February 2024

Measuring time in Prehistory

What we call a calendar is a system which, for civil or religious purposes, organises long periods of time into shorter ones (like years and months) using the day as a basic unit. We don´t know when the first calendars were made. There is no archaeological evidence that humans were able to predict seasonal changes in the Lower and Middle Palaeolithic periods, which ended about 40,000 years ago. But in the Upper Palaeolithic period we can speculate with the existence of signs which correspond to a single date, considered to be of great importance, which tends to correspond to a solstice, the day when the Sun reaches its maximum or minimum yearly height and marks the beginning of a change in its behaviour. Certain caves in Spain and France were much visited (as we can see from their decoration and remains found inside them) and chosen as cave-sanctuaries towards the end of the Upper Palaeolithic period, perhaps because of their solsticial orientation, although we cannot confirm this. For example, the inside of the Parpalló cave is lit up at sunrise on the winter solstice; the Lascaux cave is lit up at sunset on the summer solstice. However, the cave paintings show no representations of the moon, the sun or the brightest planets, and even those of stars are doubtful. What’s more, for the whole Palaeolithic period we have not identified changes to the landscape (not even small megaliths) which could have been used as markers of time or of observation of the sky. In fact, we know nothing about Palaeolithic rituals and, moreover, it is risky to try and interpret any remote clue with a modern mentality, prejudices, ideas and knowledge.

The lunar cycle was obviously important for humans: the risk of night-time attacks by predators is greater when the moon has yet to rise or is shining weakly, and smaller when the moon is full. Given that the cycle of evident changes in the moon night by night is relatively short, its evolution could be memorised. The speculative suggestion that marks found on a few bones or wooden sticks from the Upper Palaeolithic period might represent the 29 day lunar cycle does not convince many experts. They may perhaps indicate the capacity of registering symbols, but we don´t know what they refer to. We can also speculate about human capacity to predict the seasons, which might have been important for movements in search of water and food, including ancestral knowledge of the behaviour of the animals they hunted and the plants they gathered, and also for the seasonal grouping or dispersal of humans, etc. But there is no evidence that they counted moons for these purposes.

The situation changed in the Neolithic period with the development of agriculture and shepherding about ten thousand years ago, and the resulting dependence on the yearly cycle of seasons. Given that the seasons are identified more by climate than by the moon or other astronomical phenomena, the first markers of agricultural activities were probably natural, knowledge which was transmitted from one generation to another. In the Mediterranean basin, the flowering of certain plants, the migration of cranes, the arrival of storks, the cuckoo song, new sprouts on trees and other signs may have been the predictors for the sowing time of different crops. There are frequent references to this in historical texts (Hesiod, Aratus, Virgil, Columella), which may reflect ancient traditions.

On the other hand, we cannot discard the idea that the succession of lunations was used for cultic activities, in fact all the oldest known calendars in history are based on the succession of lunar months. Sometimes they were incomplete, that is, only the lunar months for the agricultural seasons were used, the rest of the solar year being left out of the calendar. (In the historical era we have the cases of the first, legendary Roman calendar or that of the Iban in Borneo. In many places the moon was so important that it was deified as one of the principal gods or goddesses: the Sumerian king Rim-Sin I of Larsa describes the moon as the god Nanna «who establishes the months and completes the year», which we can interpret to mean that the moon was the main base of calendars of that period and, presumably, the previous ones.)

The progressive settlement of populations during the Neolithic also gave rise to megalithic constructions which were sometimes oriented using astronomical criteria. These are ceremonial constructions in Egypt, Europe, the Mediterranean basin and other special places which mark, using a large stone or something similar, the orientation towards sunrise or sunset on the solstice. There were also other observation points from which, on a certain date, the sun could be seen to rise towards a mountain peak or some other specific feature on the horizon. But none of these are calendars in the true sense of the word. In fact, for most of the population there were only a few days of interest in the seasonal cycle, days which signalled the celebration of a festivity, ceremony or ritual, as well as those of importance for farming and shepherding. They did not need a calendar which detailed each and every day, like the almanac calendars we are used to. It is possible they did not even have a word for the concept of calendar, as is the case for some small communities in Oceania.

The increase in settled populations gave rise to city-states in Sumer, the invention of writing, the development of bureaucracy and legal codes. And also to the establishment of complex and complete calendars as can be deduced from tablets which specify certain dates such as «the 25th day of the month of ammar-a-a-si», from the calendar from the Sumerian city of Girsu (around 2038 BCE). In this millennium many Sumerian and Semitic lunisolar calendars had already been established, and most important cities in Mesopotamia had their own. But that is another story which would culminate in the total abandoning of the lunar cycle and the establishing of a strictly solar calendar, the Julian calendar.


Pere Planesas Bigas, now retired, has worked as a researcher in astrophysics at the Observatorio Astronómico Nacional de España (National Astronomical Observatory of Spain), the California Institute of Technology (Caltech) and the European Southern Observatory (ESO), in Chile.

Further reading:

Kelley DH, Moline EF. 2011. Exploring Ancient Skies, Ed. Springer

Planesas Bigas P. 1999. La medida del tiempo. Una aproximación a la historia de los calendarios, Ed. Andersen Consulting.

Robbins LH. 2000. Astronomy and Prehistory, in Astronomy across Cultures, Ed. Kluwer AP.