The Antikythera mechanism is an ancient Greek hand-powered orrery, a mechanical model of the solar system, described as the oldest example of an analog computer used to predict astronomical positions and eclipses decades in advance.
Captain Dimitrios Kontos and a crew of sponge divers from Symi island discovered the Antikythera shipwreck during the spring of 1900, and recovered artifacts during the first expedition with the Hellenic Royal Navy, in 1900–01. This wreck of a Roman cargo ship was found at a depth of 148 ft off Point Glyphadia on the Greek island of Antikythera. The team retrieved numerous large artifacts, including bronze and marble statues, pottery, unique glassware, jewelry, coins, and the mechanism.
In 1902, archaeologist Valerios Stais found that one of the pieces of rock had a gear wheel embedded in it. He initially believed that it was an astronomical clock, but most scholars considered the device to be too complex to have been constructed during the same period as the other pieces. Investigations into the object were dropped until ant became interested in it in 1951.
In 1971, Derek J. de Solla Price and Greek nuclear physicist Charalampos Karakalos made X-ray and gamma-ray images of the 82 fragments. The Antikythera mechanism is generally referred to as the first known analog computer. The quality and complexity of the mechanism’s manufacture suggest that it must have had undiscovered predecessors made during the Hellenistic period[1]. Its construction relied on theories of astronomy and mathematics developed by Greek astronomers during the second century BC, and it is estimated to have been built in the late second century BC or the early first century BC.
The mechanism is remarkable for the level of miniaturization and the complexity of its parts, which is comparable to that of fourteenth-century astronomical clocks. The device, housed in the remains of a 13.4 in × 7.1 in × 3.5-inch wooden box, was found as one lump, later separated into three main fragments which are now divided into 82 separate fragments after conservation efforts. Four of these fragments contain gears, while inscriptions are found on many others. The largest gear is approximately 5.1 inches in diameter and originally had 223 teeth.
It had 37 meshing bronze gears enabling it to follow the movements of the Moon and the Sun through the zodiac, to predict eclipses, and to model the irregular orbit of the Moon, where the Moon’s velocity is higher in its perigee than in its apogee.
This motion was studied in the 2nd century BC by astronomer Hipparchus of Rhodes[2], and it is speculated that he may have been consulted in the machine’s construction. There is speculation that a portion of the mechanism is missing and it also calculated the positions of the five classical planets.
On the front face of the mechanism, there is a fixed ring dial representing the ecliptic, the twelve zodiacal signs marked off with equal 30-degree sectors. Outside that dial is another ring which is rotatable, marked off with the months and days of the Sothic Egyptian calendar, twelve months of 30 days plus five intercalary days.
The months are marked with the Egyptian names for the months transcribed into the Greek alphabet. The first task, then, is to rotate the Egyptian calendar ring to match the current zodiac points. The Egyptian calendar ignored leap days, so it advanced through a full zodiac sign in about 120 years. The mechanism was operated by turning a small hand crank (now lost) which was linked via a crown gear to the largest gear, the four-spoked gear visible on the front of the fragment.
This moved the date pointer on the front dial, which would be set to the correct Egyptian calendar day. The year is not selectable, so it is necessary to know the year currently set, or by looking up the cycles indicated by the various calendar cycle indicators on the back in the Babylonian ephemeris tables[3] for the day of the year currently set, since most of the calendar cycles are not synchronous with the year. The crank moves the date pointer about 78 days per full rotation, so hitting a particular day on the dial would be easily possible if the mechanism were in good working condition.
The action of turning the hand crank would also cause all interlocked gears within the mechanism to rotate, resulting in the simultaneous calculation of the position of the Sun and Moon, the moon phase, eclipse, and calendar cycles, and perhaps the locations of planets.
What could the Antikythera Mechanism do?
- Predicted the hour, month, and day of an eclipse, accounting for leap years
- It predicted the positions of the Sun and moon against the Zodiak
- It had a black and white stone that displayed the moon phase on a certain date
- It showed the astronomical positions of Mercury, Venus, Mars Jupiter, and Saturn
- It told the Greeks exactly when to start the Olympiad games, a 4-year time span that began on the full moon closest to the summer solstice
Footnotes
- The Hellenistic period spans the period of Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium in 31 BC[1] and the conquest of Ptolemaic Egypt the following year.
- Hipparchus of Rhodes was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his incidental discovery of the precession of the equinoxes. He is known to have been a working astronomer between 162 and 127 BC. Hipparchus is considered the greatest ancient astronomical observer and, by some, the greatest overall astronomer of antiquity. He was the first whose quantitative and accurate models for the motion of the Sun and Moon survive.
- Babylonian ephemeris tables, in astronomy and celestial navigation, an ephemeris is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly velocity) over time.
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