The story of mapping time in ancient world

by Lionel Wijesiri

Celestial bodies, the sun, moon, planets, and stars have provided us a reference for measuring the passage of time throughout our existence. Ancient civilizations relied upon the apparent motion of these bodies through the sky to determine seasons, months, and years.

Little is known about the details of timekeeping in prehistoric eras, however, records and artifacts that are discovered, show that in every culture, people were preoccupied with measuring and recording the passage of time.

Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon.

Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes).

There are no written records of the creating of Stonehenge, built over 4000 years ago in England, but its alignments show its purposes apparently included the determination of seasonal or celestial events, such as lunar eclipses, solstices and so on.

The earliest Egyptian calendar was based on the moon's cycles, but later the Egyptians realized that the "Dog Star" in Canis Major, which is now called 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 in 4236 B.C., the earliest recorded year in history.

In Babylonia, again in Iraq, a year of 12 alternating 29-day and 30-day lunar months was observed before 2000 B.C., giving a 354-day year. In contrast, the Mayans of Central America relied on not only the sun and moon, but also the planet Venus, to establish 260-day and 365-day calendars.

This culture flourished from around 2000 B.C. until about 1500 A.D. They left celestial-cycle records indicating their belief that the creation of the world occurred in 3113 B.C. Their calendars later became portions of the great Aztec calendar stones.

Lunar Year

Starting sometime in the eighth century B.C. in the Roman Empire, the priest-astronomers were assigned the duty of declaring when a new month began and it was usually said to have started at the first sighting of a new moon. Month length at that time was simply the number of days that passed from one new lunar crescent to the next.

The High priest observed the sky and called out when there was a new moon and therefore a new month. For centuries afterward Romans referred to the first day of each new month as Kalends or Calends from their word calare (to announce solemnly, to call out). Obviously, the word calendar was derived from this custom.

This practice of starting a month at the first sighting of a new moon was observed not only by Romans but by Celts and Germans in Europe and by Babylonians and Hebrews. All of these peoples began their month when a young crescent was first seen in the sky. This is still done for the Islamic Calendar, but a new moon's date is calculated for traditional lunar calendars that are currently used in China and India.

During the period when month lengths were not fixed, new moons were usually sighted after either 29 or 30 days. If clouds obscured vision on the thirtieth day, a new month was declared to have begun.

When month lengths were identical with lunations, only those that lasted 30 days were considered to be normal. In many nations, months that consisted of 30 days were considered to be "full;" those that lasted only 29 days were said to be "hollow." Months containing 30 days were also called "full" in Babylon, but those containing 29 were deemed to be "defective."

In addition to their declaring the beginning of each month based upon a sighting of the new moon, priest-astronomers were also charged with pinpointing the start of a year. This is where we come across the star "Sirius" and the early Egyptians for their method of determining when the new year began.

Sirius

By this time Egyptian civil calendar were further divided into three sections called "decades" each of 10 days. The Egyptians noted that the helical rising of certain stars, such as Sirius and Orion, matched the first day of the 36 successive decades and called these stars decans.

During any one night, a sequence of 12 decans would be seen to rise and were used to count the hours. (This division of the night sky, later adjusted to account for the epagomenal days, had close parallels to the Babylonian zodiac. The signs of the zodiac each accounting for three of the decans. This astrological device was exported to India and then to Medieval Europe via Islam.)

Early Egyptians divided the day into temporal hours whose length depended upon the time of year. A summer hour, with the longer period of daylight, would be longer than that of a winter day. It was the Egyptians who first divided the day (and night) into 24 temporal hours.

Although many other stars may be used to fix the beginning of a sidereal year, the Egyptians made an excellent choice for this purpose. Sirius-Egyptians called it Sothis - not only signalled the approaching Nile flood, but is the brightest "fixed" star in the heavens. It is easy to recognize when seen in the night sky.

By observing the movement of Sirius, Egyptians came to grips with the fact that the year was longer than their venerable 360-day calendar. This resulted in a change to their method of approximating year length that had been in use for nearly a millennium. But it also caused them to wonder where the additional days came from. In order to account for these additional days, Egyptians created a myth about their sky-god, Nut.

According to the story, Nut was allowed to bear a child in "no month of no year." In answer to her pleading, her lover played dice with the moon and won an extra five days in the year. He gave them to Nut. Because these days were outside the old 360-day calendar, the prohibition against Nut bearing children did not apply. Nut's son Osiris was born on the first of them. Earth-god Geb was said to be the father.

During the reign of the Babylonian king Nabonasser (traditionally dated between 747 and 734 B.C.) priest-astronomers in that country discontinued their practice of looking for the new moon in order to name the beginning of a month. Instead, they returned to a fixed-length calendar that had 12 months of 30 days each, but with five days added at the end.

Zoroastrians used a calendar of 365 days whose era began at 389 B.C., the year of Zoroaster's birth. It consisted of 12 30-day months with five "Gatha days" added to the final month of the year. Each of the 30 days as well as each of the Gatha days has its own name.

They are referred to by that name just as we speak of a day by its number in the month. One of Alexander the Great's Generals, Seleucus Necator, founded (early in the 4th century B.C.) an empire that stretched from Asia Minor to India. He established a new calendar that was essentially the same as one that had been used for some time in Syria.

It contained 12 months of 30 days each and an extra five days at the year's end. Every fourth year an additional day for a total of six days were added at the end of the year.

In Persia under the Sassanids, and in Armenia and Cappadocia the official system of time-reckoning was 12 months of 30 days followed by five more days at the end of the year. The Sassanian year of 12 30-day months (plus five days) was kept adjusted to the seasons by observing an extra month every 120 years.

The 360-day calendar in Persia has been referred to as the "Old-Avestan" but the name was changed to "Young-Avestan" after five Gatha days were added at the end of each year. This happened between 510 and 487 B.C. depending on which historian is correct.

24-hour Day

Temporal hours were of little use to astronomers, and around 127 CE Hipparchus of Niceae, Greek astronomer and mathematician, working in the great city of Alexandria, proposed dividing the day into 24 equinoctial hours. These equinoctial hours, so called because they are based on the equal length of day and night at the equinox, split the day into equal periods.

The division of time was further refined by another Alexandrian based philosopher, Claudius Ptolemeus, who divided the equinoctial hour into 60 minutes, inspired by the scale of measurement used in ancient Babylon.

Claudius Ptolemeus also compiled a great catalogue of over one thousand stars, in 48 constellations and recorded his concept that the universe revolved around the Earth. Following the collapse of the Roman Empire it was translated into Arabic (in 827 CE) and later into Latin (in the 12th century CE). These star tables provided the astronomical data used by Gregory XIII for his reform of the Julian calendar in 1592, which we use today.