Measurement of Time M.Y. ANAND, B.A. KAGALI* Department of Physics, Bangalore University, Bangalore 560056 *email: [email protected] ABSTRACT Time was historically measured using the periodic motions of the sun and stars. Various types of sun clocks were devised in Egypt, Greece and Europe. Different types of water clocks were assembled with ever greater accuracy. Only in the seventeenth century did the mechanical clock with pendulums and springs appeared. Accurate quartz clocks and atomic clocks were developed in the first half of the twentieth century. Now we have clocks that have better than microsecond accuracy. This article gives a brief account of all these topics. Keywords: Sun clocks, Water clocks, Mechanical clocks, Quartz clocks, Atomic clocks, World Time, Indian Standard time We all know intuitively what time is. It can be civilizations relied upon the apparent motion of roughly equated with change or motions. From these bodies through the sky to determine the very beginning man has been interested in seasons, months, and years. understanding and measuring time. More We know little about the details of recently, he has been looking for ways to limit timekeeping in prehistoric eras, but we find the “damaging” effects of time and going that in every culture, some people were backward in time! preoccupied with measuring and recording the Celestial bodies—the Sun, Moon, planets, passage of time. Ice-age hunters in Europe over and stars—have provided us a reference for 20,000 years ago scratched lines and gouged measuring the passage of time. Ancient holes in sticks and bones, possibly counting the Physics Education • January − March 2007 277 days between phases of the moon. Five Peoples of various civilizations have been thousand years ago, Sumerians in the Tigris- interested in determining time with ever greater Euphrates valley in today's Iraq had a calendar accuracy as they progressed. that divided the year into 30 day months, divided the day into 12 periods, and divided these periods into 30 parts. We have no written Sun Clocks records of Stonehenge, built over 4000 years Egyptians were apparently the formally divided ago in England, but its alignments show its their day into parts something like our hours. purposes apparently included the determination Obelisks (slender, tapering, four-sided of seasonal or celestial events, such as lunar monuments) were built as early as 3500 BCE eclipses, solstices and so on. (see Figure.1). Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks also showed the year’s longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, additional markers around the base of the monument would indicate further subdivisions of time. The earliest Egyptian calendar was based on the moon's cycles, but later the Egyptians realized that the “Dog Star” in Canis Major, which we call Sirius, rose next to the sun every 365 days, about when the annual inundation of the Nile began. Based on this knowledge, they devised a 365 day calendar that seems to have begun around 3100 BCE(Before the Common Era). In ancient India time was measured in terms of respirations. The time required in pronouncing ten long syllables (Gurvakshara) Figure 1. was called a respiration. Six respirations made up one vinadi( period Another Egyptian shadow clock or sundial, of twenty four seconds), 60 vinadis made one possibly the first portable timepiece, came into nadi and 60 nadis made one day. use around 1500 BCE. This device divided a The Puranic version of the day was sunlit day into 10 parts plus two “twilight measured by taking 15 twinklings (nimesha) as hours” in the morning and evening. When the equal to one bit (kashtha), 30 bits making one long stem with 5 variably spaced marks was minute (kala). Thirty minutes made one hour oriented east and west in the morning, an (muhurta) and 30 hours made one day. elevated crossbar on the east end cast a moving 278 Physics Education • January − March 2007 shadow over the marks. At noon, the device associated with the internal workings of atoms was turned in the opposite direction to measure as their regulators. the afternoon “hours.” The merkhet, the oldest known 2) a means of keeping track of the astronomical tool, was an Egyptian increments of time and displaying the result. development of around 600 BCE. A pair of Our ways of keeping track of the passage of merkhets was used to establish a north-south time include the position of clock hands and line (or meridian) by aligning them with the digital time displays. Pole Star. They could then be used to mark off The history of timekeeping is the story of nighttime hours by determining when certain the search for ever more consistent actions or other stars crossed the meridian. processes to regulate the rate of a clock. In the quest for better year-round accuracy, sundials evolved from flat horizontal or vertical plates to more elaborate forms. One Water Clocks version was the hemispherical dial, a bowl- Water clocks were among the earliest shaped depression cut into a block of stone, timekeepers that didn't depend on the carrying a central vertical gnomon (pointer) observation of celestial bodies. One of the and scribed with sets of hour lines for different oldest was found in the tomb of the Egyptian seasons. pharaoh Amenhotep I, buried around 1500 The hemicycle, said to have been invented BCE. Later named clepsydras (“water about 300 BCE, removed the useless half of the thieves”) by the Greeks, who began using them hemisphere to give an appearance of a half- about 325 BCE, these were stone vessels with bowl cut into the edge of a squared block. By sloping sides that allowed water to drip at a 30 BCE, Vitruvius could describe 13 different nearly constant rate from a small hole near the sundial styles in use in Greece, Asia Minor, bottom. Other clepsydras were cylindrical or and Italy. bowl-shaped containers designed to slowly fill with water coming in at a constant rate. Markings on the inside surfaces measured the Elements of a Clock passage of “hours” as the water level reached Before we consider the different ways them. These clocks were used to determine employed to mark the passage of time, we hours at night, but may have been used in should broadly define what constitutes a clock. daylight as well. Another version consisted of a All clocks must have two basic components: metal bowl with a hole in the bottom; when placed in a container of water the bowl would 1) a regular, constant or repetitive process fill and sink in a certain time. These were still or action to mark off equal increments of in use in North Africa in the 20th century. time. Early examples of such processes More elaborate and impressive mechanized included the movement of the sun across the water clocks were developed between 100 sky, candles marked in increments, oil lamps BCE and 500 CE by Greek and Roman with marked reservoirs, sand glasses horologists and astronomers. The added (hourglasses), and in the Orient, knotted cords complexity was aimed at making the flow more and small stone or metal mazes filled with constant by regulating the pressure, and at incense that would burn at a certain pace. providing fancier displays of the passage of Modern clocks use a balance wheel, pendulum, time (see Figure 2). Some water clocks rang vibrating crystal, or electromagnetic waves bells and gongs; others opened doors and Physics Education • January − March 2007 279 windows to show little figures of people, or escapement invented about 725 CE. The Su moved pointers, dials, and astrological models Sung clock tower, over 30 feet tall, possessed a of the universe. bronze power-driven armillary sphere for A Macedonian astronomer, Andronikos, observations, an automatically rotating celestial supervised the construction of his Horologion, globe, and five front panels with doors that known today as the Tower of the Winds, in the permitted the viewing of changing manikins Athens marketplace in the first half of the first which rang bells or gongs, and held tablets century BCE. This octagonal structure showed indicating the hour or other special times of the scholars and shoppers both sundials and day (see Figure 3). mechanical hour indicators. It featured a 24 Since the rate of flow of water is very hour mechanized clepsydra and indicators for difficult to control accurately, a clock based on the eight winds from which the tower got its that flow could never achieve excellent name, and it displayed the seasons of the year accuracy. People were naturally led to other and astrological dates and periods. The approaches. Romans also developed mechanized clepsydras, though their complexity accomplished little improvement over simpler methods for determining the passage of time. Figure 2. Figure 3. In the Far East, mechanized astronomical/ Before 2000 BCE, the Babylonians (in astrological clock making developed from 200 today’s Iraq) used a year of 12 alternating 29 to 1300 CE. Third-century Chinese clepsydras day and 30 day lunar months, giving a 354 day drove various mechanisms that illustrated year. In contrast, the Mayans of Central astronomical phenomena. One of the most America relied not only on the Sun and Moon, elaborate clock towers was built by Su Sung but also the planet Venus, to establish 260 day and his associates in 1088 CE. Su Sung’s and 365 day calendars. This culture and its mechanism incorporated a water-driven related predecessors spread across Central 280 Physics Education • January − March 2007 America between 2600 BCE and 1500 CE, no evidence or record of the working models reaching their apex between 250 and 900 CE.