Conversation so far:
who built the jantar mantar observatories in jaipur and delhi
Maharaja Jai Singh II of Jaipur constructed five Jantar Mantar in total, in New Delhi, Jaipur, Ujjain, Mathura and Varanasi
when were the jantar mantar observatories completed
The Jantar Mantar observatories were completed between 1724 and 1735

Question: what was the purpose of the observatories
Self-contained rewrite: what was the purpose of the jantar mantar observatories