The regular motions of celestial bodies can be used to keep track of the passage of time, and observations of different cycles in the sky have underlain many different types of calendars developed and used from early prehistory to the present. The moon is an obvious tool for this purpose: it is conspicuous in the sky, and its cycle of phases is easily tracked and of a convenient periodicity (between twenty-nine and thirty days). The sun is another obvious candidate: the annual passage of its rising and setting position along the horizon, and its changing arc through the sky, which gives rise to days of different lengths, is directly related to the seasonal year. The stars follow the same track through the sky day after day, the changing time of night of their appearance and disappearance are also correlated with the seasonal year. Even the planets follow regular cycles, more complex but nonetheless recognizable and readily observable.

The diversity of human calendars, and the complexity of many of them, arises partly because these natural cycles do not fit together neatly. The length of the lunar phase cycle (synodic month) is not a whole number of days, the length of the seasonal year is not a whole number of lunar phase cycles, and so on. Sometimes nature provides, by chance, a reasonably close fit—thus five synodic cycles of the planet Venus are very close to eight seasonal years—and some human cultures seem to have gone out of their way to identify such correlations. A prime example is the Maya, as is clear from the almanac known as the Dresden Codex. Others observe certain cycles in the heavens and seem to ignore the rest. It is scarcely surprising that there is a broad correlation between calendars and latitude, since the general appearance of the skies and of the celestial cycles depends upon it. For example, the annual variation in the length of the day, and in the horizon position of sunrise (or sunset), is much greater, and hence more obvious, at higher latitudes than near to the equator.

These are convenient categorizations for us and for the modern astronomer, but they may have made no sense to peoples in the past. To understand the nature of a calendar we must understand the needs it fulfilled— practical, ideological, and social. In some cases more than one distinct calendars fulfilling different needs may have run in parallel. We must also understand that a calendar may have been conceptualized within a very different framework of understanding from our own. It then comes as less of a surprise that calendars are commonly regulated using astronomical cycles observed in combination with a multitude of other natural cycles, many of which (to our way of thinking) are less regular or reliable.

One of the greatest pitfalls is to try to identify “stages” in the development of the modern Western calendar, seeing these as a progression along an inevitable path of calendrical development. This encourages a form of intellectual imperialism (or ethnocentrism) in which we attempt to measure the achievement of others in relation to our own. This stepped approach is easily refuted by a number of indigenous calendars recorded in modern times. The calendar of the Mursi of Ethiopia is particularly valuable in this respect. What we actually find, rather than a single progression, is considerable diversity combined with remarkable ingenuity and adaptability to local circumstances.

Another pitfall is to simplify calendrical developments in the distant past, postulating the existence of calendars in common use over great swaths of the prehistoric world and/or changing little over many centuries. Notorious examples of this kind of oversimplification are the “megalithic” calendar in Neolithic Britain and the “Celtic” calendar in Iron Age Europe. Another is the proposition that the modern Borana calendar represented a calendar that had been propagated, without variation, through two millennia since being encapsulated in a set of alignments at Namoratung’a in northern Kenya. Research has shown, instead, that calendars are very often adapted rapidly to changing circumstances.

On the other hand, broad calendrical principles can be preserved with remarkable consistency, as happened amongst the scattered islands of Polynesia. Yet even here, considerable local variations developed: even between individual islands and parts of islands within the Hawaiian group. There were variations, for example, in the naming of months, the timing of months within the seasonal year, and rules for inserting intercalary months. A similar degree of variation is evident between different city-states in Classical Greece and also within pre-Columbian Mesoamerica.

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