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4. The Moon, Early Calendars and Temporal Measurement

Loveday's (2012) association of early monuments with the moon is not such a surprise and the issues surrounding the use of the moon for temporal measurement are worth discussing further. The potential to use the lunar phase cycle to provide a simple, easily observable, means of tracking convenient periods of time (29 or 30 days) and closely associated with bodily rhythms such as the human menstrual cycle, was clearly appreciated by many past societies. The sun does not possess such characteristics. An understanding of the nature of such early calendrical systems, and how this information was used, can provide us with important insights into the conceptual frameworks of early societies (Nilsson 1920; Ben Dov et al. 2012, 1-8). A key characteristic of using lunar phase cycles for calendrical purposes arises because the length of the seasonal year is not a whole number of lunar phase cycles. This means that a lack of correspondence with the seasons will inevitably ensue if the moon is used as the primary guide to the passing of time. Certain historical calendars, as well as some modern calendars, including the Islamic system, retain an unmodified lunar cycle independent of the seasonal round.

Luni-solar calendars use observations of seasonal events to correct the seasonal drift by indicating when, every two or three years, to insert or omit an intercalary month in order to guarantee continuing goodness of fit. These observations do not have to be astronomical but typically involve the heliacal rising or setting of particular stars (Nilsson 1920, chapter 4) or observations of the sun rising or setting at a given point on the horizon as seen from a fixed observing position. Other systems of measurement do exist and the Hopi, in contrast, have historically used detailed observations of the position of the sun on the horizon to provide a highly effective calendrical system that guided agricultural and ritual activities (Walton 2012, fig. 5). Such 'solar horizon calendars' are truly tied to the seasons and do not reference the moon, but are frequently associated with sedentary societies. Likewise, it is only in contexts where it was possible to consult records of meticulous observations spanning generations that one can imagine direct observations being replaced by an efficient arithmetic algorithm for adding intercalary months in a luni-solar calendar. Thus it was that centuries of lunar observation in Babylonia led to the recognition of the Metonic cycle by the 5th century BC. This provided an effective method for 'time reckoning' and time keeping based upon the moon and a known intercalation cycle that had an error of slightly more than a day in a century (Ruggles 2005, 230; Nilsson 1920, 355).

The relative ubiquity of lunar calendars at an early period of human history has led to the presumption of the historical primacy of lunar systems within the development of calendrical systems, although Ruggles (2005, xxi) has cautioned against such simplistic views. It has also been recognised for some considerable time that an appreciation of the passing of time should not necessarily be equated with a calendrical system. However, Nilsson's (1920, chapter 1) refinement of the term 'time reckoning' to describe early temporal systems or observations, remains useful when considering the variation in ethnographically or archaeologically attested time systems. These may vary from 'discontinuous systems', in which natural events may be recognised (sleeps, solstices or seasons) without any capacity to form a comprehensive or uninterrupted framework of time, through to societies exhibiting 'continuous time reckoning' in which there is no obvious gap in a sequence in which days form months and months form years in a system of near equivalence.

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