Welcome to Knowing the Universe: The History and Philosophy of Astronomy. My name's Chris Impey. I'm a distinguished professor of astronomy at the University of Arizona. And this is a course about the history of astronomy running over 5000 years and the philosophy of science that leads us to understand the universe. In this first module will be talking about ancient skies, what people before the dawn of time and before even recorded civilization knew about the sky and what they learned about it. Ancient astronomy. It's a subject that goes back very far. And we start by asking why ancient civilizations might have use for astronomy. There are a number of reasons they would keep track of time. They would keep track of the seasons and their calendar. They would be interested in the cycles of the moon and in the strange movements of the planets and the regular motions of the stars through the night sky. They would also have noticed the episodic phenomena of eclipses that were quite spectacular on occasion and much more so for these cultures, astronomy was very essential because the sky was a map. It was a clock, it was a calendar and of course in the days before the internet and tv it was entertainment. It was a book of stories. It was mythology, it was religion, it was all of these things. So we go back to the beginning of time to the first human artifacts dating back 35 or 40,000 years before Christ and we find objects that indicate that humans were keeping calendars. Now of course they didn't have a written language. They left us nothing in writing. So we're having to infer the use of a calendar by the objects that they left us. For example, we found examples of gibbon bones that were used as portable calendar sticks. And there are cave paintings including the famous paintings at Lescaux where the markings are in groups of 29 same with the markings on the gibbon bone. Now obviously on a gibbon bone that could encounter a number of things you'd killed or keep track of other things but 29 is a special number because it corresponds to a lunar cycle. So there's very good indirect evidence that these ancient cultures were keeping lunar calendars. And so we also know that they looked at the sky and they wondered what was in the sky? They must have told stories about the sky and maybe if we told the stories more recently than they did, we'd have other names for these constellations. The ones we use now are inherited from thousands of years ago. You also might wonder if you ever got lost in a boat on a big ocean. Would you be able to find your way using the stars? Most ancient cultures could do that. That was a primal skill back then to be able to navigate by the stars, a skill that most of us have lost. And what are the observations you could see in the course of a year. What are the patterns that you would notice? And in particular the motions. The patterns were those of the stars and the constellations. The motions were the circling of the sky once in 24 hours, which we know is caused by the rotation of the earth on its axis. There is also the seasonal cycle which very important to ancient cultures because the sources of their food depended on the time of the seasons, whether they're eating plants or animals. So they would look at that, but they wouldn't understand back in those days that that's caused by the Earth's orbit around the sun. There's an even subtle cycle that lasts many millennia that was noticed by ancient people, which is due to the wobbling of the Earth's axis as it orbits the sun in a very long time scale. There was the motion of the moon, our constant companion and a very bright object in the night sky, and this is the moon's orbit around the earth. And then finally, there were the mysterious planets. The word planet comes from the Greek word for wanderer, because the planets appear to move amongst the fixed stars. Strange patterns occasionally moving forward and then reversing some of the planet's for weeks at a time before recovering their forward motion. These were all the motions that they saw, and of course we know that these correspond to our divisions of time. There's the day which keeps time by the rotation of the Earth. There are the seasonal cycles which keeps time by the orbit of the earth around the sun, the moon is the lunar orbit. And finally, there is a week and you'll notice that the week is the only normal division of human time that isn't directly associated with astronomical orbits. But as we'll see, it does indeed have much to do with the planets. And it is an astronomy based division of time. Essentially all our divisions of time come from astronomy and they've been handed down over literally centuries and millennia. What ancient people didn't know and we can see here in a visualization is that the earth was spinning on its axis. The moon was orbiting the earth about once every 29 days and the earth was orbiting the sun every 365 days. Ancient people didn't know the three dimensional situation of these objects in space. They just knew the cycles that they saw in the sky. We now can explain these observations. We look at the beautiful paintings in Lascaux and realized that astronomy must have been important for these people. And it was connected somehow to their daily lives and the rituals of their myths and their cultures and they commemorated this in these gorgeous cave paintings. They're now unavailable for view because they're so fragile that the French government has limited access to the caves, unfortunately. I was lucky enough to be able to go into these caves decades ago before they were closed to the public. And we can go around the world and find examples of astronomical cycles that people kept. It doesn't matter about the culture and none of these cultures talk to each other and yet they all had observations of the sky that were important to their civilization. In Central Africa, where the seasonal cycle is not so obvious because it's a tropical zone. They actually predicted the rainy and dry seasons from the orientation of the crescent moon near the western horizon. And so we have evidence going back over 6000 years that people in Africa used this means to keep track of time. In a more classic vein we have obelisks. Here, you can see in Central London an obelisk that originally belonged to the Egyptians now transported into the center of London which represents a very rudimentary sun dial. A vertical stake in the ground is called Gnomon and it of course cast a shadow according to the sun as it moves through the sky. When the length and angle of that shadow is calibrated by observations over a year, it turns into a sundial and it's a good way of keeping track of time quite accurately. Sundials have been in use in Egypt and Babylonia since 1500 BC. So for 3500 years and like I said, well calibrated, they're quite accurate time keeping pieces. So this is a very direct use of astronomy the sun's motion in the sky to tell time. Stonehenge is perhaps very familiar to everyone from my homeland in the UK. It was completed around 1550 BC. But it was a construction project that lasted two millennia. While obviously Stonehenge has cultural and religious significance to the tribes, the Neolithic tribes that constructed it. It also has some very important astronomical markers. Narrator: 5000 years ago Stonehenge started life as an urban works enclosure and it wasn't until around 2600 BC that the first stones arrived. These were bluestones dragged all the way from west Wales. The bluestone circle was taken down 200 years later in 2400 BC. It was at this point that the greatest building project in ancient Britain got underway. The first of the huge sarsen stone trilithon were erected. The bluestones were then slaughtered into the center of that ring, hinting at their symbolic importance. Then another outer circle of bluestones was added before the massive circle of sarsen stone trilithon enclosed the whole thing. [MUSIC] This was the heyday of the monument and it lasted for about 500 years until around 1900 BC. Over the next 4000 years, Stonehenge fell into a long, slow decline neglect, theft and time producing the iconic structure we're left with today. Stonehenge was a marker of my childhood as well because when I was young it hadn't yet been hemmed in by barbed wire fences with an underground car park. And it was an evocative site as you drove across the Wilshire planes and saw it looming in the horizon. The construction involved was incredible. Some of these large bluestones were dragged literally hundreds of miles with no nearby waterways. The reason why Stonehenge was built exactly where it was is still somewhat mysterious. What's not mysterious is that in addition to its ceremonial and religious value as the center for this ancient civilization, it also was a wonderful calendar for keeping time. The heel stone seen here in this diagram was moved over time and eventually lost. But we see the hole where it was originally marking the sunrise on the longest day of the year, the summer solstice June 21st making a perfect alignment across the large stones behind it. And indeed other astronomical markings exist in the Stonehenge monument. There are even examples of it keeping track of moonrise and moon set times and it was an eclipse predictor as well on eclipse cycles of 19 years. This is a very sophisticated timekeeping piece of course it's not a modern clock or a modern observatory in any sense, but it was a very important piece of this civilization. Indicating viscerally how important astronomy was for this culture. Elsewhere around Europe, we find other indications that astronomy is important. A beautiful object is the never sky disk seen here diagrammatically on the left and the physical object on the right, the size of a shield. And this was a high status object that was found buried with someone who must have been at the level of a king or close to a king in what was eventually to be Germany. It dates back to 1600 BC. And it's a beautiful evocation of the night sky including the pleads the layout of the stars, the moon, the sun and the planets and it was there for astronomy. But of course it was also there to mark extremely high status an indication of how valued an astronomical artifact might be to an ancient culture. In China, we have even longer records going back to almost 2000 years before Christ. The earliest records of supernova explosions or transient stars in the sky come from the Chinese. Because the Chinese had a long stable civilization for two millennia, they have unbroken records of astronomical observations. It's an unparalleled data set for historical astronomy and that's the end of this topic of talking about ancient skies and ancient people before the recorded history.
