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Jantar Mantar: what the instruments actually measure

Without knowing what you are looking at, it is a field of handsome shapes. With it, it is the most impressive thing in Jaipur.

By 4 min read
The great sundial at Jantar Mantar in Jaipur, its enormous triangular gnomon rising against the sky.
Vrihat Samrat Yantra, Jantar Mantar. Photograph by Jakub Hałun, CC BY-SA 4.0, via Wikimedia Commons.

Key takeaways

  • Built by Maharaja Sawai Jai Singh II and completed around 1734, the observatory holds roughly nineteen masonry instruments.
  • The instruments are large because size, not precision engineering, was how they achieved accuracy.
  • The Vrihat Samrat Yantra is a sundial around 27 metres high that reads local solar time to about two seconds.
  • The time it shows is Jaipur local solar time, which is not the same as Indian Standard Time.
  • It was inscribed as a UNESCO World Heritage Site in 2010.
Destination
Jaipur, India

Jantar Mantar is the site in Jaipur most likely to disappoint you and most likely to astonish you, depending entirely on whether anyone has explained it. Walked round unaided it is a courtyard of large, handsome, inscrutable masonry. Understood, it is an observatory built at architectural scale for a single clear reason.

Why everything is enormous

Start with the question that answers most of the others: why build a sundial the height of an eight-storey building?

An astronomical instrument measures angles. How finely it can measure them depends on how much physical distance one degree of angle covers on the scale you are reading. On a small brass instrument, a degree is a fraction of a millimetre and you cannot subdivide it by eye. Make the instrument fifty times larger and that same degree is spread across a stretch you can mark, read and stand next to.

Jai Singh II had access to fine metalworking. What he did not have was a way to make a small instrument that could resolve seconds of time. So he built in stone and plaster instead, and bought precision with size. That is the whole design philosophy of the place, and once you have it every instrument makes sense.

The great sundial

The Vrihat Samrat Yantra dominates the site: a vast right-angled triangle of masonry, around 27 metres high, flanked by two curved quadrant scales.

The triangle is the gnomon. Its sloping edge is set parallel to the Earth’s axis, which means it points at the celestial pole and the shadow it casts sweeps around it at a steady rate as the Earth turns. The curved scales on either side are marked so that the position of the shadow’s edge can be read directly as time. It resolves to about two seconds.

Watch the shadow’s edge against the scale for a minute and you can see it move. That is the Earth’s rotation, visible in real time, on a stone wall.

Why it disagrees with your phone

People check the sundial against their watch, find a discrepancy, and conclude that one of them is wrong. Both are right; they are measuring different things.

The sundial reads local solar time at Jaipur: it tells you where the sun is over this exact spot. Your phone reads Indian Standard Time, a single zone applied to the entire country and referenced to a meridian some distance to the east. Any place not sitting on that meridian will differ.

There is a second, subtler difference. Solar days are not all exactly the same length, because the Earth’s orbit is elliptical and its axis is tilted, so the sun runs slightly ahead of or behind clock time depending on the date. Astronomers call the correction the equation of time, and over a year it swings by a quarter of an hour in each direction. Applying both corrections turns the sundial’s reading into the time on your watch, which is a satisfying thing to do while standing in front of it.

The other instruments worth finding

  • Jai Prakash Yantra. Two hemispherical bowls sunk into the ground, marked as an inverted map of the sky. A small ring suspended above casts a shadow into the bowl, and where it falls tells you the sun’s position in celestial coordinates. Built as two complementary halves so that an observer can stand in one while reading the other.
  • Ram Yantra. Two large open cylinders with a central pillar, used to read the altitude and azimuth of a body: how high it is above the horizon and which compass direction it lies in.
  • Rashivalaya Yantra. Twelve separate instruments, one for each sign of the zodiac, each aligned to its constellation. This group is unique to Jaipur among Jai Singh’s observatories.
  • Laghu Samrat Yantra. The smaller sundial, and a good one to read first, because the same geometry is easier to take in at a size you can see all at once.
  • Dhruva Darshak Pattika. A simple sighting instrument for locating the pole star, which is the reference everything else is aligned against.

Practical notes

Allow an hour. Take a guide, which is advice we would not give for anywhere else in Jaipur. Come on a clear day, because every instrument here works by casting a sharp shadow and an overcast sky renders the entire site inert.

It sits beside the City Palace in the walled city, so it belongs to the walking day rather than the fort day.

It is the anchor of day two in the two-day Jaipur itinerary. If the orbital mechanics are the part that interests you, why the Moon always shows us the same face runs on the same kind of reasoning. The gap between solar time and clock time is also the reason we have leap years.

Things to do

  • Vrihat Samrat Yantra, the great sundial
  • Jai Prakash Yantra, the two hemispherical bowls
  • Ram Yantra, for altitude and azimuth
  • Rashivalaya Yantra, twelve instruments for the twelve zodiac signs

Travel tips

  • Take a guide here. It is the one site in Jaipur where the difference is enormous.
  • Go on a clear day. Every instrument depends on a sharp shadow.
  • Allow an hour. Less than that and you will look at shapes rather than understand them.

Sources

  1. The Jantar Mantar, Jaipur (UNESCO World Heritage Centre)
  2. Jantar Mantar, Jaipur
  3. Equation of time

Frequently asked questions

Two separate reasons. The sundial shows local solar time for Jaipur, based on where the sun actually is over that spot, while your watch shows Indian Standard Time, which is a single zone applied across the whole country from a reference meridian further east. On top of that, the length of a solar day varies slightly through the year because the Earth's orbit is elliptical and its axis is tilted. Corrections for both are what convert a sundial reading into clock time.

Because scale was the available route to precision. On a larger instrument, one degree of angle occupies more physical distance, so the graduations can be spaced further apart and read more finely. A pocket sundial cannot resolve seconds; a 27-metre one can. The size is the technology.

More here than anywhere else in the city. The instruments have no moving parts and carry little explanation on site, so without someone to tell you what each one measures you are looking at abstract geometry. With an explanation, the logic of the place becomes clear very quickly.

HRITU Editorial Desk is the byline for pieces produced in-house by the HRITU editorial team. We write about cooking, travel, science and everyday knowledge, and we explain the reasoning behind what we publish rather than just stating conclusions. Every article is checked against primary or reference sources before it goes out, dated on publication, and updated in the open when we learn something is wrong. Corrections and questions are welcome at info@hritu.com.

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