Jantar Mantar, Jaipur, Rajasthan - Old indigenous observatory

the vrihat samrat yantra (the world's largest sundial).//en.wikipedia.org  

According to Wikipedi,  "The Jantar Mantar is an equinoctial sundial, consisting a gigantic triangular gnomon with the hypotenuse parallel to the Earth's axis...........The instrument is intended to measure the time of day, correct to half a second and declination of the Sun and the other heavenly bodies."

The Jantar Mantar observatory, Jaipur. Amusing Planet

Believe it or not, there are five places in India where you can see the  Jantar Mantar monuments - Jaipur, New Delhi, Ujjain, Varanasi and Mathura. At these places you can study the Astrnomy of the our solar system.  Among them, the largest observatory  is in Jaipur which has many instruments along with the world's largest stone sundial. The Vrihat Samrat yantra is  the world's largest sundial, that can give the local time to an accuracy of 2 seconds and it is a UNESCO World Heritage Site. It measures time in intervals of 2 seconds using shadow cast from the sunlight)

This  amazing scientific  structure was  built in 1720 during the reign of Maharaja Jai Singh II. The Jantar Mantar  features  various architectural and astrological instruments that have attracted the attention of  the  astronomers, historians and architects  world over because, unlike traditional clocks, this one is unique.

The Jantar Mantar observatory, Jaipur. idealista.com

 The observatory  has 19 instruments and they are designed to measure time, predict eclipses, track location of major stars as the earth orbits around the sun, ascertain the declinations of planets, etc.They help us  find out and identify the constellations and, even analyse the orbits of planets around the sun.  Some instruments are worth mentioning:

Chakra Yantra consists four semicircular arcs on which a gnomon casts a shadow; this gives  the declination of the Sun at four specified times of the day. Amazingly, the data corresponds to noon at four observatories around the world (Greenwich in UK, Zurich in Switzerland, Natka in Japan and Saitchen in the Pacific); In a way similar to a clock showing time in different parts of the world.
Dakshin Bhitti Yantra is meant to study the distances of celestial bodies. It measures meridian, altitude and zenith  Digamsha Yantra  is a pillar in the middle of two concentric outer circles. It is  used to measure azimuth of the sun, and to calculate the time of sunrise and sunset forecasts. Dhruva Darshak Pattika is used to observe and find the location of pole star with respect to other celestial bodies.
Kapali Yantra measures coordinates of celestial bodies in azimuth and equatorial systems, any point in sky can be visually transformed from one coordinate system to another. Kranti Vritta Yantra helps us measure  longitude and latitude of celestial bodies). Laghu Samrat Yantra.  is a  smaller sundial at the monument, inclined at 27 degrees. With this we can  measure time, less accurate than Vrihat Samrat Yantra. Nadi Valaya Yantra has two sundials representing  north and south hemispheres. As for the accuracy of the instrument with respect to time, it is is less than a minute. Rashi Valaya Yantra has 12  dials to measure ecliptic coordinates of stahas rs, planets and all 12 constellation systems. Shastansh Yantra, like Vrihat Samrat Yantra, is a unique instrument and has a 60 degree arc built in the meridian plane within a dark chamber. At noon, the sun's pinhole image falls on a scale below making  the observer  measure the zenith distance, declination, and the diameter of the Sun. Yantra Raj Yantra  is a 2.43 -metre bronze astrolabe, one of the largest in the world. Used only once a year, it calculates the Hindu calendar.

Located near City Palace and Hawa Mahal of Jaipur, Jantar Mantart  monument consists of masonry, stone and brass instruments that were built using astronomy and instrument design principles of ancient Hindu Sanskrit texts. The common feature is  the strange instruments make the observation of astronomical positions with the naked eye. The monument is good example of the fusion of various ideas expressed in different religion  and the prevailing social beliefs then with respect to calculation of time. The architectural innovations in 18th century India in astronomy is unique  and the observatory is an example of the the astronomy as propounded by Ptolomy  that  was shared by many civilisations.

 The name "Jantar Mantar" is at least 200 years old and was mentioned in the records from 1803. But the name Jantra is not mentioned in the  archives of Jaipur State - accounts from 1735 and 1737–1738. In the spoken language, it is said to be  corrupted to Jantar.  The word Jantra is derived from yantra which means  instrument, The suffix Mantar derived from mantrana means consult or calculate. The words jantar and 'mantar (or yantra and mantra) imply calculation instrument.