Do we have astrology to thank for clocks?

Forget your fancy smartphone and give it up for the world's first analog computer, the astrolabe.

Here's the Taurus Love Forecast for a recent week in April, according to the horoscopes page of The Huffington Post: "Saturday's New Moon in Taurus is both an opportunity and a challenge. If you're stuck in a rut and resist trying different experiences, then your hopes for romance will diminish."

Science regards astrological predictions like this one as superstitious nonsense today. But in the ancient world, astrology and astronomy were the same discipline. Many political elites hired professional astrologers and sky-watchers to monitor and catalog the motions of the sun, moon and five visible planets (Mercury, Venus, Mars, Jupiter and Saturn). To do this, they established a sort of celestial map based on the sun's apparent path in the sky. They called this path the ecliptic, and then referred to the sky extending above and below the imaginary line as the zodiac. As the sun followed the ecliptic over the course of a year, it moved through 13 constellations, 12 of which formed the signs of the zodiac.


A horoscope, originally expressed as a diagram, defined the positions of the planets and signs of the zodiac at a particular time and place. Astrologers used the diagram to predict a person's future, believing that the positions of celestial objects influenced life on Earth, especially human affairs. For example, the Taurus Love Forecast we mentioned gets its unique attributes because of the position of the new moon, the time at which the moon appears as a narrow, waxing crescent.

To map the night sky accurately, astrologers relied on detailed charts and complex mathematical formulas. This proved cumbersome until, around 400 A.D., when a few brilliant scholars invented a device known as an astrolabe. Landlubbers liked it for making astronomical measurements, while sailors found it handy at sea for determining the altitude of the sun or a star, which could then be used to calculate latitude. It was, without a doubt, the slide rule of the Middle Ages -- the world's first analog computer.


The Technologies of Time

Astrolabe diagram
©2012 HowStuffWorks

Your typical, indispensable astrolabe had five basic parts:

  • The mater (Latin for "mother") served as the base of an astrolabe and received one or more thin plates.
  • Each plate, which corresponded to a specific latitude, came engraved with a coordinate system that made it possible to locate objects in the celestial sphere -- the imaginary globe that surrounded Earth and contained the sun, moon, planets and stars.
  • A rete (sounds like "treaty") sat on top of the plates and showed a number of stars and several important constellations. As it rotated around a central pin (the north celestial pole), the rete showed the daily motion of the heavens.
  • Two clocklike hands -- the rule, on the front of the instrument, and the alidade, on the back -- allowed the user to take measurements and readings and to sight objects in the sky.

With an astrolabe, anyone with a little working knowledge of mathematics and astronomy could calculate the position of celestial objects, the time of year, the altitude of any object, the latitude and much more. Clearly, astrologers would have found such a tool indispensable, for it would have provided the data necessary to make predictions. For example, the back of the astrolabe carried both zodiac and calendar scales. Using these scales together, astrologers could find the sun's position for a given day or the day the sun is in a given zodiac position. They could also determine which constellations were visible at certain times of the year and where to look in the sky to see them.


One of the most important functions of an astrolabe was to tell the time of day or night. During the day, astrologers based their calculations on the altitude of the sun. At night, they used the altitude of a visible star. Either way, they could find the time in a four-step process that involved two operations with the alidade and then two operations with the rete and the rule. It was an accurate way to get the time, but it wasn't as easy as flipping open a pocket watch.

Luckily, interest in mechanical clocks flourished at about the same time as astrolabes. The first tick-tocking timekeepers date back to about 1300 and bear the trademark invention that made them possible -- the escapement [source: Andrewes]. The earliest examples, known as verge escapements, consisted of a crown-shaped escape wheel, a vertical shaft known as the verge, and a horizontal bar, bearing weights on each end, known as the foliot. The verge carried two rectangular projections (pallets) that took turns catching in the teeth of the escape wheel. As the wheel turned, it rotated the verge foliot, causing them to oscillate back and forth. Thus, the escapement controlled the escape wheel's rotation and sustained the clock's oscillation. This in turn regulated the speed at which the clock operated.


Great Ideas Con-verge

Prague's astrolabe clock has been imparting all sorts of info to its many observers over the centuries. Time? Check. Sun's position in the zodiac? Check. Sun's altitude? Check.
Gallup Pix/Riser/Getty Images

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.


Author's Note

My teenage son doesn't wear a watch, preferring to find the time on his smartphone. (He could also use the device to get his horoscope and an interactive map of the night sky, if he so desired.) It made me wonder if an astrolabe maker from medieval Europe would have bemoaned the slow and steady decline of his trade as mechanical watches became more popular. Perhaps the astrolabe could serve as a lesson for modern watch companies. Timex, take note!

Related Articles


  • Andrewes, William J.H. "A Chronicle of Timekeeping." Scientific American Special Collector's Edition: A Matter of Time. Spring 2012.
  • Burnett-Stuart, George. "Astronomical Clocks of the Middle Ages: A Guided Tour." Almagest. (Apr. 17, 2012)
  • Janus. "The Astrolabe: An instrument with a past and a future." (Apr. 17, 2012)
  • Jespersen, James and Jane Fitz-Randolph. "From Sundials to Atomic Clocks: Understanding Time and Frequency." The National Institute of Standards and Technology. Monograph 155, 1999 Edition. (Apr. 17, 2012)
  • Rees, Martin. "Universe: The Definitive Visual Guide." Dorling Kindersley Limited. 2008.