The Antikythera mechanism, one of the world’s oldest known gadgets, has coughed up another secret. In addition to tracking time and upcoming Olympic games, the mechanism was also great at tracking the motion of the Sun across the sky.
Because the orbit of the Earth around the Sun is an elipse, not a circle, our great life-giving star would appear at times to move more quickly across the sky than at others. The mechanism was great at correcting for this subtle “weirdness” when it involved the Moon’s elliptical orbit, thanks to a specialized series of two tiny gears. With the Sun, the effect is even more subtle from an earthbound perspective, so the gears (if they had existed) would have had to be even tinier. Too tiny to be of any practical use, it turns out, so the designer allegedly used something different: Pure geometry.
Science historian James Evans of the University of Puget Sound and colleagues suggested a simpler way to make the sun dial appear to change speed: Stretch the zodiac. If the spaces on the front wheel of the mechanism were of different widths, Evans reasoned, then the hand representing the sun would take longer to travel through the part of the year lumped under the zodiac sign of Taurus than through Libra.
The delay would make the sun look like it was moving slower at some times of year and faster at others, even though the gears turning the hand moved at a constant speed.
X-ray tests have proven this out to a certain extent, although Evans’s zodiac model discovery remains controversial. What is undeniable is this this ancient device remains a source of wonder.