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.