Archaeoastronomy

Written records are missing for many prescientific societies as they turned from noting a single celestial event to making the kinds of repeated observations that could be applied predictively to events in their own lives, such as harvesting or knowing when to expect newborns in their herds. Many ancient peoples did, however, leave physical signs of their observing activities. Among the most intriguing are the sites that, to a modern eye, seemingly could have been used as very early observatories.

Between about 3500 and 1500 B.C., Bronze age builders in Britain and the northwestern portion of France known as Brittany erected, or marked in the existing landscape, thousands of sites that invite speculation about astronomical use. The most fundamental arrangement consisted of a natural indicator on the horizon, such as a notch in a mountain (the foresight), which was aligned with another, manmade marking, such as a standing post or stone, or a hollowed-out depression in a rock (the backsight). Because distances of up to 28 mi (45 km) have been measured between foresight and backsight, these common configurations have become known as long alignments. Statistical studies show that few long alignments could have been used to establish the date of a major celestial event, such as a solstice, with certainty. Even a rough approximation, however, would have sufficed for ceremonial reasons such as sun worship.

The Neolithic builders are better known for erecting stone circles, such as Stonehenge. (A henge is an earthen mound surrounded by a low bank and a ditch; wooden or stone pillars may be arranged on the top.) Astronomical opinion is divided over the uses of Stonehenge, but the evidence for its primary use as an observatory is considered weak. Stonehenge was built in three phases over a period of about 400 years, beginning around 1700 B.C., and is one of many circles built during this period. It stands on flat Salisbury Plain, in southern England. Because the horizon lacks distinctive features, short sight lines may have been incorporated into the placement of the stones. A ritual figure such as a priest or priestess silhouetted against the rising moon, and framed between two megaliths, would also have been an impressive sight.

Astronomers argue that Stonehenge could have been used to observe the winter solstice, the time when the rising of the Sun is farthest south, and the extreme rising and setting positions of the Moon. Its primary purpose seems to have been ceremonial. That a ceremonial structure might have later evolved into an observatory in prehistoric Britain has important implications for social organization, for it suggests that an educated, elite class existed to make the observations and supervise the construction and repair of sites. Those educated observers in turn might have been the forerunners of the Druids of the Iron Age in Britain.

Culturally dissimilar groups in the American Southwest and California appear to have followed observing practices like those used in prehistoric Britain. Oral histories taken from nineteenth century Pueblan informants in the Four Corners region indicate that both horizon observations (that is, long alignments) and wall calendars were used. The position of the Pueblan observer was not marked by a standing stone or gouge mark or wall painting; rather, it appeared to be esoteric knowledge held by the priest-astronomer, who simply walked to the same spot every time that observations were to be made. Wall calendars were created as sunlight penetrated an opening in a house or residential cave to fall on the opposite wall. With the use of both horizon and wall calendars, the Pueblans could track the motions of the Sun, Moon, and stars and also events that occurred in the four sacred quadrants of the sky. Similar horizon and wall calendars were used by many of the estimated 300 tribes living in California before contact with Hispanic traders and explorers, which occurred in the 1760s and 1770s. Some precontact California tribes are thought to have had two calendars: a secular one, known to all, and a secret calendar to guide the timing of sacred rituals.

Simple observing stations that made use of existing sight lines and horizon marks stand at one end of the spectrum of early observatories. At the other end is elite, corporate architecture, such as is found in the northern Yucatan site of Chichn Itz. There, the highly elaborated architecture, with its steeply ascending steps and ornately carved and painted reliefs, reflects a complex union of political power and astronomical knowledge.

Chichn Itz was built over two periods that lasted, in aggregate, from about A.D. 700 to 1263. Its people were both numerate and literate, creating written works that detailed their astronomical culture. The Maya also had a warrior class, waged war regularly, took captives as slaves, and practiced ritual human sacrifice and bloodletting.

The Mayan calendar and Mayan life were dominated by the Sun and Venus, whose astrophysical activities are related to each other in a 5:8 ratio considered sacred by the Maya. The Sun was associated with warfare; Venus, a fearful power, was associated with warfare, sacrifice, fertility, rain, and maize. In the 1960s and later, glyphs from Mayan writings were interpreted as showing that raids were undertaken during important Venus stations, such as its first appearance as the Morning Star or the Evening Star. These raids have come to be called star war events. The Caracol, a building probably designed as an observatory, and several other important ceremonial buildings at Chichn Itz, such as the Great Ball Court, the Upper Temple of the Jaguars, and the Temple of the Warriors, are fairly precisely aligned to face significant Sun and Venus positions.

Mayan interest in genealogy made calendrics important. Their basic calendar consisted of two cycles, one of 260 named days and one a year of 365 days, which ran concurrently. Fifty-two years of 365 days each formed the "Calendar Round." The Maya developed a system for uniquely identifying every one of the 18,980 days in the Calendar Round. A table in the Dresden Codex, one of three Mayan manuscripts to have survived the Spanish conquest, indicates ability to predict solar and lunar eclipses, as well as the behavior of certain other celestial bodies. At the time of the conquest, these predictions may have been accurate to within a day, rather than to within an hour or minute, as was then possible in Europe with the aid of advanced instrumentation. The Maya, however, did not have the concept of an hour.