The Role of Astronomy
Astronomy is one of the oldest sciences, although the term science has only been in use since the 17th century when Descartes defined the principle of dualism, essentially separating religion and science. As a philosophy, astronomy (astrologia) has dealt with the definition of calendar and time systems for practical and religious purposes, the prediction of stellar positions (to be used in astrology), but also had the task to make predictions about the future, and to explain the esoteric meaning of dramatic celestial events such as eclipses and the appearance of comets.
Cylinder seal, about 3500 BC
In this module we are concerned with how astronomy dealt with the problem of defining a specific time in the year, a reference point in the cycle of the seasons. See our module “Calendar and Time” for a more elaborate historical account of our present day calendar and time systems.
One way to “measure” the time of year is to observe the height (altitude) of the Sun in the sky. A simple way is to put a stick in the ground and to watch its shadow. When at about midday the shadow is the shortest within one year of observations, it will be around midsummer, when the shadow is the longest, then you know it is around the middle of winter. The position of the shadow also tells you the time of day. Essentially this is a sundial and they go back as far as 5000 BC. More...
With the stars there are other ways of judging the time of year. The visible stars change position slightly from night to night and they move around the sky throughout the year. Different constellations are visible at different times of the year. This also depends on where you are on Earth and each local civilisation used there “own” stars and constellations. But for several reasons, both practical and spiritual, there was a need to be more precise in knowing the time in the year.
Image: Gilbert A. Esquerdo (Planetary Science Institute, Tucson, and Smithsonian Astrophysical Observatory (edited)
It is striking that many civilisations throughout the world have applied the same technique, utilising what is called the heliacal rising of celestial bodies, to define a specific time of the year with a precision of a few days.
This technique observes when a certain star or constellation is visible for the first time in the year, just before sunrise. Which stars to use depends on the location on Earth, and sometimes people were interested in different parts of the season, but the basic technique has been the same.