Can we hope to find, in the material record, evidence of astronomically timed movement through the landscape in the past? If we could determine the times of year when certain sites or locations were occupied, and if we could spot symbolic links between these places and the sun or stars, then it is certainly possible that this could give us some clues. Symbolism relating to a solstice, at a site that is occupied around the time of solstice, is one possibility. It has been suggested that CHACO CANYON in New Mexico was a focus for sacred pilgrimage for Anasazi (ancestral Pueblo) communities in the surrounding area around the eleventh century C.E. The archaeological evidence for this is supported by archaeoastronomical evidence that people in the outlying communities used astronomical observations to synchronize their convergence upon Chaco. Another, much older, example of a similar phenomenon occurs at THORNBOROUGH, a group of Neolithic henges in Yorkshire, England. Here the archaeoastronomical evidence consists of a series of alignments upon the rising position of the three stars of ORION’s belt, suggesting that it might have been the first predawn appearance (HELIACAL RISE) of this asterism that triggered an annual PILGRIMAGE for an autumn festival at the henges.
In modern indigenous societies, we see ample evidence of the various ways in which people strive to harmonize what they do with cycles of events in the nat-ural world. Studies of tally counts, astronomically related myth and ritual performance, and patterns of ritual movement have begun to reveal some of the ways in which this may have been done in the past. CALENDARS—self-consistent systems of marking time—developed from these more rudimentary perceptions of correlations between different cycles of activity in nature, and the desire to keep human activities—ones that we might identify as sacred, mundane, or having aspects of both—in tune with natural events. The cycles of change of the celestial bodies—regular, immutable, and reliable—are clearly of particular importance in this context.
It is commonly assumed that there is a natural progression of calendrical development relating to astronomical observation. The first step is to develop a simple month-by-month calendar based on the phase cycle of the moon, the most obvious cycle in the night sky. However, because there are between twelve and thirteen synodic (phase-cycle) months in a solar year, it follows that in order to keep in phase with the seasons, one needs to have twelve months in some years and thirteen in others. The second step, then, is to take note of phenomena that are related to the seasons, including astronomical ones, using them to keep lunar calendars in pace with the seasonal year by inserting or omitting a month from time to time as required (see LUNAR AND LUNI-SOLAR CALENDARS). The ancient Egyptians, for example, used the first predawn appearance (heliacal rise) of Sirius, the brightest star in the sky, to keep the start of the new year in step with the annual flood of the Nile. Sirius would have been first seen approximately eleven days later in the twelfth month each year. Whenever it did not appear until the last few days of the twelfth month, then an extra or intercalary month was added so that it would continue to rise in the final month of the following year.
A seasonally related phenomenon of particular importance is the changing rising or setting position of the sun along the horizon (see SOLSTICES, SOLSTITIAL DIRECTIONS). Once a society recognizes this, they can use it to regulate a solar calendar that is entirely independent of the moon. If the horizon is sufficiently distant and contains a good many distinctive points, then such a calendar can quite easily be kept accurate to within two or three days of the “true” solar year. One of the classic modern examples of a solar horizon calendar is that of the Hopi people of Arizona. As first recorded by the ethnographer Alexander Stephen at the end of the nineteenth century, solar horizon observations were used by the Hopi both to regulate crop-planting activities and to pinpoint events within an elaborate ceremonial calendar (see HOPI CALENDAR AND WORLDVIEW).
The final step in the development of luni-solar calendars is to replace the ad hoc insertion or omission of intercalary months by a systematic procedure. By about the fifth century B.C.E., the Babylonians had developed a fixed system whereby a thirteenth month was inserted into seven different years, in a fixed pattern, within every period of nineteen years (see BABYLONIAN ASTRONOMY AND AS-TROLOGY). This Metonic cycle, named after the Greek astronomer Meton, keeps the lunar calendar in step with the solar year to within an error of just one day in every 200 years.
The Babylonian calendar was an impressive achievement made possible by systematic astronomical observations recorded over many generations. However, ancient calendars do not inevitably follow the progression just described (and even in Babylonia itself, calendrical developments were more complicated). The Mursi yet again provide a good example. They have what, to an outsider, looks like a thoroughly haphazard calendar in which no one ever seems to know for certain what month it is, although everyone believes there are “experts” around. In practice, different opinions always exist, and the calendar is effectively adjusted “on the fly” according to various seasonal markers—though no one is aware that these adjustments are being made. The calendar is completely self-consistent in its own terms, and there is no need for intercalary months. The nearby Borana have a completely different and utterly distinctive luni-stellar calendar that reckons the time of the month and year by observing the moon in relation to the stars, completely ignoring the sun (see BORANA CALENDAR). The Works part of HESIOD’s Works and Days describes farmers’ rules of thumb in eighth-century B.C.E. Greece that related exclusively to seasonal astronomical phenomena such as the heliacal rising of stars; the lunar phases are only mentioned in the separate Days part. The Roman civic calendar, upon which the modern (Western) calendar is based, only emerged from chaos when it ignored the moon completely (see ROMAN ASTRONOMY AND ASTROLOGY). On the other hand, uncorrected lunar calendars remain of considerable importance to this day, one of the most obvious examples being the Islamic calendar (see ISLAMIC ASTRONOMY). Finally, the ancient Mesoamerican calendar, arguably the most sophisticated and complex of all the world’s calendars, operated by combining cycles as diverse as the 365-day year, a 260-day cycle (whose astronomical derivation, if it is astronomically derived at all, remains unclear), and the 584-day synodic cycle of the planet Venus (see MESOAMERICAN CALENDAR ROUND).
In short, there is no inevitable path in the development of calendars. Instead, they advance in diverse ways according to local conditions and needs. This means, for one thing, that they cannot be used as yardsticks of cultural achievement. It also means that they cannot be considered as abstractions, divorced from the social context in which they developed and the social needs that they fulfilled. The Hopi calendar, for example, had (as we would see it) both a pragmatic and a sacred function, but from the Hopi perspective it functioned holistically to ensure the well-being of the community in all respects. Different calendars can have different purposes and even run alongside one another, as—it appears—did the religious and administrative calendars in ancient Egypt (see ANCIENT EGYPTIAN CALENDARS). To the historian or archaeologist, understanding the technical aspects of an ancient calendar is often of limited interest. It is much more intriguing (and of-ten much more challenging) to understand how a calendar operated in its social context, what it meant to people, and what its social implications were.