Perhaps I could say that Jantar Mantar is one of my favorite place. As you step into the Jantar Mantar, you'll be aww with their huge size and variety of astronomical measurement instruments.

Jantar Mantar is situated at the opposite road of the city palace (note that there are a few entrances to the city palace). The entry price is around Rs 400.

The Jantar Mantar is a collection of architectural astronomical instruments, built by Sawai Jai Singh who was a Rajput king. He had constructed a total of five such facilities at different locations, including the ones at Delhi and Jaipur. 

The name is derived from jantar ("instrument"), and mantar ("formula", or in this context "calculation"). Therefore Jantar Mantar means literally 'calculation instrument'. 

The Jaipur observatory is the largest and best preserved of these. It has been inscribed on the World Heritage List as "an expression of the astronomical skills and cosmological concepts of the court of a scholarly prince at the end of the Mughal period". 

The Jantar Mantar was made by Sawai Jai Singh as he was particularly interested in learning about the sky above his head.

The observatory consists of fourteen major geometric devices for measuring time, predicting eclipses, tracking stars' location as the earth orbits around the sun, ascertaining the declinations of planets, and determining the celestial altitudes and related ephemerides. 

Built from local stone and marble, each instrument carries an astronomical scale, generally marked on the marble inner lining. Bronze tablets, all extraordinarily accurate, were also employed. For further reading, you can go here.

Photo courtesy: I Love Jaipur
These 12 monuments represents the 12 horoscope. Each one measures each horoscope.

Do you know that the Brihat Samrat (refer to photo below) 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? More facts about the Jantar Mantar here.

The Brihat Samrat is one of my favorite instrument in the Jantar Mantar. Why? Because the instrument is so huge!. Spectacular will be the word I can use to describe this one.