JANTAR MANTAR, The Astronomical Observatories of Maharaja Jai Singh II:

The Jantar Mantar Observatory was built by Maharaja Sawai Jai Singh II, as a focal point of his new capital, Jaipur, the first and earliest geometrically planned city in India. Jai Singh II was one of several powerful princes rising to power as the influence of the Mughal Empire decreased. In his attempt to become an almost independent ruler of Rajasthan, he started to build a new capital underlining the link between scientific capacities, urban planning and social control. The construction of the observatory site started in the 1720s and was completed in 1738.

Jantar Mantar is the most complete and best-preserved great observatory site built in the Ptolemaic tradition. This tradition developed from Classical Antiquity through to Medieval times, and from the Islamic period through to Persia and China. Jantar Mantar was greatly influenced by earlier great observatories inside central Asia, Persia and China.

The observatory was very active during the life of Jai Singh II, with around 20 permanent astronomers. After his death in 1743, this key landmark in the centre of the capital city of Rajasthan remained in use almost continuously until around 1800. This is evident from the fact that repairs were carried out at least twice during this period. Nevertheless, during the 19th century the site ceased to function permanently as an observatory, being re-opened from time to time between periods of low activity or complete abandonment. Some important restorations occurred at the end of the 19th century, and mainly in 1902, under British rule. This started a new life for the observatory as a monument of Rajasthan. Other campaigns of restoration occurred during the 20th century and the most recent took place in 2006–07.

The most significant instruments (yantras) among the collection are:

- Brihat Samrat, probably the largest gnomon-sundial ever built. With a gnomon arm 22.6m high and two lateral quadrants of radius 15.15m, it measures local time to an accuracy of 2 seconds.

- Sasthamsa, which has four large meridian dials inside two high black chambers.

- Jai Prakash, a highly innovative sundial made of two hemispherical bowls that produce an inverse image of the sky and allow the observer to move freely around inside to take readings.

- Great Ram is a rare, and perhaps unique, double-cylinder instrument to record the azimuth of celestial bodies;

- Raj comprises a bronze astrolabe 2.43m in diameter, probably the largest in the world.

- Kapala is able to record the co-ordinates of celestial bodies in both the azimuth-altitude and equatorial systems, and permits a direct visual transformation of the co-ordinates of any point in the sky between the two systems;

- Rasivalaya is a unique group of 12 gnomon-dials to measure the ecliptic co-ordinates of celestial objects, each becoming operative when a different one of the 12 zodiacal constellations straddles the meridian.

The Jantar Mantar of Jaipur was inscribed on the World Heritage List in July 2010. It is truly an "Architecture in the Service of Science".