One of the most ancient and enduring uses of astronomy lay in the construction of religious calendars to ensure that particular acts of worship took place at the correct time of the year. The eighth-century BC Greek writer Hesiod, in his Works and Days brings home to us the importance of astronomy to both religion and agriculture in the pre-scientific Greek world.
Indeed, modern scholars, with the aid of computer programs which enable them to back-calculate the heavens to a given date in the past, have been able to use astronomy to explain details in many ancient texts.
In their 2005 paper Knowing When To COnsult The Oracle At Delphi (Antiquity, 79 (2005), p564-572), Alun Salt and Efrosyni Boutikas of Leicester University cast a new light on the consultation of the famous Oracle at Delphi in Greece.
At that time, around 750 BC, different parts of the Greek world had different calendars. One wonders at how people from across that world were about to coordinate their travels to arrive at the mountainous shrine of Delphi for the seventh day of the month Bysios. This was the god Apollo's birthday, upon which the Oracles statements were issued to the faithful. To confuse matters further, the Greeks used a lunar calendar within a solar year that began and ended with the summer solstice. Thus Bysios could move depending on when the new moon fell, between in our calendar, late January and mid-March.
It has been calculated however that around 750 BC the stars in the constellation of Delphinus rose just before the Sun - displaying a heliacal rising at the very end of December. This would have produced an astronomical event which could have been seen across the Greek world and elsewhere, and would have acted as an international time signal.
In the lunar calendar of the Greeks, this would have signalled that the next month would be Bysios and that the time of the year had arrived to set out for Delphi, in the foothills of Mount Parnassus, above the northeastern end of the Gulf of Corinth. Travellers could then be present for the god's birthday and could seek his advice at this, the most sacred oracle in Greece.
It would have given worshippers a months travelling time. Of course, the heliacal rising of Delphinus does not happen after the winter solstice today. A slow change in sky alignment has taken place, caused by the precession of the equinoxes over 2770 years.