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A Chronicle of Timekeeping Our Conception of Time Depends on the Way We Measure It by William J

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A Chronicle of Timekeeping Our Conception of Time Depends on the Way We Measure It by William J 50 Scientific American William J. H. Andrewes is a museum consultant and maker of pre- cision sundials who has specialized in the history of time measure- ment for more than 35 years. He has worked at several scholarly institutions, including Harvard University. In addition to writing arti- cles for popular and academic journals, Andrewes edited The Quest for Longitude and co-wrote The Illustrated Longitude with Dava Sobel. INVENTION A CHRONICLE OF TIMEKEEPING Our conception of time depends on the way we measure it By William J. H. Andrewes umankind’s efforts to tell time have helped drive the evolution of our technology and science throughout history. The need to gauge the divisions of the day and night led the ancient Egyptians, Greeks and Romans to create sundials, water clocks and other early chronometric tools. Western Europeans adopted these tech nologies, but by the 13th century, demand for a dependable timekeeping instrument led medieval ar- Htisans to invent the mechanical clock. Although this new device satisfied the requirements of monastic and urban communities, it was too inaccurate and unreliable for scientific application until the pendulum was employed to govern its operation. The precision timekeepers that were subsequently developed resolved the critical problem of finding a ship’s position at sea and went on to play key roles in the industrial revolution and the advance of Western civilization. Today highly accurate timekeeping instru- began to measure time at least 5,000 years ago, IN BRIEF ments set the beat for most of our electronic de- introducing calendars to organize and coordi- vices. Nearly all computers, for example, contain nate communal activities and public events, to Devices for measuring time have been a quartz-crystal clock to regulate their opera- schedule the shipment of goods and, in particu- around for at least 5,000 years and tion. Moreover, not only do time signals beamed lar, to regulate cycles of planting and harvesting. today are essential for coordinating the operation of everything from cell ) down from Global Positioning System satellites They based their calendars on three natural cy- phones to power-distribution grids. calibrate the functions of precision navigation cles: the solar day, marked by the successive peri- The earliest mechanical clocks, in- equipment, they do so as well for cell phones, in- ods of light and darkness as the earth rotates on vented just before 1300, told time by stant stock-trading systems and nationwide its axis; the lunar month, following the phases of striking a bell. digital question marks question digital N ( power-distribution grids. So integral have these the moon as it orbits the earth; and the solar By the early 1500s inventors had de- A time-based technologies become to our day-to- year, defined by the changing seasons that ac- veloped spring-driven mechanical MERIC A day lives that we recognize our dependency on company our planet’s revolution around the sun. clocks that were small enough to be them only when they fail to work. Before the invention of artificial light, the portable. Modern quartz and atomic clocks ); SCIENTIFIC ); SCIENTIFIC moon had greater social impact. And, for those have made it possible to keep extreme- RECKONING DATES living near the equator in particular, its waxing ly accurate time, which has opened watch face watch according to archaeological evidence, the Bab- and waning was more conspicuous than the the door to new applications. MY ( A AL ylonians, Egyptians and other early civilizations passing of the seasons. Hence, the calendars ScientificAmerican.com 51 WHEELS AND SPRINGS Development of Early Mechanical Clockworks Verge and Foliot Escapement Verge The innovative component of the first mechanical clocks (circa 1300) was the escape- ment, a device that both controlled the escape wheel’s rotation and transmitted the Cords power needed to sustain the motion of the oscillator, which in turn regulated the speed Pallet at which the timekeeper operated. The escape wheel, a sawtoothed crown, is driven by a Pallet gear train powered by a weighted cord wound around the axle. The clockwise rotation of the escape wheel is obstructed by two pallets protruding from a vertical shaft, called a Escape wheel verge, which carries a bar known as a foliot. When the top pallet checks the escape wheel’s rotation (causing a “tick”), Foliot the engaged wheel tooth gradually Verge forces the pallet back until it is free Cycloidal to escape. The wheel’s movement, Pendulum Clock cheeks however, is stopped almost immedi- Although Galileo and other 16th-cen- ately when the lower pallet arrests tury scientists knew about the poten- Weight another tooth (causing a “tock”) and tial of the pendulum as a timing in- strument, Christiaan Huygens was the then pushes the verge in the oppo- Pallet site direction. Driven by the escape first to devise a pendulum clock. Huy- wheel, the to-and-fro oscillation of gens soon realized that a pendulum the verge and foliot continues until swinging in a small arc would perform Crutch the cord fully unwinds. The rate at Escape its oscillations faster than one moving which the mechanism operates can wheel in a large arc. He overcame this prob- be adjusted by moving the weights lem by installing two curved “cycloidal cheeks” (shown in side view) at the Rod on the foliot arms out (for slower) Pallet and in (for faster). pendulum’s suspension point. Acting on the suspension cords, these curved stops reduced the effective length of the pendulum as its arc increased so Fusee that it maintained a cycloidal rather The use of coiled springs as the motive force for timekeepers was made practical by than a circular path. Thus, in theory, the invention of the fusee in the early to mid-1400s. Although a spring is a compact the pendulum completed every swing power source, its force varies, increasing as it is wound more tightly. The fusee, a cone- in the same time period, regardless of shaped grooved pulley, was devised to compensate for the variable strength of a time- amplitude (swing distance). In Huy- Bob (swings keeper’s mainspring. The barrel, gens’s clock, the gravity-influenced perpendicular which houses the spring, is con- motion of the pendulum replaced the to the plane nected to the fusee by a cord or purely mechanically driven oscillation of the page) chain. When the mainspring is ful- of the horizontal foliot. Now it was the ly wound, the cord pulls on the Mainspring pendulum’s beat that regulated the narrow end of the fusee, where a action of the verge escapement and Barrel short torque arm produces rela- the rotation of the wheels, which in tively little leverage. As the clock turn delivered this far more reliable Fusee runs, the cord is gradually drawn and accurate time measurement to back onto the barrel. To compen- the hands of the clock dial. sate for the mainspring’s diminish- ing strength, the cord’s spiral track on the fusee increases in diameter. Cord Thus, the force delivered to the Anchor Escapement gear wheels of the timekeeper re- Developed around 1670 in England, the anchor escapement is a lever-based de- mains constant despite the chang- vice shaped like a ship’s anchor. The motion of a pendulum rocks the anchor so ing tension of its mainspring. that it catches and then releases each tooth of the escape wheel, which, in turn, provides an impulse to maintain the pendulum’s oscillation. Unlike the Anchor verge escapement used in early Spiral Balance Spring pendulum clocks, the anchor es- In 1675 Huygens invented the spiral capement permitted the pendulum balance spring. Just as gravity con- to travel in such a small arc that trols the swinging oscillation of a maintaining a cycloidal swing path pendulum in a clock, this spring became unnecessary. Moreover, regulates the rotary oscillation of a this invention made practical the balance wheel in portable timepiec- use of a long, seconds-beating pen- es. A balance wheel is a rotor that spins dulum and thus led to the de- one way and then the other, repeating the velopment of a new, floor- cycle over and over. Depicted here is a mod- Balance standing case design, which Spring ern version, finely balanced with adjustable wheel became known as the long- Escape timing screws. case, or grandfather, clock. wheel VID PENNEY A D 52 Scientific American developed at the lower latitudes were in- land. That the Roman Catholic Church these fractions to be included on dials un- fluenced more by the lunar cycle than by should have played a major role in the in- til the 1660s, when the pendulum clock the solar year. In more northern climes, vention and development of clock technol- was developed. Minutes and seconds de- however, where seasonal agriculture was ogy is not surprising: the strict observance rive from the sexagesimal partitions of important, the solar year became more of prayer times by monastic orders occa- the degree introduced by Babylonian as- crucial. As the Roman Empire expanded sioned the need for a more reliable instru- tronomers. The word “minute” has its or- northward, it organized its calendar for ment of time measurement. Further, the igins in the Latin prima minuta, the first the most part around the solar year. To- Church not only controlled education but small division; “second” comes from se- day’s Gregorian calendar derives from also possessed the wherewithal to employ cunda minuta, the second small division. the Babylonian, Egyptian, Jewish and Ro- the most skillful craftsmen. Additionally, The sectioning of the day into 24 hours man calendars. the growth of urban mercantile popula- and of hours and minutes into 60 parts The Egyptians formulated a civil cal- tions in Europe during the second half of became so well established in Western endar having 12 months of 30 days, with the 13th century created demand for im- culture that all efforts to change this ar- five days added to approximate the solar proved timekeeping devices.
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