Runboard user emeritus
Location: Athens, Hellas
The Astrolabe of Antikithera
One of the most impressive things found in this period was the Astrolabe of Antikithera. Some sponge collectors found it in 1901 near the Greek island of Antikithera. In the Greek research centre 'Demokritos' professor Derek de Solla Piere (from Yale university) and professor Har. Karakalos (from Demokritos Research center) examined it under X-rays and found an amazingly complex construction. It is the most complex mechanical construction until 1200 A.D.!!!
The function of a common astrolabe is to measure the altitudes of celestial bodies, from which time the observer's latitude could be determined, too. The measurement of the altitude of the North Star yields the latitude and the altitude of the Sun and stars yields the time.
The instrument found on the island of Antikithera has a lot of metal wheels arranged in a way that simulates the movement of the stars and does the required calculations too! Who designed it and who made it with such accuracy remains a mystery. But it is clear enough that is not a plain astrolabe but a kind of astrological calculator.
The found relic and the reconstruction of its mechanism
In the astrolabe there were 27 different circular gears that were connected all together and were put into movement by a hand shaft. Everything was inside a wooden box with possible dimensions of 35 x 17 x 10 cm. On the front face there were two disks with indications about the date of the month according the sun, and the date according the moon. On the back face there were two other disks, one showing the moon month and the other the moon eclipses.
You may think that these things seem simple but if you want to calculate them be prepared for calculations with six-decimal-digit accuracy! Doors hinged to the box served to protect the dials, and on all available surfaces of box, doors and dials there were long Greek inscriptions describing the operation and construction of the instrument. At least 20 gear wheels of the mechanism have been preserved, including a very sophisticated assembly of gears that were mounted eccentrically on a turntable and probably functioned as a sort of epicyclic or differential, gear-system. Many of the Greek scientific devices known to us from written descriptions show much mathematical ingenuity, but such "hard fact" knowledge was so unexpected that some scholars at first thought that the fragments of the astrolabe must belong to some modern device...
Another technique appeared on the astrolabe for the first time in the history of engineering, was the use of the differential gear. The rotation speed of the in shaft equals to the difference of the speed of the two output shafts. Nothing like that reappeared in any known machinery until 19th century! The differential gear was used on that Astrolabe in order to allow the connection of sun to solar eclipses!!!.
The astrolabe was probably first used by the ancient Greek astronomer Hipparchus. [Hipparchus discovered the precession of the equinoxes. His calculation of the length of the year measured by the sun was within 6.5 minutes of modern measurements. Hipparchus devised a method of locating geographic positions by means of latitudes and longitudes. He catalogued, charted, and calculated the brightness of perhaps as many as 1000 stars. Hipparchus also compiled a table of trigonometric chords that became the basis for modern trigonometry.] Some people say that Archimedes has used some things like this, but a lot different from what it is found.
Ok... how old is this relic, you may now be wondering... Well, if the slip ring has not moved from its last position, it was set in 80 B.C. Furthermore, if we are right in supposing that a fiducial mark near the month scale was put there originally to provide a means of setting that scale in case of accidental movement, we can tell more. This mark is exactly 1/2 degree away from the present position of the scale, and this implies that the mark was made two years before the setting. Thus, although the evidence is by no means conclusive, we are led to suggest that the instrument was made about 82 B.C.!!! (and it's been under the sea for almost 2,000 years)...