Short answers, pulled from the story.
Maharaja Jai Singh II of Jaipur commissioned the construction of five distinct observatories between 1724 and 1735. He designed these massive stone structures to serve as permanent arguments against the astronomical errors of his time.
Maharaja Jai Singh II commissioned the five complexes across India between 1724 and 1735. The name Jantar Mantar appears in an account from 1803, though the original term Jantra was used in Jaipur State archives from 1735, 1737, and 1738.
The surviving Jantar Mantar sites are located in Jaipur, New Delhi, Ujjain, and Varanasi. The observatory in Mathura and its protecting fort were destroyed before 1857, leaving the other four locations as enduring testaments to the ruler.
The Samrat Yantra is the largest of the Jantar Mantar structures and functions as the world's largest stone sundial. Its triangular gnomon has a hypotenuse that runs parallel to the Earth's axis, allowing observers to determine the time of day and the declination of the sun with an accuracy of about 20 seconds.
The observatory in Mathura and the fort that protected it were destroyed before 1857 due to conflicts and changes that swept through India during the 19th century. This loss occurred while the structures in Delhi, Jaipur, and Varanasi survived to become enduring testaments to Jai Singh II.