Antikythera Mechanism rendering of the gears with the pin-and-slot device.
The museum explanation reads: "THE ANTIKYTHERA MECHANISM FUNCTION: The turning of a handle through the side of the casing moved all the pointers simultaneously by means of gears and axles connecung them,
By selecting a specific day on the front dial with the calendar and the desired year and month on the upper back dial, the rest of the pointers supplied all the corresponding astronomical information. Conversely, by setting the pointer on an astronomical event, its date in the future or in the past
was estimated.
For instance, the user could directly check the pasition and phase of the Moon, and the eclipses that might occur for a given day of the selected month. But the most remarkable ability of the Antikythera Mechanism is to show the variable motion of the Moon, realized through an extraordinary epicyclic gear train."
From the 2013 "Το Ναυάγιο των Αντικυθήρων. Το Πλοίο, οι Θησαυροί, ο Μηχανισμός" (The Antikythera Shipwreck. The Ship, The Treasures, The Mechanism) at the National Archaeological Museum in Athens, Greece.