The name of the observatory means "instrument and formula". There is a huge sun dial called "Samrat Yantra" and the accuracy of this instrument is such that it can measure and calculate the time of the day with an accuracy of half a second. It is said that Jai Singh was the person who designed the machine. There are two pillars at the south-western part of the observatory which determines the longest and the shortest day. In the winter it is observed that the shadow of one pillar falls over the other completely while in the summer you will see that no shadow falls at all.

The calculating algorithms for celestial object positions had not been corrected for more than thousand years due to lack of precessions of earth's axis of rotation. This effect was observed as far back as the Vedic times ad is called Ayanamasa. Whenever there was a difference in the calculated and predicted elements due to precessions, ad-hoc corrections called Bija corrections were applied so that the calculations and the observations match each other. These corrections were never organized and classical treatise does not mention these. Jai Singh felt that corrections were needed to be made to the existing methods of calculations with precise observations. Jai Singh constructed a huge observatory of masonry for precise observation. It is made of brick and plaster and consist of extruded, interlocked shapes externally. He started with the brass instruments in his observatory but soon gave them up because of several inherent flaws. They were too small and their axes were unstable displacing the center often. He then decided to follow the style of the renowned Arab astronomer, Prince Ulugh Beg, builder of the famous 15th century AD observatory at Samarkand, Uzbekistan. The massive masonry instruments at the observatory at Samarkand suited Jai Singh's architectural tastes and were far more accurate because of their size.

Jantar Mantar has

• Samrat Yantra
  An equinoctial Sun dial - the largest and the most imposing structure at the Observatory.
• Misra Yantra
  A mixed instrument which incorporates many interesting functions.
• Jayaprakash
  The 'Crest Jewel' of instruments consisting of two complimentary concave hemispheres, reflecting the dome of the sky.
• Ram Yantra
  Two imposing circular complementary buildings, designed for very easy determination of local celestial co-ordinates.