I'm thinking specifically of the Jantar Mantar sundial in Jaipur, India, the Samrat Yantra. It is apparently accurate to within 2 seconds, and was built around 1740. What would such an accurate clock have been used for at that time in India? Was the instrument accurate simply because it was built so big, or was the accuracy important for something else?

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    If the technologists of the time were anythng like modern engineers, the answer is "because they could" . We often do stuff like that just to show it's possible -- or for ego points. – Carl Witthoft Oct 4 at 12:52

The Jantar Mantar monument in Jaipur was built on the orders of the Rajput king Sawai Jai Singh II, and completed in 1734. Aside from the sundial it has an observatory with various other instruments for astronomical observations with the naked eye. Apparently the zīj at the time, Indian/Islamic version of ephemerides, were not very accurate, and the king noticed. Jantar Mantar was one of several observatories he ordered built to improve on them. The resulting ephemerides, called Zij-i Muhammad Shahi, were used in India for about a century, but in Europe astronomers made far more accurate ephemerides by using telescopes.

  • And to the original question, was time accurate to within 2 seconds required to make an accurate ephemeris? I suppose the question would boil down to: was it necessary to have such accurate timing, or did other factors play into the inaccuracy of the zij? – Michael Stachowsky Oct 4 at 12:10
  • @MichaelStachowsky Of course, other factors also play a role, like precision of the observational instruments. One would want to have the timing of observations as precise as one can get and can use. Less than 2 seconds is probably not very usable with naked eye observations, perhaps even 2 seconds was an overkill. – Conifold Oct 4 at 17:40

Indian and Chinese calendars are astronomical, in that for instance dates are based on the exact moment of astronomical events. So wheter for instance a new Moon falls one second before or after midnight (in the Chinese case) or one seconde before or after sunrise (in the Indian case), can change the date of the start of a lunar month. That's why astronomical precission has been of paramount importance in both countries .

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