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.