The history of a science – in this case Astronomy – is the history of attempts to find a theoretical, rational model that is capable of explaining, or at least accounting for, a set of observational facts or data.

Appearances to be explained

Apparent rotation of the stars One of the data that an astronomical theory has to collect is the apparent daily rotation of the stars around the celestial pole. The stars appear to describe a complete circumference every 23 hours and 56 minutes.

The word “planet” is a Greek term that means “globetrotter.” This is what they called the stars that “wandered” among the stars that keep their relative positions fixed. The Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn, the seven planets of antiquity, enjoy a daily movement towards the West that is exactly what the fixed stars have. But, in addition, they move slowly towards the East. This last movement –except in the case of the Sun and the Moon- is sometimes replaced by a backward movement towards the West, called «retrogradation». Astronomers had, in this particular movement of the planets, the main challenge at the time to formulate a theoretical model.

Apparent movement of the Sun

Throughout the year the Sun always rises at some point on the horizon located towards the East and sets towards the West. But the exact place where it comes out, the number of hours of light, the length of the shadows and the exact place where it sets, changes from day to day. The winter solstice (December 22) is the day the Sun rises and sets further south, the shadows at noon are longer than any other day of the year. On the spring and autumn equinoxes the Sun rises exactly in the East and sets in the West. At the summer solstice (June 21) the Sun rises and sets further north, the shadows at noon are the shortest of the year. Likewise, the length of the day changes, that is, the number of hours of light.

The four seasons of the year

The variations of the Sun’s position on the horizon at sunrise and sunset correspond to the cycle of the seasons of the year. For this reason, most of the ancient peoples believed that it was the Sun that controlled the four seasons. The apparent path of the Sun and its relationship with the four seasons are also data that must be incorporated and explained in any astronomical theory. In summary, these are the four main appearances that must be saved in an astronomical theory: First, the annual movement of the Sun: if the point where the Sun rises each day is recorded on a stellar map, along the year, a circumference will be drawn: the ecliptic. Secondly, the so-called lower planets, that is, Mercury and Venus, are always found at the point of the ecliptic where the Sun is located. Thirdly and fourthly, the most difficult data for all astronomers in the world to integrate. antiquity: the retrogradation of the planets