2. Ancient Astronomy and Celestial Phenomena

Transcript: In the year 584 B.C., on the coast of Asia Minor, two warlike tribes were engaged in a fierce battle: the Medes and the Lydains. As written by the Greek poets, these two cultures were hacking away at each other on the battlefield with burnished swords and shields, when suddenly the sky darkened. The temperature dropped five or ten degrees. Animals started acting strangely, and the warriors, seeing no explanation for the darkening of the Sun, wandered, dazed and confused, from the battlefield. They were ignorant as to the cause of what had surrounded them, but, unbeknownst to them, a man called Thales had used Egyptian eclipse records to predict this eclipse of the sun. Ancient cultures like the Babylonians and the Egyptians made careful observations of the sky. They observed patterns. They had accurate calendars. But they could never answer the fundamental questions that we would ask as scientists: how far away are the objects? What’s their fundamental nature? What are the distances and sizes of the things that you see in the night and daytime sky? They could not answer these questions. The answers first started to come from a Greek group of philosopher-scientists in the 6th century B.C. who lived in the place that is now Greece and Turkey. These philosopher-scientists were able to speculate about the true nature of astronomical objects and the physical nature of the universe for the very first time.

Transcript: Thales was a philosopher who lived in the 6th century B.C. in Miletus, in what is now Turkey. No written work by Thales survives, but we know that he kept accurate eclipse records and he speculated about astronomy. He decided that the source of all things was one thing, and that thing was water. It may seem naive to try and explain the natural world in terms of the substance water, but it is a sophisticated notion to decide that the diversity of the natural world does indeed stem from one thing, and all things have this source. Thales was a philosopher and a statesman. He is also subject to the first stereotype of scientists that we may see as familiar to us even now. It was written that a servant girl mocked Tales, because he had been walking along at night staring up at the stars and wondering about their nature and he fell into a well, because his head was so up in the stars that he couldn’t pay attention to what was below his feet.

Transcript: The apparent motions of the stars in the night sky depend on your position on the Earth’s surface. At a northern temperate latitude, the stars rise in the east and set in the west, and they travel on slanting paths across the sky. The north celestial pole sits in the northern sky and the elevation of the pole, or the bright star Polaris, is the same as your latitude on the Earth’s surface. Some stars are visible throughout the night as they orbit the north celestial pole; they are called circumpolar stars. If you were positioned at the Earth’s equator, stars would appear to rise directly out of the east and set directly into the west. The north celestial pole would be down on the horizon. That represents the region around which the stars are rotating in the night sky. If you moved to the pole of the Earth, the north pole of the Earth, the north celestial pole would be directly overhead; imagine yourself standing on the top of a spinning top staring upwards. All of the stars would be circumpolar; those near the horizon would be orbiting parallel to the horizon, and all stars would appear to circuit around the north celestial pole, or the star Polaris, which would be directly above your head.

Transcript: At the summer solstice in the northern hemisphere, the northern pole of the Earth is tilted as much towards the Sun as it can. The Sun is overhead at noon at the Tropic of Cancer, the Sun never sets north of the Arctic Circle, and the Sun never rises south of the Antarctic Circle. At winter solstice, December 22, the northern pole of the Earth is tilted as far away from the Sun as it can be. The Sun is overhead at noon at the Tropic of Capricorn, the Sun never sets south of the Antarctic Circe, and the Sun never rises north of the Arctic Circle. The midpoints between the winter and summer solstices are called the equinoxes, spring equinox March 21, fall equinox September 21. At these times, the Sun is overhead at noon the equator and the Sun is just visible at the north and south poles of the Earth’s surface.

Transcript: Solar eclipses are among the most spectacular phenomena that can occur in the sky. During a solar eclipse, the Earth darkens substantially during broad daylight; the temperature can drop 5 or 10 degrees. A solar eclipse occurs when the Moon passes between the Earth and the Sun, and the Moon casts its shadow on the Earth. Because the Moon shadow is much smaller than the Earth’s shadow, solar eclipses are much rarer than lunar eclipses. During a solar eclipse, the point or full shadow of the Moon, the umbra, casts darkness on the Earth’s surface. A larger annular region around the umbra called the penumbra has partial shadow. Because the Earth is rotating during a solar eclipse, the shadow tracks across the Earth’s surface at speeds approaching 1000 miles per hour. Therefore, as seen from any point on the Earth’s surface, solar eclipses last only a few minutes, at most, 6 or 7 minutes.

Transcript: Many cultures have used a solar calendar, and, in fact, Sun worship was a basic part of ancient civilizations. Solar calendar divides the year into seasons using 4 fixed points. The longest day in the year, the summer solstice, June 21, the shortest day in the year, December 21, the winter solstice, and the two midpoints, March 21 and September 21, the spring and autumn equinoxes, equinox from the Latin word “equal night,” equal times of day and night. These are the four markers of a solar calendar, but ancient cultures also used to celebrate the 4 midpoints between the solstices and the equinoxes, and celebrate festivals on those days as well. We can even see residues of this in some European cultures. For instance, in Ireland, they still celebrate Imbolc on February 1, Beltane on May 1, Lughnasadh on August 1, and Samhain on November 1. Two of these festivals, the 8 points of the cardinal points of the solar calendar, are festivals that are celebrated widely. May Day in many parts of Europe, November 1, of course, marks All Hallows Eve, or Halloween, or the Day of the Dead, a widely celebrated holiday. The solar calendar is still with us in our religious and festival days.

Transcript: A sidereal day is a period of the Earth’s rotation with respect to the celestial sphere, the time it takes for a star to appear at the same angle in the sky from one day to the next. A solar day is a period of the Earth’s rotation with respect the Sun, the time it takes for the Sun to appear at the same angle in the sky from one day to the next. The solar day is 4 minutes longer than the sidereal day. You can see this if you realize that the Earth is spinning as it orbits the Sun, so it takes an extra little bit of time for the Earth to rotate to the point where the Sun is overhead in the sky compared to the celestial sphere. This extra motion corresponds to the distance the Earth travels in its orbit of the Sun in one day, about one degree. One degree on the rotating Earth is 1/360 of the motion. 24 hours divided by 360 is 4 minutes, so the solar day is 4 minutes longer than the sidereal day.

Transcript: The fundamental issue of calendars comes down to two astronomical numbers. One is the solar year, the time it takes the Earth to go a complete orbit of the Sun, or the time it takes the Sun to reappear at the meridian, at the highest point in the sky from one year to the next; a full cycle of the seasons. This is 365 ¼ days, roughly. A lunar month, or the time it takes between two consecutive similar phases of the Moon, is 29 ½ days. The problem of calendars comes from that fact that neither of these numbers is a whole number, and one does not divide into the other evenly. Thus, cultures in the world throughout the history humans have followed two different methods of calendars. Some people have followed the solar calendar, usually those who practice agriculture in northern latitudes. They follow the cycles of the Sun, and try to make a calendar that approximates as well as possible the solar year. Other cultures, primarily Arab cultures, have followed a lunar calendar. They ignore entirely the cycles of the Sun, and regulate their affairs according to the cycles of the Moon. Because 12 lunar months is only 354 days, or 11 days short of a solar year. If you keep a lunar calendar, your seasons and festivals will migrate through a solar calendar; one complete cycle every 33 years. This cultural difference between those who follow a lunar calendar and a solar calendar is also tied to religion. Most of the Islamic countries of the world still follow a lunar calendar. You see who they are by looking on their flag. You will see the crescent Moon, because that is how they keep track of their festivals, by direct observation of the lunar cycle.

Transcript: Socrates was an enormously influential Greek philosopher even though he wrote nothing down. Socrates was not a scientist. In fact, he speculated that the most important thing to do was to understand your own thoughts and motivations. As he said, “The unexamined life is not worth living,” and he gave the Delphic injunction to “Know thyself.” Although he was not a scientist, Socrates’ questioning nature is at the heart of modern science because he believed that it was not worth taking people’s opinion just because they were senior figures in the community or your elders. His idea of questioning everything that you hear is the basis of logic in the scientific method, and it got him into sufficient trouble that he was killed for it.

Transcript: The width of your fist at arm’s length is about ten degrees. The width of your thumbnail at arms length is about one degree. The angular diameter of the Moon or the Sun is half a degree. For angles of a degree or smaller astronomers can use a very useful equation called the small angle equation. This equation relates the angular diameter of an object to its distance and true diameter. If any two of these quantities are known the third can be deduced. Fro example, the Sun and the Moon subtend the same angle in the sky, half a degree. But the Sun is vastly farther away than the Moon, and so its size is substantially larger too. The small angle equation is heavily used in astronomy for measuring linear sizes once the distance and the angular size are known.

Transcript: The seasons are caused by the tilt of the Earth’s spin axis as it orbits the Sun. If there were no tilt of the axis there would be no seasons because the illumination at every point on the Earth’s surface would not vary throughout the year. In the winter in the Northern Hemisphere, the Northern pole of the Earth tips away from the Sun. So Northern parts of the Earth receive less direct sunlight, and the day is shorter than twelve hours. In the Northern Hemisphere summer, the Northern pole of the Earth tips towards the Sun. The day is longer than twelve hours, and Northern parts of the Earth’s surface receive more direct sunlight and therefore more heating. The cause of the seasons is entirely this tilt. Many people think that the seasons are caused by the changing Earth-Sun distance. Not true. This effect is very small, only a few percent, and in any case it could not possibly explain the fact that the Southern Hemisphere experiences the opposite seasons to our seasons in the Northern Hemisphere.

Transcript: The word “planet” comes from the Greek root for the word “wanderer.” The planets move through the fixed stars from night to night. This motion was known to ancient people for the five planets that can be seen with the naked eye. Mercury and Venus are never seen very far away from the Sun. Their orbits of the sun are interior to the Earth’s, so they always appear within about twenty-five and about forty-five degrees from the Sun respectively. The planets in orbits exterior to the Earth’s display what is called retrograde motion occasionally. That is, for a period of weeks or months at a time, their systematic motion around the stars will reverse and then change again. This backward, or retrograde, motion comes because the Earth is on an interior orbit and moving faster in its orbit. And so the Earth appears to overtake on the inside a planet like Mars causing Mars to apparently move backwards for a period of time. Retrograde motion is in principle observable on all the planets with exterior orbits to the Earth’s, but retrograde motion is most apparent for the planet nearest to the Earth, Mars.

Transcript: Pythagoras was one of the most influential thinkers in history. This Greek philosopher and mathematician came up with the idea that numbers were the basis of everything. There is no written record, and nothing about Pythagoras survives in writing. He essentially ran a secret society of mathematicians, and later in his career his entire group was hounded by the authorities of Greece and had to leave the Greek mainland. In cosmology he believed that numbers were the basis of everything that happened in the celestial sphere. On Earth he derived things as important as the Pythagorean theorem. The statement, “A-squared plus B-squared equals C-squared,” is a fundamental statement not just of geometry but of algebra, and it’s also a statement of the geometry of space. The Pythagorean Theorem is only true in the space described by Euclidian mathematics. Pythagoras was impressed enough with his discovery of his theorem that he sacrificed a hundred oxen to the gods. Pythagoras discovered the rules of musical harmony by dividing a string and listening to the notes that emerged, and he actually believed in the harmony of the spheres, that sounds could be heard by enlightened people from the heavens due to its fundamental basis in number and harmony.

Transcript: A spinning top or gyroscope that has not pointed straight up will wobble. That is, its axis of rotation traces out a circle. This is called precession. The spinning Earth does exactly the same thing. It’s tilted by twenty-three and a half degrees on its axis, and the North Celestial pole traces out a large circle on the sky with a 26,000 year motion. This causes in a very subtle way the position of the North Celestial pole to change among the fixed stars. As subtle as this motion was, it is detectable by observations over many centuries and was known to ancient cultures thousands of years ago. This means that the North Celestial pole has not always pointed at the bright star Polaris. In the time of the ancient Egyptians it pointed at a different star called Thuban, and in the future the North Celestial pole will point at a different star.

Transcript: Plato was a disciple of Socrates. He founded the world’s first university in an olive grove outside Athens in 387 BC. Plato was a hugely influential philosopher. He was a rationalist. He believed that we could conceive of the natural world and the way the universe works entirely in theory within our own heads. He believed that the observational phenomenon of the world were unreliable representations of what was truly going on. So Plato was not a scientist in the modern sense. However, his creation of a university, a place where people could think deeply about mathematics and logic, spurred Greek science onward. Written over the portal of Plato’s academy were the words, “Let none but Geometers enter here.”

Transcript: The moon does not emit its own light. All the light we see from the moon is reflected sunlight. The phases of the moon are related entirely to the changing angle between the Sun, the Earth, and the Moon. As the Moon orbits the Earth, it moves occasionally between us and the Sun and occasionally on the opposite side of the sky. When the Moon is nearly between us and the Sun we see the new Moon because the lit face of the Moon is facing back towards the Sun. As it continues in its orbit, after about a week, we see a first-quarter Moon. The Moon is still half lit, but we only see half of the lit surface, a quarter. Another seven days and the Moon is now on the opposite side of the sky to the Sun. We see its entire half-lit surface facing back at us, the full Moon. Seven days later, a third quarter moon. Once again, the Moon is half-lit by the Sun, and we see only a half of the lit face, a quarter.

Transcript: The patterns and motions of the stars in the night sky can be used for navigation. In the Northern Hemisphere the stars all appear to move about a fixed point in the sky called the Northern Celestial Pole. There happens to be a fairly bright star in this direction called Polaris. The elevation of Polaris above the horizon gives your latitude on the Earth’s surface. For thousands of years navigators have been able to use the motions and patterns in the night sky to navigate. Islanders in small boats in the South Pacific were able to travel distances of several thousand miles with an accuracy of fifty or sixty miles purely using the motions and patterns of the night sky.

Transcript: What would you observe if you looked at the sky for a year from somewhere in the Northern hemisphere?  You'd notice that the stars rose in the east and set in the West and appeared to move about a fixed point in the Northern sky.  You'd notice that the Sun, the Moon, and the planets all traverse the same strip of the sky.  You'd notice that the stars rise and set slightly earlier everyday and that the constellations move through the entire sky in the course of a year.  You'd notice that the Sun during the summer rises slightly North of due-East, and the day is longer than twelve hours.  In the winter you'd notice the Sun rising South of due-East, and the day is shorter than twelve hours. You'd notice the constellations rising in the East and setting in the West and always preserving a fixed pattern with respect to each other.  You’d notice the regular pattern of the Lunar phases repeating every 28 or 29 days.  You’d notice the rare phenomena of eclipses and the fact that Solar eclipses are much rarer than Lunar eclipses, and neither occurs every month.  And you would notice that some of the planets follow irregular motions with the fixed constellations.  All of this you would notice by careful, naked-eye observations over the course of the year.

Transcript: We always see the same features on the surface of the Moon.  This means that the Moon's rotational period equals the time that it takes to orbit the Earth.  This is called synchronous rotation.  The time that it takes for one phase of the Moon to recur in the night sky is called the Moon Synodic period.  It's twenty-nine and a half days.  This is different from the Moon's sidereal period which is the time that it takes for a fixed phase of the Moon to reappear amongst the fixed stars; this is only 27.3 days.  The phases of the Moon are one of the most prominent patterns in the night sky. They have been known to civilizations and fairly accurately measured for tens of thousands of years.

Transcript: By the late 16th Century, the Julian calendar was out of sync with the seasons. With an average length of 365.25 days, it is in fact a hundredth of a day longer than a true solar year.  After centuries, these hundredths of a day had added up to days, and the solar calendar was in fact ten days off.  Pope Gregory, representing the Catholic Church, instituted a calendar reform, adding essentially one rule to the Julian calendar: that you would skip the leap year in century years unless divisible by 400. With this extra rule the calendar will now very closely approximate the Solar year, and this calendar will be good enough for us to use for thousands of years to come.  Non-Catholic countries, the Protestant countries of England and America in particular, did not adopt the Catholic calendar for another hundred years, by which time their calendars were eleven days off.  Essentially, in one swoop, the governments of the United States and Britain lost eleven days going directly one year from April eleventh to April first.  Benjamin Franklin wrote in Poor Richard's Almanac to reassure people about the loss of the eleven days.  The French, a Catholic country, decided to mock the Americans by creating April Fool's Day to mark the day that the British and the Americans finally adopted the Gregorian calendar.  Have calendars never been sensible or rational?  At the time of the French Revolution, for twelve years the French instituted a decimal calendar with twelve thirty-day months, not named after Pagan gods, five festival days, a leap year, a week divided into ten days, the day into ten hours, each hour into a hundred minutes and each minute a hundred seconds, the world's only decimal calendar.  But because no other culture would follow this calendar, it was dropped by Napoleon after only 12 years.

Transcript: A Lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth casts a shadow on the Moon.  Lunar Eclipses are much more common than Solar eclipses because the Earth's shadow is much larger than the Moon's shadow, and the probability depends on the size of the shadow.  Neither Solar nor Lunar eclipses occur every month because the Moon's orbit of the Earth is tilted five degrees with respect to the Earth's orbit of the Sun.  So there are only two times in the year, the so-called "nodes," when eclipses can occur.  If you have ever seen a Lunar eclipse, you will notice that the Moon does not become entirely dark despite being in the Earth's shadow.  That's because light can refract through the Earth's atmosphere around the Earth leaving some dim illumination on the Moon itself.

Transcript: The apparent position of the Sun in the sky depends on your position on the Earth's surface.  The regions between plus twenty-three and a half degrees Northern Latitude, the Tropic of Cancer, and minus twenty-three and a half degrees latitude, the Tropic of Capricorn, is the zone within which at some points in the year the Sun can be seen directly overhead.  The Arctic Circle, plus sixty-six and a half degrees North latitude, and the Antarctic Circle, minus sixty-six and a half degrees latitude, define the regions within which at sometimes of the year the Sun never rises and at other times of the year the Sun never sets.

Transcript: Major calendar reform occurred around the time of Julius Caesar in 44 BC.  The early Roman calendar was very imprecise.  The Romans were superstitious. They disliked odd numbers, for example.  In their calendar February was the month with the fewest days; it was essentially the "bad luck" month.  Julius Caesar rationalized the calendar.  He added a leap year giving an average length of 365 and a quarter days, a good approximation, within one-hundredth of a day, of a solar year.  He also decided, because he had a fairly large ego, to take the next month that was not named after a god and name it after himself, and so we had July named after Julius Caesar.  The Emperor that followed, Augustus, actually messed up the nice pattern of Caesar’s calendar.  Augustus wanted a month named after himself, and so he took the next numbered month and named it August.  Noticing that his month had fewer days than Cesar's month he added a day to it and then messed with the rest of the sequence thereby making it relatively difficult to remember.  The Julian calendar sufficed for hundreds of years in keeping track of the seasons and keeping track of time, but eventually even it got out of sync with the seasons.

Transcript: Empedocles supposed that all material was composed of four fundamental substances, earth, air, fire and water, and that everything we see in nature was either these primordial substances alone or seen in combination.  This ancient idea is the first signs of something that we would today recognize as the periodic table.  The Greeks also imagined there was a fifth, ethereal element that corresponded to the outer Celestial Sphere: quintessence.

Transcript: Eclipses occur due to the coincidence that the sun and the moon have the same apparent angular size in the sky, about half of a degree.  Eclipses can only occur when the Earth, the sun, the moon are in the same plane.  A lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth casts a shadow on the Moon.  Solar eclipses occur when the Moon passes between the Earth and the Sun, and the moon casts a shadow on the Earth.

Transcript: When you think of the calendar that we use every day some questions arise.  Why do you have to count on your knuckles or remember a rhyme to figure out how many days there are in a month?  Why does the year start in January as opposed to any other time? Why is December named after the Latin word for "ten" when in fact it is the twelfth month?  To answer these and other questions about the quirks in our calendar we have to go back to the early Romans.  The calendar we use is based on a Roman calendar.  The first Roman calendar was far inferior to the Babylonian calendar several thousand years previously.  It was very inaccurate; the Roman calendar started sometime in March when the Romans felt that the snow had melted enough in the Alps to send their legions off into the other countries around them.  This calendar started in March and had ten months of unequal length and was a poor approximation to the Solar year.  Eventually they added two months to the front end so that it started near the Winter Solstice.  The names of the months were partly numbers for the latter part of the year and named after their gods for the first part of the year.  And so, we have January after Janus, the god of doorways and beginnings, February after Februa, the god of purification, March after Mars, the god of war, April after Aphrodite, the goddess of beauty, May after Maya, an ancient fertility god, and June after Juno, the goddess of women.  Many of the practices we still follow today originated with the Roman calendar about 2000 years ago.

Transcript: The early Greek philosophers had none of the tools of modern science.  They did not have the machines with which to probe the atom.  They did not have telescopes. They didn't have modern technology of any kind, and yet with logic and mathematics they were able to make some striking speculations and discoveries about their universe.  They speculated as to the existence of atoms.  They speculated that the Earth was round and imbedded in a large cosmos.  They speculated as to the true nature of eclipses and the cause of the seasons.  These speculations were born out by fact many hundreds of years later.

Transcript: A calendar is a way of keeping track of the days in a year or the time it takes for the Earth to go from one place in its orbit to the same place one orbit later.  The first calendars date back 5000 years to the Babylonian and Egyptian Cultures.  The earliest calendars had 360 days, and this fact became the basis for our system of measuring angle and of keeping track of time, the subdivisions of an hour and of a minute.  The Babylonians eventually had a calendar that was accurate to about thirty minutes in a year, or one part in 10,000.  They had a leap-year; that is, they added an extra day to their calendar every fourth year to approximate a Solar year of 365 and a quarter days.  These early calendars allowed the Babylonians and the Egyptians to run their systems of agriculture with good reliability.

Transcript: The simplest division of time used by hunter-gatherers was to divide the day into the time when the sun was rising, ahead of the Meridian, and the time the sun was setting, beyond the Meridian.  We still use that terminology.  "AM" means "Anti-Meridian," or before the Meridian. "PM" means "Post-Meridian," or past the Meridian. So the first people divided the day into two, roughly equal parts.  The practice of dividing a day into hours originated 4 or 5000 years ago with the Egyptians.  They kept track of time at night using "decans" or time-keeping stars, and they roughly divided the day and the night into twelve equal portions using these time-keeping stars.  That's the basis of our hours.  In the northern latitude of the Earth, if you divide the day throughout the season into equal amounts, hours will not be of equal length.  The hour will vary if you divide daylight into twelve pieces, from 75 minutes in the summer to about 45 minutes in the winter.  So hours did not have equal length.  The first uniform time-keeping dates from the time of the first clocks.  The first clocks were mechanical devices, enormously expensive for the medieval cultures who created them from the 14th and 13th century.  The Great Clock in Salisbury has been working continuously since 1365.  The word "clock" comes from the word for "bell."  The earliest clocks had no hands and no faces and only bells because almost nobody could read.  Division of time within an hour dates back to the Babylonians, who chose to divide the hour into sixty minutes and the minute into sixty seconds.  But the practice of keeping time this way relied on the invention of the pendulum as a time-keeping device during the time of Galileo.

Transcript: It is fundamental to how science works, and to our understanding of the everyday world, that events have causes.  This is called Determinism.  Sometimes our belief that events have causes leads us to confuse causation and correlation. Bertrand Russell told the story of a chicken growing up on a farm. Every day the Sun rose and the chicken was fed.  The chicken came to associate the Sun rising with being fed and believed that one caused the other.  One day the farmer comes out after sunrise and strangles the chicken for the dinner table. Obviously, induction has failed in this case because one event does not cause the other.  Imagine one morning you try to start your car, and it won't start. You know a little about cars, but after inspecting under the hood and looking everywhere you can you can't figure it out.  You go up to your roommate who knows more about cars, and after getting himself dirty he can't figure it out either.  Your car just won't start.  Eventually, you get your mechanic over and then the dealer of the car itself, and after using complicated test equipment, the dealer says, "I'm sorry. I can't find anything wrong. The car just won't start."  If you could imagine this situation you would clearly decide that this is an unreasonable outcome.  There has to be a reason that your car won't start. We fundamentally believe that events have causes.  Thus is the strength of our belief in determinism.

Transcript: Suppose you have a small particle or a grain of sand and a fine knife and you divide the grain of sand in half, and then in half again, and then in half again.  Is there an end to this process, or must you reach a limit, an indivisible particle?  Democritus, the fifth century BC Greek philosopher, imagined that it was illogical that this process of subdivision of matter could continue without end, and so he proposed that all matter was made of atoms, indivisible, microscopic units of matter comprising all the things we know in the natural world.  This was a very advanced idea because Democritus had no way of isolating or seeing individual atoms. But the idea was very modern because it is the basis of modern science, that the properties of matter such as color, taste, odor, are secondary properties, and the fundamental properties apply to atoms themselves.  Democritus also speculated that the fundamental particles of matter, atoms, were in constant motion, something we also know to be true today.  And so Democritus, 2000 years before atoms were actually seen, hypothesized the microscopic nature of the natural world.

Transcript: Every human culture has felt the need to subdivide time. In addition to the seasons, which follow from the Earth's motion around the sun, and months, which are tied to the lunar cycle, almost every culture has divided time into weeks. "Week" comes from the old German word meaning "to change." Weeks have had different numbers of days in different cultures through history. The modern tradition of a seven-day week actually dates back to the Babylonian times.  The word “Sabbath” is a Babylonian word meaning "to rest."  On the original Sabbath day, only the ruler rested, while the other people had to work.  The idea of a weekend is a modern invention only a few hundred years old.  Why does a week have seven days?  Because there are seven visible, moving objects in the sky: the Sun, the Moon, and the five nearest planets.  If you consider the names of the days of the week you will see that.  However, you usually have to go into a Romance language, French, Italian, or Spanish, to see it clearly.  Sunday, obviously, the day named after the Sun.  Sun worship was big for ancient cultures. This is the day of rest.  Monday, named after the Moon. Tuesday, not obvious, but in a Romance language, obviously associated with Mars.  In the English culture, this word came from the Norse god Tue. Wednesday, in a Romance language, you will see that relates to Venus.  Our name of Wednesday comes from the Norse god Woden.  Thursday: in Romance languages you will see this relates to Jupiter.  In our language, it comes from the Norse god Thor.  Friday relates to Mercury, in our language, the Norse god Frig.  And Saturday, is of course, for Saturn. Seven has always had mystical significance for cultures throughout history, so the number of days of the week relates to mysticism and the ancient practice of astrology.

Transcript: The constellations of the night sky are among the oldest human artifacts.  For thousands of years humans have been noticing patterns and using them to navigate, or to keep track of the sky, or to tell myths and legends.  There are 104 constellations in the modern sky.  Some of them are extremely old. There is evidence that Ursa Major, the great bear, which includes the asterism of the Big Dipper, dates back to at least 10,000 years.  Asian tribes named this constellation, and knowledge of this constellation traveled from Asia through to Europe and to North America. The thirteen original, and then twelve, constellations where the sun, moon, and planets travel, were divided into the Zodiac, the circle of animals.  Those are the familiar twelve constellations of the star signs.  But in general, the constellations are a rich source of human history as myths and legends have been posited in the constellations for many, many years.  The constellations don't look like the things they are supposed to represent; you have to use your imagination.  Remember, the constellations were used as shorthand by people who depended on the night sky for their very existence.

Transcript: The celestial sphere is an imaginary sphere surrounding the Earth onto which are projected the objects of the night sky.  There are several fixed points on the celestial sphere that are important.  The Zenith is the point directly over your head.  The Nadir is the point directly below your feet.  The line drawn across the sky that represents the highest elevation of the sun or any other object as it traverses the sky is called the Meridian.  The sun rises through to its highest point and then starts setting as it passes the Meridian.  The other important points are to define two angles anywhere on the celestial sphere that give uniquely the position of any star or object.  Altitude and Azimuth are two angles that can do this.  Altitude is the angle with reference to the horizon, and Azimuth is the angle along the horizon.  Astronomers use a different coordinate system based on Right Ascension and Declination.  The other important track on the sky is the Ecliptic, the path traveled by the sun. This is also the region of the sky within which you will see the motions of the moon and the planets.

Transcript: Humans has been anatomically modern for about 40 or 50,000 years.  If you can imagine what life would have been like 30 or 40,000 years ago for hunter-gatherers somewhere in Europe or Africa or the plains of Asia, you can realize that the sky must have been important to them.  The first use of astronomy was not in the modern, scientific sense.  Astronomy and the stars and the sky were a fundamental part of people's lives. The sky was a map, a clock, a calendar, a source of myth and legend, and more.  If you lived in those times you would have needed to know the motion of the sun to be able to go on a journey, a hunting journey, to be able to return before dark. Nobody ever stayed out before dark in the time before electricity and lights.  If you were living and subsisting off natural vegetation and fruits and berries you would need to be able to keep track of the seasons to know where your food supply was or follow the migrating herds, so you needed to use the sky as a calendar.  If you were navigating on the open ocean, as many tribes in Polynesian areas or tropical areas of the Earth did, you would need to use the sky as a map for navigation.  Cultures over thousands of years have been able to travel thousands of miles in small boats by their knowledge of the sky as a map.  For these reasons and more the sky has been important to cultures throughout the centuries.  In fact, it is unfortunately only in the modern age that people have become detached from the night sky because so many of us live in cities.

Transcript: Aristotle, who lived in the fourth century BC, was the most influential and famous Greek philosopher.  He founded a second University, the Lyceum, near Athens. He was a pupil of Plato.  He wrote and lectured on many subjects, including marine biology, botany, anatomy, economics, politics and meteorology.  He developed the tools of logic that are the basis of the scientific method.  He had his own observatory, and he made observations of stars and planets.  In cosmology he developed a system that would stay in place as the current theory for 2000 years.  Even though it was wrong, in Aristotle's cosmology the Earth was round, but it was stationary at the center of a universe where the stars formed the outermost sphere.  This is called the geocentric cosmology.

Transcript: Aristarchus, who lived in the third century BC, was a skilled geometer, and he anticipated the Copernican heliocentric model by over 1800 years.  He deduced by application of logic and geometry that the sun must be 19 or 20 times larger and further away than the moon.  Knowing that the sun was larger than the Earth it made no sense for the sun to go around the smaller Earth.  Rather, Aristarchus supposed that the smaller object went round the larger object.  He used the analogy of a hammer-thrower to make this clear.  In Aristarchus's heliocentric model the stars had to be extremely far away so that we would not see the variation in their relative position and brightness as the Earth moved around the Sun.  Aristarchus also measured a highly accurate solar year.

Transcript: It took a long time for people to figure out the true sizes and distances of celestial objects.  Part of the reason is psychological. It was always difficult for humans to imagine that there might be things so much larger then them or so much larger than the Earth.  The other reason is that when you look at something in the sky all you can really see is its angular extent, and its angular extent tells you nothing about its true distance or true physical size.  For example, when people talk about observing things in the sky, they might say, "It looked as big as a dinner plate," when in fact the object might be very far away and very much larger than a plate.  Without a measure of distance, it is impossible to guess, or estimate, the true physical size of an object in space.

Transcript: For tens of thousands of years humans have used the sky, the daytime sky and the night sky, as a map, a clock, and a calendar.  The first astronomy thus is directly tied into human culture, into the needs for hunter gatherers to move around, follow the migrating herds, garnish food sources, and protect themselves from the extremes of climate.  The first astronomers were not scientists in the modern sense of the word; they used careful observations made systematically, in cases over many years and decades, to understand the patterns of the night sky and of the seasons.  They were in tune with the motions of the objects in the night sky and the motions of the sun and the moon.  They derived fairly accurate calendars.   They made monuments out of stones that can still be found in many parts of Europe to make calendars and to predict the seasons. These first uses of astronomy do not conform to the modern rules of science because these people were not interested in scientific explanations for what they saw.  They used the sky as part of their mythic and religious culture, and that of course gave it much of its importance.

Transcript: Anaximenes was a student of Anaximander and also from Aletes in Turkey.  He believed that the source of all things was air and that the diversity arose from changes in the primordial substance, that the universe had an underlying homogeneity.  That is also a strikingly modern idea because we believe that the universe today had all of its structure emerge from an initially uniform, hot state. Anaximenes believed that the Earth was flat and that it floated on air.  This is not a very modern idea, but it was consistent with what he knew and could observe.  He believed that the stars were like nails in the celestial vault, hammered through and allowing light to leak through.

Transcript: The oldest surviving fragment of philosophy dates back to Anaximander in the sixth century BC. Anaximander wrote that the universe was made of a primordial substance and that this primordial substance was infinite in extent.  This is the first known use of the concept of infinity in science.  He also supposed that the primordial substance had no intrinsic color, odor, or weight, that these were all secondary attributes, another strikingly modern idea.  He also wrote about how the universe was subject to the warring opposites of order and chaos.  These words come down to us today.  The Greek word cosmos comes from the word for order, and the word chaos comes from the Greek word meaning disorder or disharmony. To the earliest Greek philosophers the universe was a balance between the forces of order and disorder.

Transcript: In the 5th century BC Anaxagoras deduced the true cause of eclipses. He realized that the curved shadow of the Earth on the Moon during a lunar eclipse supported the idea that the Earth was round.  In fact, a sphere is the only three-dimensional object that, whatever its orientation, always casts a circular shadow.  He was aware of a meteorite that had fallen in his native Greece and deduced from it that objects could move between the celestial and terrestrial spheres.  He also speculated as to the true size of the sun, saying that it might be an incandescent stone larger than the Peloponnesian peninsula. Such ideas were heretical in Greece at the time, and so he was banished for impiety.