Jantar Mantar

When it was built: 1725
By whom it was built: Maharaja Jai Singh II
Nature of the building: Observatory

Maharajah Jai Singh II of Jaipur incidentally, constructed five astronomical observatories in west and central India between 1727 and 1734. The observatories, or "Jantar Mantars" comprise of multiple buildings, each with a particular function for astronomical measurement. Jantar Mantar in Delhi-with a collection of structures inside it, was built by Maharaja Jai Singh II in 1725. This observatory in New Delhi is located to the east of Hanuman Mandir in Sansad Marg. The Mughal emperor, Muhammad Shah gave the famous astronomer Maharaja the responsibility to revise the calendar and correct astronomical tables used by the then priests. So even before the construction began, astral observations were made daily for years.
  -   There are unique instruments to trace the course of heavenly bodies and predict eclipses.
  -   Jantar Mantar in Delhi has a variety of information sources so that one can explore various historic sites through interactive panoramic "VR" photographs, time lapse sequences, 3D models as well as articles, drawings, and historic texts.
  -   The observatory also says about the technological achievements under the Rajput kings and their attempt to resolve astronomical mysteries.
  -   Instead of using brass instruments in this observatory, the style of the renowned Arab astronomer, Prince Ulugh Beg, was followed.
  -   The two pillars on the southwest of Mishra Yantra decide the shortest and longest days of the year. Interestingly, one pillar completely covers the other with its shadow in December while it does not cast any such shadow in June.