Jantar Mantar is an astronomical observatory located in the heart of New Delhi, India. It is one of the five observatories built by Maharaja Jai Singh II of Jaipur during the early 18th century. The Jantar Mantar in Delhi was constructed in 1724 and is the largest among the five observatories.

The observatory has several large instruments that were used to measure astronomical and astrological data. The primary instruments at the observatory include the Samrat Yantra, Jayaprakash Yantra, Ram Yantra, and Misra Yantra. The Samrat Yantra is the largest of these instruments and is a massive sundial that stands 27 meters tall. The Yantra can measure the time of day with an accuracy of up to two seconds.

The Jayaprakash Yantra is another important instrument at the observatory, consisting of two concave hemispheres made of marble. These hemispheres are used to measure the altitude of celestial bodies as they move across the sky. The Ram Yantra is a cylindrical structure that was used to measure the altitude and azimuth of celestial objects. The Misra Yantra is a combination of four different instruments and is used to measure the altitude and azimuth of celestial objects, as well as to determine the local time.

The Jantar Mantar in Delhi was declared a UNESCO World Heritage Site in 2010, and it remains a popular tourist attraction today. The observatory is a testament to the incredible scientific knowledge and skill of the people of India during the early 18th century.