The quest for time: How did ancient civilizations use astronomy?

The secrets of time in ancient civilizations

Astronomy, a vital necessity

Human societies of the past always sought to orient themselves in time, driven by the anxiety of tomorrow but also by the need to understand the world in which they lived. As today, thousands of years ago, people were already asking questions about their environment. Without astronomy, they would have been lost in the immensity of the Universe. Astronomy is a necessity for humankind, and it was vital for past human civilizations.

A contrast with our modern era

Today, the notion of time is well integrated into our civilization. We no longer ask ourselves the date: seasons, months, days, and hours are constantly displayed before our eyes. These data no longer need to be calculated; 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

Celestial cycles as first benchmarks

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

• The seasons are marked by the cyclical appearance of stars.
• The Sun delimits 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. Incidentally, the word "mens" in Greek, in its primitive sense, meant "moon".

Constellations and seasons

Celestial figures to organize the year

Our ancestors noticed that the seasons were linked to the stars. They drew constellations, seeing in them reassuring imaginary figures, often inspired by familiar animals. The configuration of the stars changes over the seasons, because the Earth's tilt exposes us to a different sky throughout its 365-day journey on its orbit. This cyclical journey is a wonderful one-year clock by 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 on the stars.
• Chaco Canyon: An ancestral Pueblo site in the United States, marking solar and lunar cycles.

The solstices: Benchmarks for the seasons

The stability of solar benchmarks

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 in its orbit on the same date, and the celestial vault always displays the same figures at the same time of year.

Fixed dates for agriculture

This "divine" precision of the celestial clocks helped past civilizations (such as the Incas, Mayans, Egyptians, and Greeks) to organize themselves and plan their tasks, mainly agricultural. By observing the sky, these civilizations explored the concept of time. The Sun, the Moon, and the 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

An experiment in solar geometry

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

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

A highly precise geometric method

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 its time, was 39,689 km, very close to the current value of 40,075 km.

N.B. :
The exact values attributed to Eratosthenes vary according to ancient sources (250,000 or 252,000 stadia), due to uncertainty over the precise length of the Greek stadion used and the actual distance between Syene and Alexandria, estimated at the time by the walking time of caravans. The order of magnitude of the result remains remarkable for a measurement made without modern instruments.

Other clocks of the sky: Babylon, China, and the Maya

The Metonic cycle and the Babylonian calendar

Far from the Mediterranean basin, other civilizations solved the same problem in their own way: making the lunar month (about 29.5 days) coincide with the solar year (about 365.25 days). Babylonian astronomers discovered, as early as the end of the 6th century BCE, that a cycle of 19 solar years corresponds almost exactly to 235 lunar months. By adding a thirteenth intercalary month to seven years out of nineteen, their lunisolar calendar remained aligned with the seasons for decades. This 19-year cycle is now known as the "Metonic cycle", after the Greek astronomer Meton of Athens who described it in the 5th century BCE, although the Babylonians used it first.

The Chinese calendar, a similar luni-solar compromise

Independently, ancient China developed a lunisolar calendar of the same type, regulated by the 24 solar terms (jiéqì), markers indicating the Sun's position on the ecliptic and setting the rhythm for agricultural work. Intercalary months were also inserted to keep the calendar in phase with the seasons. This system, still mentioned today to set the date of the Chinese New Year, illustrates how several civilizations, without contact with each other, arrived at very similar mathematical solutions when faced with the same astronomical challenge.

The Mayan Long Count, a different conception of time

The Maya, on the other hand, developed a radically different approach with the Long Count, a vigesimal (base 20) system counting the days elapsed since a mythical creation date corresponding to August 11, 3114 BCE in the proleptic Gregorian calendar. Unlike the cyclic calendars mentioned above, this system does not repeat itself: each day has a unique date, making it a privileged tool for dating monuments and historical events. It coexisted with two other cyclic calendars, the Tzolk'in (260 days, for ritual use) and the Haab' (365 days, civil calendar), combined into a 52-year "calendar round".

N.B. :
The exact correlation between the Mayan Long Count and the Gregorian calendar has been the subject of scientific debate for over a century; the date of August 11, 3114 BCE corresponds to the most commonly accepted correlation (known as "GMT").

The Antikythera mechanism: the oldest known astronomical calculator

A chance discovery in a shipwreck

In 1901, Greek sponge divers discovered, in the shipwreck of a Roman vessel sunk off the small island of Antikythera, a lump of corroded bronze that went unnoticed for a long time. It was not until 1902 that the object was recognized as a complex mechanical device. Dated to the end of the 2nd century BCE, this Greek machine, now fragmented into 82 pieces, remained for decades one of archaeology's great enigmas.

A mechanical calculator of unexpected sophistication

Surface imaging and X-ray tomography analyses conducted in the early 2000s made it possible to reconstruct the operation of this mechanism, consisting of at least 30 bronze gears. The device calculated and displayed the phases of the Moon, a lunisolar calendar, as well as the position of several planets visible to the naked eye; it also predicted solar and lunar eclipses, relying on arithmetic cycles of Babylonian origin. Technically, no device of comparable complexity would reappear for at least a millennium. For this reason, the Antikythera mechanism is now considered the oldest known analog calculator in human history: proof that some ancient civilizations were not content merely to observe the sky, but were already seeking to automate its calculation.

The division of the week

A lunar origin still in effect today

The division of the week into seven days corresponds roughly 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), Monday is considered the first day of the week. The weeks of a given year are numbered from 01 to 52 or 53, depending on the number of Thursdays. Week 01 is the one that contains the first Thursday of January.

The astronomical origin of the days of the week

The planets visible to the naked eye

A week contains seven days, whose names are related to astronomy. The astronomers of the Roman era observed six luminous objects moving across 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).

Celestial movements and modern discoveries

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 apparently, but observers can see, 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 other cultures, however, 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. Whether it be the geometric precision of Eratosthenes, the lunisolar cycles of Babylon and China, the linear conception of time of the Maya, or the mechanical ingenuity of the Antikythera mechanism, these discoveries testify to the same universal curiosity towards the sky. Today, we inherit this knowledge, which continues to guide our understanding of time and the Universe.