Jantar Mantar:

The observatory is consisted of eighteen large instruments between 1728 and 1734 by Jai Singh. Although he was influenced by the works of foreign astrologers and the advice of his teachers, one of them was his mother, many of the devices were of his own invention.

The most impressive construction is the sundial, Samrat Yantra casting shadows onto curved stone faces that are graduated in hours. Each hour is divided into thirty parts, so the time calculated accurate to within two minutes.

A more original device, the Jaiprakash Yantra, consists of two hemispheres laid in the ground, each composed of six curving marble slabs with a suspended ring in the center whose shadow marks the day, time and zodiac symbol. This is vital for the calculations of auspicious days for marriage, when unfavorable planets are in influential position, e.g. between August and October, marriages have always been avoided.

This was used until the modern observatories came to use.