They knew for a fact, without any shadow of a doubt that certain activities on earth are synchronised with activity in the heavens. This does include human, animals and plants. Human women have periods roughly synchronised with the movements of the moon, animals and plants have reproductive cycles synchronised with the seasons and movements of the sun and stars. These are facts. Priests and philosophers used astronomical knowledge to predict changes in the seasons, weather, the flooding of the Nile, to navigate at sea, so they had an extensive skills set in this area.

Given that there are known synchronisations, it therefore follows there are likely to be unknown synchronisations, so the logical step is to start looking for them. Unfortunately we know that if you start looking for patterns in phenomena that aren't correlated, human beings have a proclivity to find them anyway. People back then did not know this, and did not have the conceptual tools to critically analyse evidence to the extent that we do.