How Did People in Ancient times Measured the Passing of Time?
The earliest natural events to be recognized were in the heavens, but during the course of the year there were many other events that indicated significant changes in the environment. Seasonal winds and rains, the flooding of rivers, the flowering of trees and plants, and the breeding cycles or migration of animals and birds, all led to natural divisions of the year, and further observation and local customs led to the recognition of the seasons.
Ever since man first noticed the regular movement of the Sun and the stars, we have wondered about the passage of time. Prehistoric people first recorded the phases of the Moon some 30,000 years ago, and recording time has been a way by which humanity has observed the heavens and represented the progress of civilization.
Measuring time by the Sun, the Moon and the Stars
As the sun moves across the sky, shadows change in direction and length, so a simple sundial can measure the length of a day. It was quickly noticed that the length of the day varies at different times of the year. The reasons for this difference were not discovered until after astronomers accepted the fact that the earth travels round the sun in an elliptic orbit, and that the earth’s axis is tilted at about 26 degrees. This variation from a circular orbit leads to the Equation of Time (see ‘Note 2’ below) which allows us to work out the difference between ‘clock’ time and ‘sundial time’.
Another discovery was that sundials had to be specially made for different latitudes because the Sun’s altitude in the sky decreases at higher latitudes, producing longer shadows than at lower latitudes. Today, artists and astronomers find many ways of creating modern sundials.
The oldest image of a star pattern, the constellation of Orion, has been recognised on a piece of mammoth tusk some 32,500 years old. The constellation Orion is symbolized by a man standing with his right arm raised and a sword at his belt and can be seen throughout the world at different times of the year. Orion was the sun god of the Egyptians and Phonecians and called the ‘strong one’ by the Arabs. In parts of Africa, his belt and sword are known as ‘three dogs chasing three pigs’ and the Borana people of East Africa based a sophisticated calendar on observations of star clusters near Orion’s belt. Orion contains some of the brightest stars in the southern part of the winter sky in the northern hemisphere and can be seen later in the southern hemisphere.
The earliest Egyptian Star Map is about 3,500 years old and shows the most unusual conjunction of the planets (Venus, Mercury, Saturn and Jupiter) in the constellation of Orion and the occurrence of a solar eclipse that happened in 1534 BCE. The Egyptians also used a clepsydra or water clock. This was a basin-shaped, alabaster vessel filled with water that ran out through a hole in the bottom. The time was indicated by the level of water remaining inside. Monks were the first to operate clocks by wheels and weights. Clocks of this type, found in monasteries, date back to the 14th century. The first spring clock is dated about 1500.
It is believed that the Babylonians first used a pole fixed in the ground to measure the passing of time. They noticed that the position of the shadow changed during the hours of sunlight. They found that the shadow was long at sunrise and that it slowly grew shorter until it reached a point when it started to lengthen again. They learned to judge the time by looking at the shadow. The simple shadow and pole arrangement was the basis of the various shadow clocks or sundials used by the ancient Egyptians. Eventually sundials were provided with the hour figures engraved on a metal plate.
Babylonian records of observations of heavenly events date back to 1,600 BCE. The reason for adopting their arithmetic system is probably because 60 has many divisors, and their decision to adopt 360 days as the length of the year and 3600 in a circle was based on their existing mathematics and the convenience that the sun moves through the sky relative to fixed stars at about 1degree each day.
The constellation Taurus, the bull, a symbol of strength and fertility, figures prominently in the mythology of nearly all early civilizations, from Babylon and India to northern Europe. The Assyrian winged man-headed bull had the strength of a bull, the swiftness of a bird and human intelligence.
From about 700 BCE the Babylonians began to develop a mathematical theory of astronomy, but the equally divided 12-constellation zodiac appears later about 500 BCE to correspond to their year of 12 months of 30 days each. Their base 60 fraction system which we still use today (degrees / hours, minutes and seconds) was much easier to calculate with than the fractions used in Egypt or Greece, and remained the main calculation tool for astronomers until after the 16th century, when decimal notation began to take over.
The earliest archaeological evidence of Chinese calendars appears about 2,000 BCE. They show a 12 month year with the occasional occurrence of a 13th month. However, traditional Chinese records suggest the origin of a calendar of 366 days depending on the movements of the Sun and the Moon as early as 3,000 BCE. Over such a long period of observation, Chinese astronomers became aware that their calendar was not accurate, and by the second century CE it was recognised that the calendar became unreliable every 300 years. This problem is called Precession and was recorded by Chinese historians in the fourth and fifth centuries CE. In the fifth century CE the scholar Zu Chongzi created the first calendar which took precession into account, and the most comprehensive calendar was the Dayan Calendar compiled in the Tang Dynasty (616-907 CE) well ahead of any such development in Europe.
Precession is due to the gradual movement of the Earth’s rotational axis in a circle with respect to the fixed stars. This movement produces a slow ‘wobble’ which means that the positions of the stars complete a cycle of about 26,000 years.
In the Mediterranean, Hipparchus made the earliest calculations of precession in about 160 BCE. The problem was taken up by astronomers in the Middle East and India who recognized that precession gradually altered the length of the year. Calendars have had to be altered regularly. In 325 CE the spring (vernal) equinox had moved to March 21. The Emperor Constantine established dates for the Christian holidays, but Easter is based on the date of the vernal equinox which varies every year because the equinox is an astronomical event. By 1582 the vernal equinox had moved another ten days and Pope Gregory established a new calendar, and this change is the reason for having an extra day in every leap year. However, there are still small changes accumulating, and one day we shall have to adopt a new calendar!
Inventions for measuring and regulating time
The early inventions were made to divide the day or the night into different periods in order to regulate work or ritual, so the lengths of the time periods varied greatly from place to place and from one culture to another.
Oil lamps: There is archaeological evidence of oil lamps about 4,000 BCE, and the Chinese were using oil for heating and lighting by 2,000 BCE. Oil lamps are still significant in religious practices, symbolic of the journey from darkness and ignorance to light and knowledge. The shape of the lamp gradually evolved into the typical pottery style shown. It was possible to devise a way of measuring the level in the oil reservoir to measure the passing of time.
Candle Clocks: Marked candles were used for telling the time in China from the sixth century CE. There is a popular story that King Alfred the Great invented the candle clock, but we know they were in use in England from the tenth century CE. However, the rate of burning is subject to draughts, and the variable quality of the wax. Like oil lamps, candles were used to mark the passage of time from one event to another, rather than tell the time of day.
Water Clocks: The water clock, or clepsydra, appears to have been invented about 1,500 BCE and was a device which relied on the steady flow of water from or into a container. Measurements could be marked on the container or on a receptacle for the water. In comparison with the candle or the oil lamp, the clepsydra was more reliable, but the water flow still depended on the variation of pressure from the head of water in the container.
Astronomical and astrological clock making was developed in China from 200 to 1300 CE. Early Chinese clepsydras drove various mechanisms illustrating astronomical phenomena. The astronomer Su Sung and his associates built an elaborate clepsydra in 1088 CE. This device incorporated a water-driven bucket system originally invented about 725 CE. Among the displays were a bronze power-driven rotating celestial globe, and manikins that rang gongs, and indicated special times of the day.
Hour Glasses or Sandglasses
As the technology of glass-blowing developed, from some time in the 14th century it became possible to make sandglasses. Originally, sandglasses were used as a measure for periods of time like the lamps or candles, but as clocks became more accurate they were used to calibrate sandglasses to measure specific periods of time, and to determine the duration of sermons, university lectures, and even periods of torture.