Beyond The Barricades: Jantar Mantar’s Stone Instruments And Our Windows To The Cosmos

Jantar Mantar in the heart of Delhi, which has served as a dynamic venue for agitations—protests, rallies and campaigns on social and political issues—is once again in the news, making global headlines. Independent of what each of our opinions is on the agitation or on the government's response, many of us don’t seem to know much about the significance of this historic site of agitation and what it stands for.

In 1724, Maharaja Sawai Jai Singh II built Jantar Mantar, the Delhi astronomical observatory, and he went on to build others at Jaipur, Ujjain, Mathura and Varanasi. These large observatories, meant for naked-eye observations, were built to prepare astronomical tables and improve the accuracy of astronomical observations—predicting eclipses, measuring time and tracking the movements of the Sun, Moon and stars. Jai Singh built these large instruments to achieve higher accuracy.

The instruments at Delhi's Jantar Mantar include a variety of ingeniously constructed positional astronomy instruments—Samrat Yantra, Ram Yantra, Jaiprakash Yantra and Misra Yantra. Naked-eye observations from these observatories resulted in a significant set of star tables, the Zij-i Muhammad Shahi. The significance of these observatories led to the Jaipur observatory being recognised as a UNESCO World Heritage Site in 2010.

Teaching Astronomy Through Ancient Instruments

My late friend Dr Rathnasree, director of the Nehru Planetarium in Delhi, used these observatories to teach positional astronomy to school students. Moving through these instruments gave students a feel for walking through spherical trigonometry. She regularly brought students and amateur astronomers to Jantar Mantar and had them observe what these instruments could teach them that a textbook cannot. She would have been dismayed that the place is now known mostly for slogans.

“Almost everything that distinguishes the modern world from earlier centuries is attributable to the progress in science.” This statement by Bertrand Russell, in some sense, describes the importance of Jantar Mantar observatories—a precursor to modern Indian science, particularly in radio astronomy, which is used to study the cosmos. The Ooty Radio Telescope (ORT) is the best example.

From Ooty To The Giant Metrewave Radio Telescope

The ORT was conceived by Dr Govind Swarup, who had worked at CSIRO in Australia and at Stanford before returning to India in 1963 to commission the ORT in 1970 on a hillside near Udhagamandalam. It is a cylindrical parabolic reflector about 530 metres long and 30 metres wide. Its beauty lies in its geography. Ooty is at roughly 11° north latitude, and the hill slopes at nearly the same angle. That aligns the ORT’s long axis with the Earth's rotation axis, so a single tilt lets it follow a radio source across the sky without an expensive mount. Dr Swarup ingeniously used the natural slope of difficult terrain to his advantage.

Swarup’s success with the ORT resulted in the commissioning of the Giant Metrewave Radio Telescope (GMRT) near Pune. Its thirty dishes, each 45 metres across, are spread over 25 kilometres, and they were built using a lightweight mesh design at modest cost. The GMRT has helped astronomers from across continents study pulsars, distant galaxies and the early universe. In recognition of its scientific and technological achievements, the GMRT has been recognised by the IEEE as an engineering milestone.

Bringing Mega-Science To The Public

In 2019, when I was director of the Nehru Science Centre in Mumbai, we hosted the first edition of Vigyan Samagam, a mega-science exhibition. It was organised jointly by the Department of Atomic Energy, the Department of Science and Technology, and the National Council of Science Museums, a collaboration of three government institutions with a single aim: to bring mega-science to ordinary people. It opened in Mumbai on May 17, 2019, travelled to the Visvesvaraya Museum in Bengaluru and Science City in Kolkata, and concluded at the National Science Centre in Delhi in 2020.

The exhibition presented India's role in mega-science projects, projects larger than any one country could build. CERN's Large Hadron Collider, FAIR in Germany, the India-based Neutrino Observatory, ITER, and LIGO, with a detector planned for India, constituted some of these projects. Also on display were the Thirty-Metre Telescope, in which India is a partner, and the Square Kilometre Array, the world's most ambitious radio telescope, with Indian engineers and astronomers, many trained at the GMRT, helping to design it.

Incidentally, the Nobel Prize in Physics, 2026, has been awarded to Francis Halzen for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. The prize recognises the work of hundreds of scientists across the world, including India. In the 1960s, physicists from the Tata Institute of Fundamental Research, led by BV Sreekantan, placed detectors deep in the Kolar Gold Fields and recorded neutrinos, among the first ever seen.

The Vigyan Samagam exhibition provided an opportunity for students to ask what is out there in the universe and how we know. Jai Singh built Jantar Mantar to try to answer these very questions ahead of time, and the quest continues.

One Pale Blue Dot, One Shared Sky

In 1990, Voyager 1 photographed Earth as a speck of light less than a pixel wide. Carl Sagan called it a “pale blue dot” and wrote that the image was a reminder of our responsibility to deal more kindly with one another and to preserve that dot. Everyone who has ever stood at Jantar Mantar, for any cause and on any side, must realise that they have stood on that dot—planet Earth.

Science does not ask whose side we are on when confronted with a disagreement. That does not mean we should stop disagreeing, because democracy, like science, is built on disagreement.

The next time Jantar Mantar appears in the news, look beyond the crowds at the instruments built to measure the sky more accurately and advance our understanding of science.

Jai Vigyan.

The writer is a Senior Advisor at CSMVS, a trustee of Gandhi Smarak Nidhi and former director of the Nehru Science Centre and NGMA Mumbai.

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Published on: Monday, October 26, 2026, 09:58 PM IST