How Jantar Mantar read the cosmos

The Archaeological Survey of India began repairs on Delhi's iconic Jantar Mantar late last year, a process that has cast the spotlight on the astute science behind the 18th century observatory built by Sawai Jai Singh and his kin

- 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 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.

Samrat Yantra