I take that you are asking 'astronomical observatories'. 'Observatory' is a noun, observatories is the plural form. Astronomical is simple a modifier (adjective, in this case.)

Siegfried Marx has written:

'Sternwarten der Welt' -- subject(s): Astronomical observatories

'Observatories of the world' -- subject(s): Astronomical observatories

observetories

Roger Hutchins has written:

'British university observatories, 1772-1939' -- subject(s): Astronomical observatories, History

The Mayans had, apparently, been expert astronomers, but the first chain of astronomical observatories would be the Muslims in the 9th century. However, Stonehenge is, apparently according to a concept, an observation centre built by the prehistoric people who inhabited Europe.