The original name
Yantra (instrument)
mantra (formula) has been corrupted to Jantar Mantar.

Situated at
Connaught Place area of
New Delhi, Jantar Mantar is one of the five observatories built by
Maharaja Sawai Jai Singh II, the ruler and founder of Jaipur, in India.
He built
five Jantar Mantars in India, located at
Delhi, Jaipur, Varanasi, Ujjain and Mathura.
Jantar Mantar of New Delhi was built with a view to help the practicing astronomers in observing the movements of the Sun, Moon and all other planets.
When compared to others, this
observatory is the largest and the best preserved today.
But, after its
erection in 1724, it remained functional only for seven years. Many experts in this field are of the view that these observatories fell into disuse, because of lack of thought on the part of the king.!!
I have never been to Jantar Mantar....
Some one please tell me if i should plan to go ....