The Jantar Mantar is a collection of 19 astronomical instruments built by the Rajput king Sawai Jai Singh, the founder of Jaipur, Rajasthan.

Early restoration work was undertaken under the supervision of Major Arthur Garrett, a keen amateur astronomer, during his appointment as Assistant State Engineer for the Jaipur District.

The astronomical tables Jai Singh created, known as the Zij-i Muhammad Shahi, were continuously used in India for a century.

However, the observatory soon became neglected again, and was not restored until 1901 under Madho Singh II (r. 1880–1922) [8] The observatory consists of nineteen instruments for measuring time, predicting eclipses, tracking location of major stars as the Earth orbits around the Sun, ascertaining the declinations of planets, and determining the celestial altitudes and related ephemerides.

[15] Its shadow moves visibly at 1 mm per second, or roughly a hand's breadth (6 cm) every minute, The Hindu chhatri (small cupola) on top is used as a platform for announcing eclipses and the arrival of monsoons.

However, the penumbra of the sun can be as wide as 30 mm, making the 1mm increments of the Samrat Yantra sundial devoid of any practical significance.

[4] The earliest discussion of astronomical instruments, gnomon and clepsydra, is found in the Vedangas, ancient Sanskrit texts.

[4][17] The gnomon (called Śaṅku, शङ्कु)[18] found at Jantar Mantar monument is discussed in these first millennium BCE Vedangas and in many later texts such as the Katyayana Sulbasutras.

The Zij Muhammad-Shahi, an astronomical handbook compiled under Jai Singh's patronage, does not appear to rely heavily on data collected from his observatories.

The Zij Muhammad-Shahi essentially adapts these tables by incorporating adjustments for the precession of the equinoxes and the difference in longitude between Paris and Delhi.

[20] It was photographed by Julio Cortázar with the collaboration of Antonio Gálvez for the book Prosa del Observatorio (Editorial Lumen: Barcelona, 1972).