The Jantar Mantar is a collection of architectural astronomical instruments, built by Maharaja Jai Singh II between 1727 and 1734. It is modelled after the one that he had built at the then Mughal capital of Delhi. He had constructed a total of five such labs at different locations, including the ones at Delhi and Jaipur. The Jaipur observatory is the largest of these.
The observatory consists of fourteen major geometric devices for measuring time, predicting eclipses, tracking stars in their orbits, ascertaining the declinations of planets, and determining the celestial altitudes and related ephemerides. Each is a fixed and 'focused' tool.
Laghu Samrat Yantra : This is a small sundial. It is constructed on Jaipur's Latitude 27° North, and is used to calculate Jaipur's local time, up to an accuracy of 20 seconds.
Narivalaya Yantra: These are sundials to calculate time using the solar cycle. They consist of two hemispheres inclined at 27°. The second picture is a zoomed in view of the centre of the yantra
Jai Prakash Yantra: These look like two bowls sunk into the ground. It is believed that Maharaja Jai Singh invented this instrument himself. It is used to verify the accuracy of all the other instruments in the observatory.
Rashivalaya Yantra: This instrument comes in a set of 12 pieces. Each piece represents a different sign of the zodiac, so they face different angles and constellations. It is used by astrologers to make accurate horoscopes.
Samrat Yantra: Over 30 metres in height, the big sundial is the most impressive and the biggest device. The ramp that forms the indicator is orientated to the north. It's gradient of 27 degrees is equivalent to the latitude of Jaipur. Thus the ramp points exactly to the Celestial North Pole. The shadow of the indicator falls of the wing-shaped western and eastern scales. They are made of marble as the side edges of the indicator are, and covered with delicate measuring divisions. According to the size and architectural precision, the shadow shows the local time accurate within four minutes. Using a sighting stick accuracy lies within seconds.