with support from the Wisconsin Space Grant Consortium Watching the sky comes naturally to people who live outdoors without electricity. More systematic time keeping demands more careful observation and is usually the responsibility of professionals or priests. Typically, the two groups that the Bible says noticed and followed the Christmas star were shepherds and the magi, who were probably Babylonian priests and professional astronomers.
Almost all known societies seem to have observed the stars and taken an interest in the motions of the heavens. There are many reasons for this passion. First and most practically, telling the time of year is critical for planting. It is notable that the development of priests responsible for the calendar among the prehistoric Anasazi of the southwestern U.S. seems to have coincided with increasingly dry conditions that made planting crops on time critical to the survival of the trube. (Williamson) Second, many peoples consider themselves a part of the greater cosmos and observe the natural order in the grand sweep of the sky. Finally, many cultures believe that the stars, planets, sun and moon influence the destinies of people and nations so that it is important to follow their motions to predict the future. Because it is most vital to the survival of a society, the development of a calendar will be considered first.
Days, Months, and Years The day is measured using a fixed position of the sun. We measure our days from the time that the sun is directly at the nadir (beneath us). Other cultures measure days from sunrise, sunset or when the sun is at the zenith (directly overhead). Sunrise and sunset have variable times during the year so sophisticated cultures generally measure the day relative to the zenith. A day can also be measured relative to the fixed stars. (This is called the sidereal day.) It can also be measured relative to the horizon or a fixed position relative to the earth like the zenith (the solar day). Because the sun moves relative to the stars, the sidereal day and the solar day are not equivalent. The solar day is about 4 minutes longer than the sidereal day because the earth must turn a bit more east to catch up with the sun whose position changes as the earth moves along its orbit. The solar day also varies in length several minutes over the year because the earth’s speed changes as it moves closer and farther from the Sun along its elliptical orbit.
Years and months are measured in terms of days. The year is the time it takes the Earth to complete an orbit around the sun. From the point of view of an observer on the ground, this is the time it takes for the sun to move around the ecliptic, its path through the stars, and return to its original position. It is not simple to make observations that define the year because the stars are not visible when the Sun is in the sky. Also the year is not an even number of days long. Observers must make careful notes of the positions of the stars just before sunrise and just after sunset. Accurate determination requires years of observations in conditions where the observers can make records of what they saw.
The earth’s axis of rotation is tipped at an angle of 23½ relative to the perpendicular to the earth’s orbit around the sun. The earth’s orbit is very nearly circular, but because its axis is tipped, different parts of the earth receive more direct light from the sun at different times of the year. In the northern hemisphere, the north pole is tipped directly towards the sun on the summer solstice, about June 21, and directly away from the sun on the winter solstice, about December 21. From the point of view of an observer on earth, days get longer as the summer solstice approaches and shorter as the winter solstice approaches. As the days grow longer, sunrise and sunset move north along the horizon and the weather grows warmer. Near the winter solstice, the sun moves further and further south and winter deepens. The problem in determining the dates of solstices is that the sun’s motion along the horizon slows down near the date of the solstices so careful observations over several years are needed to narrow down a date. In societies without writing, keeping records overtime could be a problem so sun watchers coded their observations with massive buildings and monuments.
Commonly, observers watched sunrise and sunset from vantage points that allowed views of horizons that had features such as mountains, notches or other markings. Near their capital of Cuzco, the Incas constructed a massive system of pillars along the horizon for use in tracking the course of the sun. The Inca called themselves the Children of the Sun and celebrated a cycle of festivals timed to mark solar events. The celebration, Inti Raymi, marked the summer solstice, and delegates from around the empire brought gifts to the sun and his children, the ruling Inca. Solar observations were conducted from the central plaza at Cuzco as well as from other sites around the empire. The island, Titicaca, was the site of a major solar observatory that may predate the Inca. (Dearborn)
Twice each year, the tip of the earth’s axis lies tangent to its orbit around the sun. The sun seems to rise and set directly over the equator, and day and night are equal in length every where in the world. These days, which fall close to March 21 and September 21, are called equinoxes. The Inca marked the autumnal equinox with the festival Citua during which the city of Cuzco was purified. (Dearborn) The year can be defined as the time between the same solstice or equinox. However, it is also possible to consider the sun as positioned in a particular constellation. Observers could carefully watch the stars immediately before sunrise or after sunset. Many cultures measured the year as the time between the sun’s arrival at a particular position relative to the stars.
Years are based on the sun’s path through the fixed stars. Months are based on the path of the moon through the fixed stars. Unlike the sun, the moon is not a source of light. We see the sun’s light reflected from the moon. The moon’s orbit is tipped 5o relative to the earth’s equator. Thus the sun, the earth and the moon rarely form a straight line.
As the moon goes around the earth, we see different portions of the half of the moon illuminated by the sun. The moon appears to change shape, a phenomenon called phases of the moon. At full moon, the moon rises as the sun sets, and we see a full disk. At new moon, the moon rises and sets with the sun, and we do not see it. The synodic month, 29½ days, is the time from new moon to new moon. It is clearly based on the position of the moon relative to the sun. However, the sun also moves along its orbit so we could also measure the time it takes the moon to complete one orbit around the earth measured against the background of the stars. This sidereal month is about 27¼ days long, clearly much shorter than the synodic month.
27 days, 7 hours, 43 minutes
29 days, 12 hours, 44 minutes
Calendars An obvious problem in building a calendar is that the year is not a whole number of days. Furthermore, the year is not evenly divisible by months. Calendars were first developed to help determine times for planting and other agricultural activities. If the calendar was based on a year with an integral number of days, the time for planting moved from year to year. Over a hundred years, planting could occur at a completely different date thus eliminating the calendar’s utility for agriculture. Different societies solved this problem in various ways.
The development of calendars occurred many times over. Here it will be illustrated with examples from Native American cultures. There are two major problems in using examples from Native American societies. First, the pre-Spanish cultures were extremely diverse so there are almost no general examples. Second, European settlers and the diseases they brought decimated the native populations so that the priests who understood and practiced astronomy died before their ideas could be recorded.
The ancestors of the modern Pueblo Indians of the American Southwest are known as the Anasazi. They built huge cites throughout southern Colorado, Arizona and New Mexico which were abandoned before 1300 A.D., probably because of a long term drought. They had no written language, but the ruins of their cities strongly suggest that they carefully observed the motions of the Sun, the Moon and the stars.(The Anasazi World)
The new discipline of archaeoastronomy involves the collaboration of astronomers and archaeologists in the study of ancient buildings and their relationship to ancient astronomy. For example, the Anasazi ruins at Chaco Canyon contain several buildings that seem to be aligned with important solar events. For example, the great kiva, underground ceremonial chamber, at Casa Rinconada is a circle perfectly aligned north and south. Near the date of the summer solstice, an aperture in the roof allows the light of the rising sun to illuminate a niche in the wall. This must have been connected with some sort of sun ceremony. Pictographs on the canyon walls seem to mark good positions for observing the sun’s motion along the horizon. Finally, a site has been discovered in which a dagger of sunlight illuminates pictographs on the back wall of a room at the time of the solstice. This is perhaps the most spectacular example of a number of markers that use configurations of light and shadow to mark the solstices in Chaco. (Williamson)
The best understood American calendar is that of the Maya who lived in Central America. A codex is a Mayan written record. The Spaniards burned most of these, but enough have survived to allow scholars to decipher the symbols in which they are written. In addition, Mayan cities were filled with carvings, particularly on stelae, stone pillars carved to commemorate important events. The Aztecs of Mexico used a calendar that was similar to that of the Maya. The calendar was based on two individual cycles. The “vague” year consisted of 365 days divided into 18 months of 20 days each with the 5 extra days added to keep the year synchronized with the sun. This calendar was used for agriculture and civil affairs like tribute payments. There is evidence that the priest astronomers periodically adjusted the vague calendar to keep it synchronized with the sun.
The ceremonial calendar used a 260 day year divided into 13 months of 20 days each. The calendar held astrological rather than physical significance and was used to schedule religious and civil ceremonies. Every 52 (of our) years, the two calendars coincided, and that unit was a major date marker. The final important cycle in the Mesoamerican calendar was the rotation of the planet Venus whose Synodic period is 584 days. Five years of Venus revolution correspond to 8 solar years, another important coincidence to the Maya. Thus the calendar used in Central American before the Europeans arrived was far more complex and precise than its counterparts in used in Europe. Aztec and Mayan cities contain many astronomical alignments similar to those at Chaco Canyon but far more complex and sophisticated. For example, the observatory at the Mayan city of Chichen Itza, the Caracol, has a platform whose diagonals point towards the winter solstice sunset and the summer solstice sunrise as well as windows and building alignments with important points in the orbit of Venus. (Astronomy of the Mayans, Broda, Jacobs )
Ethnologists have studied the astronomy of North American Indians with difficulty since Native Americans were understandably distrustful of European Americans. The Skidi band of the Pawnee who lived in what today is Nebraska developed an elaborate cosmology based on four stars as well the sun, and they ordered their lives according to the motion of the sun, the stars and possibly the planets. They had sets of interlocking rituals that included human sacrifice as late as 1838. We know a little of their ritual because James R. Murie whose mother was Skidi managed to learn about many of the rituals and communicate them to other scholars.
The Navajo primarily base their calendar on the rising of particular constellations. For example, the reappearance of the Pleiades in the northeast after its absence of several months signaled that it was too late to plant. Their life was ordered around the four cardinal directions. It is probable that they also tracked the sun and used the appearance of shadows and light in predictable positions to determine the time of the year much as the Anasazi did. However, the Navajo never built buildings that served this purpose. (Chamberlain, Williams)
Our own calendar was developed under the auspices of Pope Gregory VIII in 1582. The Gregorian calendar consists of 12 months of varying lengths, more or less based on the synodic lunar period of 29½ days. To accommodate the fact that the solar year is slightly longer than 365 days, an extra day is inserted in every year divisible by 4 as the 29th of February. To keep the calendar better matched to the sun’s motion, leap year is skipped in every year divisible by 100 unless the century is divisible by 4. This somewhat complex system will keep the months in line with the seasons for thousands of years. England did not adopt the Gregorian calendar until 1752 when George Washington was a young man in the American colonies. By that time, the calendar in use was eleven days behind the sun. Citizens were very upset over the loss of their eleven days. Imagine the complexities of working out rents and interest payments, not to mention suddenly being 11 days closer to all existing deadlines.
Other Native American Astronomical Observations Almost all of the Native American cultures recognized patterns of stars and constructed elaborate myths to explain them. The Pleiades were particularly prominent in many mythologies as was Orion and the Big Dipper. The moon also played a prominent role in these cosmologies, usually in association with a goddess or a mother of the race possibly because of the relationship of the month to the menstrual cycle.
It is also evident that many cultures marked the movement of the planets. The well-documented Skidi Pawnee cosmology, for example, describes how the Red Star traveled across the sky to Evening Star so that their union could produce a daughter who would be the mother of the people of earth. The journey of Red Star across the sky could well describe the motion of the planet Mars. (Chamberlain) The Maya knew the periods of Venus, Mars and Mercury although there is no record that they studied Jupiter and Saturn. They also determined the average length of the month with nearly modern accuracy. They understood how to calculate the dates when the moon’s orbit crossed the ecliptic and were able to predict eclipses of the sun and the moon. Over time, they also developed and used a base 20 mathematics for complex computations. (Astronomy of the Mayans)
Finally, archaeoastronomers have searched for signs that the priest astronomers of the Americas observed such events as the supernova of 1054 or meteor showers. There has as yet been no mention of the super nova discovered in the Maya codices. Because so many of them were destroyed and not all of the ones we know have been translated, this does not mean the Maya did not observe the supernova. Surprisingly, the Anasazi may have left records in their rock art. There are two cases found in Arizona that show the crescent moon being over taken by a star. Paintings under a ledge at Chaco Canyon also seem to be a record of the appearance of the supernova. However, rock art is very difficult to date, and the best one can say is that towns near the rock art sites were occupied at the time of the supernova. (Williamson) It is well established that many Native American societies were well aware of meteor showers and the fact that meteorites fell from the sky. (Kronk)
At the time of the European arrival in the Americas, native societies had developed sophisticated calendars and had substantial knowledge of the motion of the planets and the moon. The European arrival decimated the civilizations of native people stopped their development of astronomy leaving a time capsule of the way in which astronomy developed in cultures around the world. Recent finds include a site 4200 years old in the foothills of the Andes that was probably an observatory of the same age as Stonehenge. A giant circle of 127 stones probably 2000 years old has been found in the Amazon jungle of Brazil. The stones which weigh tons each are evenly spaced, and the site’s discoverers think it may have served to pinpoint the date of the winter solstice.(Holden) Clearly, the native peoples of the Americas had more knowledge of astronomy than has been traditionally thought.
“Astronomy of the Mayans” downloaded from http://physics.unr.edu/grad/welser/astro/mayan.html on August 2, 2006.
Broda, Johanna, “Mesoamerican Astronomy and the Ritual Calendar,” in Selin, Helaine, editor, Astronomy Across Cultures: The History of Non-Western Astronomy, Dordrecht: Kluwer Academic Publishers (2000), 225 – 267.
Chamberlin, Von Del, “Native American Astronomy: Traditions, Symbols, Ceremonies, Calendars, and Ruins” in Selin, Helaine, editor, Astronomy Across Cultures: The History of Non-Western Astronomy, Dordrecht: Kluwer Academic Publishers (2000), 269 – 301.
Dearborn, David S. P., “The Inca: Rulers of the Andes, Children of the Sun,” in Selin, Helaine, editor, Astronomy Across Cultures: The History of Non-Western Astronomy, Dordrecht: Kluwer Academic Publishers (2000), 197 – 224.
Holden, Constance, editor, “Early American Astronomy” in Random Samples section, Science 312 (2006) 1115.
Jacobs, James Q., “Mesoamerican Archaeoastronomy: A Review of Contemporary Understandings of Prehispanic Astronomic Knowledge” download from http://www.jqjacobs.net/mesoamerica/meso_astro.html on August 2, 2006.
Kronk, Gary W., “Meteors and the Native Americans” downloaded from http://comets.amsmeteors.org/meteors/metlegends.html on August 2, 2006.
“The Anasazi World” downloaded from http://www.hao.ucar.edu/Public/education/archeoslides/slide_1.html on August 2, 2006
Williamson, Ray A., Living the Sky: The Cosmos of the American Indian, (1984) Norman, OK: The University of Oklahoma Press.