Introduction

Even in early antiquity, the observation of particularly striking astronomical events called for careful astronomical measurements. This can be traced back to the Sumerian period around 3350 BC. One of the earliest discoveries of an astronomical regularity was the 18-year eclipse cycle around 3000 BC. It is formed by the apparently long-term stable coupling of the periods of the Moon and the Sun. The numerical values of the different periods form fixed relations. From today’s perspective, we can refer to them as the equivalence of these periods. The concept of equivalence as such has a long tradition that goes back to the entire history of astronomy. An idea of the movements of the celestial bodies was not necessary for this.