Astro Science, Friday
"Today, we're gonna be talking about where astronomy comes from," Kaidan said, "And we're going to look at some of the astronomical models we've been using for a long time."
"Astronomy is an old science," he said. "Back before anyone understood physics or chemistry, it was easy to look up into the sky and see the stars - to figure out that sometimes it meant something, like the way the stars reflected the seasons. There's proof that the humans of old managed to keep track of eclipses using elaborate structures. Chinese civilisation even managed to track a lot of irregular objects as they shot across the sky."
He stood up. "Aristotle, an ancient Greek, came up with an idea for an elaborate model of how the universe was supposed to work. Since humans could look up into the sky and see the stars circle past them, he decided that Earth had to be the center of the universe. The four elements dictated how that universe worked." He called up an image on the screen behind him, Earth in the middle, the other planets rotating around it. "Earth itself was stationary. These days, that sounds like a ridiculous idea. The Earth wouldn't have its seasons the way it does if it stayed put in the middle of the galaxy-- and I doubt we have enough gravity to keep all the other planets in orbit around us."
He pointed at the model. "The first to figure out that the sun was at the center of the solar system was actually a Greek, too, but his work was lost when Alexandria's library burned down," he said. He tapped the image, and it warped into a different version of itself. "Ptolemy then went and established another Earth-centric model, which wouldn't be dispelled until centuries later, when Copernicus observed that Ptolemy's model no longer fit the position of all the stars and planets in the sky. Copernicus established that the sun was at the center of the solar system..."
With a swipe of his hand, he put a list up on the board.
1. There is no one center of all the celestial circles or spheres.
2. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere.
3. All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe.
4. The ratio of the earth's distance from the sun to the height of the firmament (outermost celestial sphere containing the stars) is so much smaller than the ratio of the earth's radius to its distance from the sun that the distance from the earth to the sun is imperceptible in comparison with the height of the firmament.
5. Whatever motion appears in the firmament arises not from any motion of the firmament, but from the earth's motion. The earth together with its circumjacent elements performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged.
6. What appear to us as motions of the sun arise not from its motion but from the motion of the earth and our sphere, with which we revolve about the sun like any other planet. The earth has, then, more than one motion.
7. The apparent retrograde and direct motion of the planets arises not from their motion but from the earth's. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.
"While Copernicus's work incited some controversy, it would also be used by countless astronomers after him," Kaidan continued. "Another big name is Tycho Brahe, who built the first astronomy tower and, more importantly, created a significant body of work with rights to astronomical observations. Fifty years later, the invention of the telescope followed, and we discovered something very, uh, impressive..."
He called up another image. "The Milky Way," he said. "Viewable to us on clear nights, a band of stars and galactic dust clouds. It was the first galaxy we ever truly saw, and, uh, that makes sense - it's ours." He tapped parts of the image. "It has a bulge and a halo, filled with old stars, and a disk full of young stars - including our own, Sol.
"As a result of all of this research, an astronomer called Kepler devised several common laws of planetary motion. Number one: orbits are elliptical, not circular." He called up a picture of the solar system and drew the lines. "Two, when planets are nearer to the sun, they move faster. And three, getting mathematical: If you know the period of a planet's orbit, in other words, how long it takes for the planet to orbit aroudn the sun, then you can determine how far away it is from the star itself."
He glanced towards the class. "From this point on, a lot of discoveries started piling up," he said, "The outer planets, comets, Einstein's theory of relativity... and the Doppler effect. All things we're going to go into later."
He scraped his throat. "Time for your presentations from last week," he said, "when you're done, there's some astronomical models for you to mess around with - they should have all the details."
"Astronomy is an old science," he said. "Back before anyone understood physics or chemistry, it was easy to look up into the sky and see the stars - to figure out that sometimes it meant something, like the way the stars reflected the seasons. There's proof that the humans of old managed to keep track of eclipses using elaborate structures. Chinese civilisation even managed to track a lot of irregular objects as they shot across the sky."
He stood up. "Aristotle, an ancient Greek, came up with an idea for an elaborate model of how the universe was supposed to work. Since humans could look up into the sky and see the stars circle past them, he decided that Earth had to be the center of the universe. The four elements dictated how that universe worked." He called up an image on the screen behind him, Earth in the middle, the other planets rotating around it. "Earth itself was stationary. These days, that sounds like a ridiculous idea. The Earth wouldn't have its seasons the way it does if it stayed put in the middle of the galaxy-- and I doubt we have enough gravity to keep all the other planets in orbit around us."
He pointed at the model. "The first to figure out that the sun was at the center of the solar system was actually a Greek, too, but his work was lost when Alexandria's library burned down," he said. He tapped the image, and it warped into a different version of itself. "Ptolemy then went and established another Earth-centric model, which wouldn't be dispelled until centuries later, when Copernicus observed that Ptolemy's model no longer fit the position of all the stars and planets in the sky. Copernicus established that the sun was at the center of the solar system..."
With a swipe of his hand, he put a list up on the board.
1. There is no one center of all the celestial circles or spheres.
2. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere.
3. All the spheres revolve about the sun as their mid-point, and therefore the sun is the center of the universe.
4. The ratio of the earth's distance from the sun to the height of the firmament (outermost celestial sphere containing the stars) is so much smaller than the ratio of the earth's radius to its distance from the sun that the distance from the earth to the sun is imperceptible in comparison with the height of the firmament.
5. Whatever motion appears in the firmament arises not from any motion of the firmament, but from the earth's motion. The earth together with its circumjacent elements performs a complete rotation on its fixed poles in a daily motion, while the firmament and highest heaven abide unchanged.
6. What appear to us as motions of the sun arise not from its motion but from the motion of the earth and our sphere, with which we revolve about the sun like any other planet. The earth has, then, more than one motion.
7. The apparent retrograde and direct motion of the planets arises not from their motion but from the earth's. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.
"While Copernicus's work incited some controversy, it would also be used by countless astronomers after him," Kaidan continued. "Another big name is Tycho Brahe, who built the first astronomy tower and, more importantly, created a significant body of work with rights to astronomical observations. Fifty years later, the invention of the telescope followed, and we discovered something very, uh, impressive..."
He called up another image. "The Milky Way," he said. "Viewable to us on clear nights, a band of stars and galactic dust clouds. It was the first galaxy we ever truly saw, and, uh, that makes sense - it's ours." He tapped parts of the image. "It has a bulge and a halo, filled with old stars, and a disk full of young stars - including our own, Sol.
"As a result of all of this research, an astronomer called Kepler devised several common laws of planetary motion. Number one: orbits are elliptical, not circular." He called up a picture of the solar system and drew the lines. "Two, when planets are nearer to the sun, they move faster. And three, getting mathematical: If you know the period of a planet's orbit, in other words, how long it takes for the planet to orbit aroudn the sun, then you can determine how far away it is from the star itself."
He glanced towards the class. "From this point on, a lot of discoveries started piling up," he said, "The outer planets, comets, Einstein's theory of relativity... and the Doppler effect. All things we're going to go into later."
He scraped his throat. "Time for your presentations from last week," he said, "when you're done, there's some astronomical models for you to mess around with - they should have all the details."