Summer hours were long, winter ones short only at the spring and autumn equinoxes were the hours of daylight and darkness equal. These periods became known as temporal hours because their duration varied according to the changing length of days and nights with the passing of the seasons. The cosmic significance the Egyptians placed in the 12 decans led them to develop a system in which each interval of darkness (and later, each interval of daylight) was divided into a dozen equal parts. At the rise of the star Sirius just before sunrise, which occurred around the all-important annual flooding of the Nile, 12 decans could be seen spanning the heavens. Each period of 10 days was marked by the appearance of special star groups (constellations) called decans. The Egyptians formulated a civil calendar having 12 months of 30 days, with five days added to approximate the solar year. Today's Gregorian calendar derives from the Babylonian, Egyptian, Jewish and Roman calendars. As the Roman Empire expanded northward, it organized its calendar for the most part around the solar year. In more northern climes, however, where seasonal agriculture was important, the solar year became more crucial.

Hence, the calendars developed at the lower latitudes were influenced more by the lunar cycle than by the solar year. And, for those living near the equator in particular, its waxing and waning was more conspicuous than the passing of the seasons. They based their calendars on three natural cycles: the solar day, marked by the successive periods of light and darkness as the earth rotates on its axis the lunar month, following the phases of the moon as it orbits the earth and the solar year, defined by the changing seasons that accompany our planet's revolution around the sun.īefore the invention of artificial light, the moon had greater social impact. So integral have these time-based technologies become to our day-to-day lives that we recognize our dependency on them only when they fail to work.ĪCCORDING TO archaeological evidence, the Babylonians and Egyptians began to measure time at least 5,000 years ago, introducing calendars to organize and coordinate communal activities and public events, to schedule the shipment of goods and, in particular, to regulate cycles of planting and harvesting. Moreover, not only do time signals beamed down from Global Positioning System satellites calibrate the functions of precision navigation equipment, they do so as well for cellular telephones, instant stock-trading systems and nationwide power-distribution grids. Nearly all computers, for example, contain a quartz-crystal clock to regulate their operation. Today highly accurate timekeeping instruments set the beat for most of our electronic devices. 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. 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. Western Europeans adopted these technologies, but by the 13th century, demand for a dependable timekeeping instrument led medieval artisans to invent the mechanical clock. 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. Humankind's efforts to tell time have helped drive the evolution of our technology and science throughout history.

Pendulum clock