Measuring Time
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Oceans of Truth San Francisco University High School 11 Chapter One Watching the Sky: Measuring Time 1.1 Ancient Astronomers Throughout the World It is hard to imagine what life was like before there were clocks to tell us what time it was, electricity to light up the night, and libraries full of books with the accumulation of thousands of years of learning. Even though we spend half of our lives in nighttime, many of us almost never see stars in the sky. From today’s cities with bright lights and the inevitable haze of pollution, the night sky is generally a washed-out glow. Since we know from our calendars and smartphones what season we are in and what the time of day is, the motion of the Sun in the sky is irrelevant. Few of us are ever aware of the current phase of the Moon, and whether or not it will be up in the sky that night, brightening the darkness. News articles inform us of upcoming meteor showers and eclipses of the Sun and the Moon, and we take it for granted that we know what they are and when they will occur. We have a sense of control over our environment that is unprecedented in the history of our species. We can’t really understand the perspective of early herders and farmers, who every night faced a sky filled with mysterious lights that slowly swept out subtle and complex patterns of change. For a farmer in the Nile Valley in 5000 BC, being able to anticipate seasonal changes was a matter of survival. It must have seemed obvious that the motions of the unreachable, mysterious objects in the sky were connected to changes on Earth. If one could read the motion of celestial objects to predict events on Earth, then one had some control over his or her destiny. Indeed, everywhere in the world that ancient buildings and artifacts have survived, from even before there was writing, there are traces of an interest and need to measure and seek alignment with the heavens. The map in Figure 1.1 shows the locations of some ancient examples from every continent that humans inhabited. Figure 1.1 Some examples of astronomical artifacts and sites from ancient, pre-literate cultures around the world. Oceans of Truth San Francisco University High School 12 These example archaeoastronomy sites and artifacts fall into a few broad categories, and seem to have had a variety of uses. Since these examples are so old, there is no written record of their use or significance, so archaeologists and scientists have had to deduce their purpose from the patterns in their construction. Tally marks The oldest artifacts linked to an astronomical purpose have been determined to be over 35,000 years old. They appear to be an early form of calendar based on observations of the Moon. The Moon is the most striking object in the night sky, and certainly the one that demonstrates the most dramatic change from night to night. The “phase” of the moon, ranging from the narrowest sliver of a crescent to a bright full disc, shifts noticeably and predictably from one day to the next, in a regular cycle of approximately 29.5 days. It would not be surprising, then, if the earliest evidence of calendar systems are tools for counting days relative to the phases of the moon. As we will see in a later section, sophisticated and precise lunar calendars were important developments of later civilizations, and some are still in use today. Three striking examples of what seem to be early calendars are the “Lebombo Bone”, found in the Lebombo Mountains between Swaziland and South Africa, the “Blanchard Bone”, found in what is now France, and the the “Ishango Bone”, found on what is now the border between Uganda and Congo. The Lebombo bone, which has been dated to 35,000 BCE, has 29 straight notches, which suggests that it is a tally to keep track of the days of the lunar cycle. The Blanchard Bone, believed to be from about 30,000 BCE, has 58 small holes in it, which may correspond to a record of the days in two successive lunar cycles. The shapes of the holes themselves, some round and some more crescent shaped, are suggestive of the shape of the Moon at different points in its cycle, lending credibility to the theory that it was a calendar of some sort. The Ishango bone, believed to be from about 20,000 BCE, has a more elaborate sets of notches over all four of its faces, some in consecutive groupings that double in number. It has been suggested that it was a tool to assist with simple multiplication and division, before those concepts were fully developed. But there are also theories that it was a more complex version of a lunar calendar or a longer record of lunar cycles. The “Lemombo Bone”, 35,000 BCE1 The Blanchard Bone, 30,000 BCE2 The Ishango Bone, 20,000 BCE3 Figure 1.2 Ancient Bones showing markings that appear to be an early form of calendar 1 https://trueddotorg.files.wordpress.com/2014/04/lebembo.png 2 http://www.cabinetmagazine.org/issues/28/assets/images/rosenberg1.jpg 3 http://users.ox.ac.uk/~some3056/images/ishango.jpeg Oceans of Truth San Francisco University High School 13 Observatories Another type of artifact, typical of more recent cultures on most continents, is a structure that seems designed to observe the positions of celestial objects as they rose or set on the horizon; a kind of early astronomical observatory. In many cases these take the form of a circular arrangement of stones, earth, or logs that create sight-lines for marking the rising or setting points of the Sun, the Moon, or certain stars. The interpretation of the purpose of these sites is more complicated than that of the tally marks, and involves reconstructing what the night sky looked like thousands of years ago, when these circles were made. Nevertheless, there is compelling evidence in each case that they could have functioned for this purpose, and that they accurately marked key seasonal turning points making them useful for keeping precise calendars. In the case of one of the examples listed below, Stonehenge, there is even the possibility that the earliest parts of the structure were capable of tracking the changing alignment of the paths of the Sun and the Moon and could predict lunar and solar eclipses. 4 Figure 1.3a Nabta Playa, in what is now the Western Desert of Egypt, had highly-organized settlements that pre-dated the Nile Valley civilization by over a thousand years. One of the finds at the site is an arrangement of stones, believed to be from around 5000 BCE, that seems to be an observatory for measuring the rising points of key stars and tracking the seasonal changes in the rising point of the Sun. This image is of a reproduction of the site at the Nubia Museum in Aswan, Egypt.5 Figure 1.3b The Goseck Circle, in what is now Germany, was in use from 4900-4700 BCE. It appears to be a solar observatory, with paths that align the center of the circle with the rising and setting points of the Sun at the winter solstice.6 4 Hoyle, F. From Stonehenge to Modern Cosmology. W.H. Freeman and Company, San Francisco. 19 5 https://commons.wikimedia.org/wiki/File:Calendar_aswan.JPG#metadata 6 http://www.ancient-origins.net/sites/default/files/field/image/Goseck-Circle.jpg Oceans of Truth San Francisco University High School 14 Figure 1.3c Stonehenge, near Salisbury, England, is the most well-known of the ancient observatory circles. The mound of the outer ring, dating from 2700 BCE, features post holes and a heelstone that have no sighting function, but are capable of predicting eclipses. The inner ring of stones, from later construction in 1600 BCE, is a functional solar observatory.7 Figure 1.3d The Chankillo Complex dates from around 400 BCE, and is located in what is now Peru. It is a set of thirteen towers built on a ridge, and an observation site to the west of the ridge. From the observation site, the towers provide a scale with which to measure the rising point of the Sun on the horizon, like a giant protractor. This allowed people to chart the progress of the solar year with great precision. 8 Figure 1.3e The Bighorn Medicine Wheel, located in Wyoming, USA, has built in alignments that are believed to correspond to the rising points of important stars, as well as the rising and setting points of the Sun at the summer solstice. It’s age is uncertain, but judging from the star alignments, it was most likely built between 1000 and 1500 CE. 9 7 https://static01.nyt.com/images/2015/11/10/science/10Stonehenge-front/10Stonehenge-front-superJumbo.jpg 8 https://en.wikipedia.org/wiki/Chankillo#/media/File:ThirteenTowersOfChanquilloFromFortress.JPG 9 https://commons.wikimedia.org/wiki/File:MedicineWheel.jpg Oceans of Truth San Francisco University High School 15 Star Maps More evidence of the importance of astronomy in ancient cultures comes in the form of star maps carved in stone. Three examples are shown in Figure 1.4 below. In aboriginal Australia an important constellation, the Emu, was made up of the dark patches in the milky way, which we now know to be interstellar dust blocking the light from more distant parts of our galaxy.