Gears model Hipparchus's lunar motion, Metonic cycle and Saros eclipses

▶

Key facts

• In the 2nd century BC the astronomer Hipparchus argued the Moon moves in one large circle with the Earth slightly off-centre, so it appears faster when close and slower when far away.
• The maker implemented this with two slightly off-centre gears, a pin on one sliding inside a slot on the other, so the output sped up and slowed down to mimic the real Moon.
• The upper spiral on the back of the mechanism spans 235 lunar months, the 19-year Metonic cycle in which 7 of every 19 years carry a 13th intercalary month, adding 7 months to 228 to keep the calendar aligned with the seasons.
• A smaller dial beside the upper spiral counts the four-year Olympic cycle and carries engraved names of other Greek festivals: Olympia, Nemea, Isthmia and Pythia.
• Twelve month names identified on the larger Metonic spiral belong to the Corinthian calendar tradition, whose related calendars were used in parts of northwestern Greece and in Syracuse, Sicily; this narrowed possible intended users but does not prove where the device was made.
• The lower back spiral spans 223 lunar months, the Saros eclipse cycle of roughly 18 years, known to Babylonian astronomers long before.
• Glyphs around the Saros spiral mark months in which a solar or lunar eclipse was possible, give an expected hour, and note obscuration amount, direction and predicted colour, though the forecasts were not always accurate.
• Because each Saros repeat brings the eclipse about 8 hours later in the day, the Exeligmos dial tracks three successive Saros cycles and indicates whether to add 0, 8 or 16 hours to the listed eclipse time.
• The video notes that in 2nd century BC Greece and Babylon an unexpected eclipse could panic a city, halt battles, or in Babylon signal the king's death, sometimes leading to the use of a substitute king who was later killed.

← All stories

AI-generated text · images from the original sources · always verify with the sources.