duckie talks computers

Interesting computers: The Antikythera Mechanism

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The earliest known computer is the Antikythera mechanism. The mechanism is an ancient computer, used to predict astronomical positions and eclipses. It was built sometime between 205 and 100 BCE, depending on who you ask, and found in 1901. It’s a complex clockwork mechanism, made of at least 30 gears.

It was found in the Antikythera shipwreck, off the Greek island of Antikythera.

Between when this device was made and the fourteenth century, there are no known technical artifacts getting anywhere near the complexity and workmanship of this thing.

And that’s weird! The quality and complexity suggests that something of this sort had to be made prior to it. It relied heavily on theories of math and astronomy developed by Greek astronomers.

The mechanism has a wooden case, with doors on the front and back. The case would’ve had two circular faces with rotating hands, much like a clock. There was a hand crank on the side for winding the mechanism forward or backward. When you turned the crank, gearwheels drove at least seven hands at different speeds.

These hands showed the position of the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. There was also a rotating ball showing the phase of the moon.

The front dial has two circular scales. The outer ring shows the day on the Egyptian calendar. The inner ring has the Greek signs of the Zodiac, and is divided into degrees.

The rear face has five dials. They calculate the Metonic cycle, the Callippic cycle, the Exeligmos, the Saros cycle, and the timing of the ancient Olympic games.

The Metonic cycle is built around approximately nineteen year long cycles, which is very close to a common multiple of the solar year and the lunar month. This is useful for converting between solar and lunar calendars.

The Callippic cycle is four Metonic cycles, and the Callippic dial indicates which of those four Metonic cycles is the current one.

The Exeligmos is a period of 54 years and 33 days, which can be used to predict eclipses that have similar properties and occur in a similar location. In other words, given a specific lunar eclipse, the lunar eclipse one exeligmos later will be viewed from the same part of the earth.

The Saros is a period of roughly 18 years and 11 days, which can also be used to predict eclipses. One Saros period after an eclipse, a nearly identical eclipse will occur.

In summary, the Antikythera mechanism is what many consider to be the earliest known computer. The construction implies that whoever made it had prior experience with similar devices, but none of those have been found. Whoever made it really liked eclipses.