The Quest for Time: How Ancient Civilizations Used Astronomy?

The Secrets of Time in Ancient Civilizations

Past human societies have always sought to keep track of time, driven by anxiety about the future and the need to understand the world they lived in. Like today, thousands of years ago, people were already asking questions about their environment. Without astronomy, they would have been lost in the vastness of the Universe. Astronomy is a necessity for humans, and it was vital for past civilizations.

Today, the concept of time is well integrated into our civilization. We no longer wonder about the date: seasons, months, days, and hours are constantly displayed before our eyes. We no longer need to calculate these data; clocks do it for us. But for our ancestors, understanding and measuring time was a complex task that required meticulous observation of the sky.

Observation of the Sky: A Natural Clock

Our ancestors studied the skies to find recurring events to establish a clock. Indeed, the sky is a natural reference:

• Seasons are marked by the cyclical appearance of stars.
• The Sun delineates the day.
• The Moon marks the month and the week.

With the cycle of the Moon, our ancestors identified a first repetitive pattern: the month. Moreover, the word "mens" in Greek, in its primitive sense, meant "moon."

Constellations and Seasons

Our ancestors noticed that the seasons were related to the stars. They drew constellations, seeing reassuring imaginary figures, often inspired by familiar animals. The configuration of the stars changes throughout the seasons because the Earth's tilt exposes us to a different sky throughout its 365-day journey around its orbit. This cyclical journey is a wonderful clock of a year on which we organize ourselves.

Understanding their environment was a considerable advantage for ancient civilizations. They therefore built gigantic monuments to mark time, such as:

• Stonehenge: A megalithic site in England, aligned with the solstices.
• The Pyramids of Giza: Aligned with astronomical precision with the stars.
• Chaco Canyon: An ancestral Pueblo site in the United States, marking solar and lunar cycles.

Solstices: Markers for the Seasons

The solstices, the longest and shortest days of the year, are essential references for marking the seasons and the year. The date of the solstices varies very slowly, not changing significantly over a few hundred years. Thus, the Earth always finds itself in the same place on its orbit on the same date, and the celestial vault always displays the same figures at the same time of the year.

This "divine" precision of celestial clocks helped past civilizations (such as the Incas, Mayas, Egyptians, and Greeks) to organize and plan their tasks, mainly agricultural. By observing the sky, these civilizations explored the concept of time. The Sun, Moon, and stars played a crucial role for them, allowing them to measure time and explain the mysteries of their planet.

Eratosthenes and the Measurement of the Earth

Thanks to the knowledge of the solstices, two centuries before Christ, Eratosthenes of Cyrene (-276 -194 BC) demonstrated that the Earth was enormous and, moreover, round. With a simple stick, he remarkably measured the diameter of the Earth, proving it was a sphere.

• He planted a stick in the ground in Syene, in southern Egypt.
• At the summer solstice, when the Sun was at its zenith, the stick had no shadow.
• At the same time, in Alexandria, the stick had a shadow.

By measuring the size of this shadow, Eratosthenes calculated the angle (7.2°) relative to the vertical. He thus obtained one-fiftieth of a circle. Knowing the distance between Syene and Alexandria, he multiplied this distance by 50 and obtained the equatorial circumference of the Earth. This measurement, extraordinarily precise for the time, was 39,689 km, very close to the current value of 40,075 km.

The Division of the Week

The division of the week into seven days approximately corresponds to a quarter of a lunar month, which lasts 28 days. In reality, the sidereal revolution of the Moon is 27 days, 7 hours, 43 minutes, and 11.5 seconds. According to the International Organization for Standardization (ISO 8601 standard), Monday is considered the first day of the week. The weeks of the same year are numbered from 01 to 52 or 53, depending on the number of Thursdays. Week number 01 is the one that contains the first Thursday of January.

The Astronomical Origin of the Days of the Week

A week contains seven days, whose names are related to astronomy. The astronomers of the Roman era observed six luminous objects moving in the sky: Saturn, Jupiter, Mars, Venus, Mercury, and the Moon. These celestial objects inspired the names of the days of the week:

• Monday: Day of the Moon (Luna in Latin).
• Tuesday: Day of Mars (Mars in Latin).
• Wednesday: Day of Mercury (Mercurius in Latin).
• Thursday: Day of Jupiter (Jovis in Latin).
• Friday: Day of Venus (Venus in Latin).
• Saturday: Day of Saturn (Saturnus in Latin).

The planets Uranus and Neptune were only discovered in the 18th and 19th centuries, thanks to the invention of the telescope. The other celestial objects, seen from Earth, do not move in an apparent manner, but observers can note, month by month, the displacement of the planets in the sky. Among these objects, it is the Moon that moves the fastest.

Sunday: A Historical Addition

Sunday, the seventh day of the week, was added by Emperor Constantine I in 321. In France, Sunday is not linked to the name of a celestial object, but it is considered the "Lord's Day" (dies dominicus in Latin) for Catholics. In contrast, in other cultures, such as in English, Sunday is clearly associated with the Sun (Sunday).

The Astronomical Legacy of Ancient Civilizations

Ancient civilizations used astronomy to understand and measure time, thus organizing their lives around natural cycles. Their discoveries, such as those of Eratosthenes, testify to their ingenuity and curiosity. Today, we inherit this knowledge, which continues to guide our understanding of time and the Universe.