Sun Wheel

Sun Wheel, originally uploaded by ashish100.

Built by Maharaja Sawai Jai Singh II, designer of Jaipur city, this was one of five astronomical observatories built between 1727-1734 in west central India. Multiple colourful stone and metal structures scatter the park, some functioning as simple sundails, others using the shadow and angle calculations to discern the year, date, month and time. Positions of various stars were also tracked and mapped. This was of great significance in India, where astronomy was well developed relative to the time, and worked hand in hand with astrology, still highly respected and practiced by Hindu pundits (priests) and used to determine auspicious dates for weddings and major events, both at a societal and personal level.

Jai Singh built the first Jantar Mantar in Delhi for the Mughal emporor (as he was aligned to them, and thus indebted to them - serving as chief of the Mughal Army, and ensuring peace and protection from war with their fierce forces). The Jaipur JM is the largest of the five built.

The name is said to originally have been 'Yantra Mantra', 'yantra' meaning instrument, and 'mantra' meaning chanting. Mantra can also refer to a formula.

There are 14 instruments in the observatory, which measure time, predict eclipses, track stars in orbit, ascertain declinations of planets and celestial altitudes and more. There is a Chhatri (Hindu small domed cupola) used as a platform to announce important events like the arrival of monsoonal rains and eclipses.

Mostly built of marble inside, and with bronze metal components. Wikipedia notes "An excursion through Jai Singh's Jantar is the singular one of walking through solid geometry and encountering a collective weapons system designed to probe the heavens.
samrat yantra

The instruments are in most cases huge structures. They are built on a large scale so that accuracy of readings can be obtained. The samrat yantra, for instance, which is a sundial, can be used to tell the time to an accuracy of about two seconds in Jaipur local time. Today the main purpose of the observatory is to function as a tourist attraction."

It was declared a National Monument in 1948.