It didn't take long for a shrewd inventor to marry the mechanical clock to the astrolabe. But mechanical astrolabes, or astronomical clocks, didn't appear immediately after the verge escapement was invented. It took some tinkering to perfect timekeeping technology so that it could accurately control multiple faces and hands.
- First came the 24-hour clock, which had a fixed hand and a rotating dial bearing the hour of the day. As the dial turned, the hand pointed to the time.
- Next, timekeepers built sun clocks. These devices had two parts -- an outer, fixed dial and an inner rotating plate known as the chapter ring. The chapter ring bore the image of the sun, which aligned with a number corresponding to the time of day.
- Clocksmiths then developed the sun-moon clock to keep track of the moon's motion, too. The sun and the moon plates could rotate independently. The moon plate lagged behind the sun's until, after 29.5 days, the two lined up, indicating the passage of a month.
- With the sun-moon clock working well, cocky clocksmiths added a third plate, bearing zodiac symbols, which ran slightly faster than the sun plate. When days and months were added to the zodiac dial, such a clock could tell the date and the astronomical position of the sun.
- Finally, fully functioning mechanical astrolabes began appearing. These complex devices had a rete, which bore the zodiac, an altitude plate, a sun dial and a chapter ring, revealing where the sun was placed in the zodiac, as well as its position relative to the horizon.
Fine examples of these astronomical clocks exist today. For example, the Prague astrolabe clock in the Czech Republic dates back to 1410, although it's undergone a few renovations over the years. With a quick glance, an observer can get the time, the sun's position in the zodiac, sidereal time (based on Earth's position relative to distant stars) and the sun's altitude. All of this comes with a juicy legend: that the people of Prague blinded the clockmaker who designed the instrument so he couldn't replicate the feat in another town.
Astronomical clocks like the one in Prague may seem extraordinarily complex by today's standards, but they were required to do more. Because time in the ancient world was intimately tied to the motion of the sun, moon, stars and planets, clocks had to keep track of these celestial cycles. And because people also believed that the arrangement of the heavens influenced life on Earth, monitoring the zodiac constellations was equally important. Astrologers may not have thrown away their astrolabes when the first astronomical clocks were built, but they certainly would have appreciated how easy it was to get the time -- and the Love Forecast for Taurus in a particular week in April.