Astronomers are an important part of many ancient societies, for they are able to determine religious calendars and predict the seasonal rising of rivers. From at least Babylonian times, the sister art of astrology has also been used to predict more nebulous events and determine auspicious days for various undertakings. Therefore, both astronomers and astrologers often find themselves employed in royal courts or temples. ground and some skills useful in the manufacture of pretelescopic astronomical instruments.