Short answers, pulled from the story.
Jantar Mantar refers to five assemblies of stone-built astronomical instruments constructed in India between 1724 and 1735. The five sites were built in New Delhi, Jaipur, Ujjain, Mathura, and Varanasi, though the Mathura observatory was destroyed before 1857.
All five Jantar Mantar observatories were built at the command of Maharaja Jai Singh II of Jaipur, who had a keen interest in mathematics, architecture, and astronomy. Construction was completed between 1724 and 1735.
The Samrat Yantra is the world's largest stone sundial, located at the Jaipur Jantar Mantar. It features a gigantic triangular gnomon with its hypotenuse parallel to the Earth's axis, allowing a skilled observer to measure the time of day and solar declination to an accuracy of about 20 seconds.
The Jaipur Jantar Mantar is a UNESCO World Heritage Site. It is the largest of the five observatories and is home to the Samrat Yantra, the world's largest stone sundial.
Jantar derives from yantra, meaning instrument, while Mantar comes from mantrana, meaning to consult or calculate. The name is at least 200 years old, appearing in a written account from 1803, though earlier Jaipur State archives from 1735 used the spelling Jantra.
The primary purpose of the instruments was to compile astronomical tables and to predict the times and movements of the sun, moon, and planets. The observatories held multiple devices including the Jai Prakash Yantra, Rama Yantra, Chakra Yantra, and others, all designed for naked-eye observation.