misra
Yantra with its semicircular arcs. Believed to have been added later by one of Jai Singh’s sons, it had
multiple purposes, some of which are not yet fully understood.
The other three instruments not only
tell us about Jai Singh’s keen interest in astronomy but also serve as a lesson for any student starting
out in the subject. These are common to the observatories in Delhi and Jaipur: a sundial called the Samrat
Yantra, a pair of hemispherical bowls called the Jai Prakash Yantra, and a pair of cylindrical structures
called the Ram Yantra.
We know that a sundial tells the time with its shadow, cast by the sun as it
moves across the sky. To ensure that the shadow moves equal distances at equal intervals, the sundial
needs to be aligned (or calibrated) perfectly. Because of the earth’s tilt, a vertical sundial at any
location (other than the poles) will not accurately reflect the movement of the sun
To make it
effective, the sundial needs to be tilted so that its shadow-casting part, called the gnomon, is aligned
with the earth’s north-south axis. This tilt needs to be at an angle equal to the location’s latitude:
elementary geometry shows that this will ensure the gnomon points towards one of the celestial
poles.
And indeed, the gnomon of the Samrat Yantra points towards the pole star.