# What is the Mechanism and what does it consist of?

The astronomers, following systematic observations and mathematical calculations, ended in establishing certain periods of time in which there was repetition of astronomical phenomena. The main function of these composite cycles, was to compute the ratio of the solar to the lunar calendar.

They observed that because a solar year does not contain a whole number of lunar months similarly so the dates diverge as time advances. Some of these cycles served to provide the exact calculation of phenomena such as eclipses.

Press **here **to find out information

on some composite cycles of ancient astronomy.

Using the arrows as a guide, match the functions of the mechanism to the correct points on the front and back.

* If you want, first get all the information for each function by clicking on the corresponding icons ⓘ

The Metonic cycle (430 B.C.): every 19 years (or 235 lunar* months) the Sun, the Moon and the Earth return to the same position relative to each other. The deviation between lunar and solar calendar amounts just to 5 days every 19 years.

The Callippic cycle (370-300 B.C.): every 76 years (or 940 lunar* months) the Sun, the Moon and the Earth return to the same position relative to each other.

The Callippic cycle consists of four Metonic cycles, with a different though distribution of the complete (30 days) and the incomplete (29 days) months, a fact that reduces the deviation between lunar and solar calendar to 4 days every 76 years.

The Saros cycle: the Babylonians had observed that every 223 lunar months* (a period corresponding to 18 years and approximately 11 days) the eclipses of the Sun and the Moon are repeated, but not exactly with the same coordinates.

Because this time span does not contain a whole number of days, has as a consequence the displacement of the recurring eclipse by 8 hours.

The Exeligmos cycle: The time span that equals 669 lunar* months (namely three times the Saros cycle), contains a whole number of days and contributes to the correction of the time of the eclipse.

* lunar or synodic month: the time span that intervenes from one new Moon to the next (29,5 days approximately).