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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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....

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...

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. ...

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...

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...

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...

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...

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...

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....

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.