The first memory I have of hearing the name Jantar Mantar is from when I was 7 or 8 years old – my mother bought me an India-themed general knowledge book during a book exhibition in school (during open day!) I don’t think I knew what it was about, just that it had a cute rhyming name.
The name is derived from jantar (‘instrument’), and mantar (‘formula’ or ‘calculation’). There are a five of them in India & the one is Jaipur was built between 1727 and 1734.

There are a total of fourteen major geometric devices on the grounds for measuring time, predicting eclipses, tracking stars’ location as the earth orbits around the sun, ascertaining the declinations of planets, and determining the celestial altitudes and related ephemerides. And don’t for a second think that these tools are primitive – if used correctly, the giant sundial can predict accurate time with just a 0.6 second margin of error.