The History of Astronomy http://www.phys.uu.nl/~vgent/babylon/babybibl_intro.htmhttp://mason.gmu.edu/~jmartin6/howe/Images/pythagoras.jpghttp://www.russellcottrell.com/greek/aristarchus.htmhttp://www.mesopotamia.co.uk/astronomer/homemain.htmlplato.lib.umn.edu/http://web.hao.ucar.edu/public/education/sp/images/aristotle.htmlhttp://web.hao.ucar.edu/public/education/sp/images/ptolemy.htmlhttp://www.windows.ucar.edu/tour/link=/people/ancient_epoch/hipparchus.htmlhttp://copernicus.atspace.com/http://www.danskekonger.dk/biografi/andre/brahe.htm/http://antwrp.gsfc.nasa.gov/apod/ap960831.htmlhttp://www.lucidcafe.com/library/95dec/newton.html
The Model of the Cosmos • It’s a natural thing for people to want to understand the world around them. • People build models, a (simplified?) conceptual framework that represents the real world and operates in a manner consistent with observations. • To be successful, a model MUST be able to do two things…
Model Building • A model must accept and incorporate all careful, accurate observations. No observations may be conveniently discarded, simply because they contradict the model. [The model must be refined!] • A model must be able to make accurate predictions of future events.
Models Change • Take note of how the model of the cosmos changes as new observations become available. • Take note also of the forces which resisted logical changes: mental inertia, political censorship and religious dogma.
The First Astronomers • The first astronomers were the Babylonians. • Other cultures observed the stars, but the Babylonians were the first to keep records of the positions of objects in the sky.
Babylonian Astronomy • The Babylonians kept careful records on clay tablets for 1400 years, from 1600 B.C. until 200 B.C., even though they were repeatedly conquered, and their culture declined. The tablet aboverecords a lunar eclipse.
Religious Motivation • To the Babylonians, the objects moving regularly or erratically through the sky were deities, gods who could influence the events and futures of men. • Marduk was their chief. Mardukhttp://www.ancientneareast.net/religion_mesopotamian/gods/marduk.html
Religious Motivation • Because the motivation for the Babylonian’s astronomy was religious rather than scientific (they wouldn’t even have recognized the word!), the Babylonians never bothered to make a model of what the cosmos (universe to them, solar system to us) was like.
The Ancient Greeks • The Greek culture overlapped the end of the Babylonian period (600 to 100 B.C.) • Certain Greek sub-cultures (notably the Athenians) came to prize logic and philosophy as the way to understand the world. • You might recognize some of the names: Pythagoras, Plato, Aristotle, Democritus, Aristarchus, and Hipparchus.
Pythagoras • The philosopher Pythagoras was the first to propose a model of the cosmos. • Because of their symmetry, Pythagoras thought that some shapes were more perfect than others.
Pythagoras (2) • Because the sphere was perfectly symmetrical in every direction, it was considered to be the most shape. • The sky and everything in it appeared to be spherical. It was also thought that the earth, being the center of life and thought, was spherical and therefore perfect. • It follows that things that are perfect don’t ever need to change….
Pythagoras (3) • Pythagoras proposed a series of nested, concentric, crystalline spheres which rotated at constant speeds around the earth. • Each sphere would carry a single object on its inner surface. • The moon was on the closest sphere, followed by Mercury, Venus, the Sun, the outer planets. • The stars were “painted” on the outermost sphere. • The spheres were each able to rotate independently at different, but uniform speeds.
Plato • Plato was an extremely influential person in ancient Greece. Because he was so highly regarded, what he said was often taken as absolute truth. http://plato.lib.umn.edu/Images/plato.jpg
Plato (2) • Plato didn’t change Pythagoras’ model of the cosmos, but Plato’s contribution to the model-building process was his insistence that any model “save the appearances.” By this, Plato meant that any model of the universe had to accurately match the positions of the objects in the sky.
Plato (3) • Plato’s belief in the Pythagorean model and his insistence that a model must “save the appearances” became driving forces in the early understanding of the arrangement of the cosmos. • (Note, however, that Plato himself couldn’t live up to his own pronouncements: Pythagoras’ model couldn’t explain retrograde motion of the planets!)
Aristotle • Aristotle was a student of Plato’s. • He was the first to try to understand not just the way the cosmos was arranged, but the “why and how” of its functioning. http://www.seanet.com/~realistic/chpt4.html
Aristotle (2) • A Greek philosopher, Democritus, had proposed that every different substance in the universe had its own type of atom. Imagine “cheese”, “wood”, “hair” atoms! • A competing idea was that there were only 5 elements, fire, air, water, earth, and ether and that every different substance was composed of different ratios of these elements.
Aristotle (3) • Aristotle believed that the elements would always try to return to their sources. • No forces were necessary for this to occur; rather it was natural for these movements to happen.
Aristotle (4) • Some examples: • Smoke rises, because it is mostly air. • Flames rise because they were trying to return to the sun. • Water flows downhill because it’s attempting to return to the sea. • A thrown rock falls because it’s returning to the earth.
Natural Motion • Aristotle used arguments like these to explain that Pythagoras’ crystalline spheres turned naturally; no force or engine was required to drive the revolution. • Today, we reject natural motion and accept something equally non-intuitive: action at a distance, forces like gravity and magnetism, transmitted with no visible mechanism or connection.
Aristotle (5) • Aristotle also had other reasons for believing that the earth didn’t move, but that the heavens did: • He reasoned that if the earth were moving, a stone thrown into the air would fall back to the earth along a parabolic track, not a vertical track.
Aristotle (6) • An arrow fired directly north, would appear to veer to the west as the earth under the arrow rotated eastward. • Since neither of these things appeared to happen, it was quite natural for Aristotle to believe that the earth did not rotate.
A competing idea • Aristarchus of Samos proposed a heliocentric model of the cosmos. • It incorporated a rotating earth which revolved around the sun, along with all the other planets. http://www.krzysiek.finka.pl/szkoly/foto/rel02_fot10_aristarchus.jpg
A Prediction • Why wasn’t Aristarchus’ model accepted? • It made the prediction of heliocentric stellar parallax, which wasn’t observed. • Parallax is the apparent movement of a nearby object that is really due to the movement of the observer.
http://www.physics.carleton.ca/~watson/410_notes/History_of_Astronomy/410_Astro_history.htmlhttp://www.physics.carleton.ca/~watson/410_notes/History_of_Astronomy/410_Astro_history.html Angle a > Angle b
A Modern Explanation of Parallax http://www.yourdictionary.com/images/ahd/jpg/A4paralx.jpg
Try these animations • http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html • http://physics.bgsu.edu/~layden/Anim/Parallax/parallax.htm
Parallax (2) • Today, with large telescopes, we are able to detect and measure the parallax of nearby stars. • Even the nearest stars show parallaxes of less than about 0.3 arcsecond, an apparent motion far too small to see with the naked eye. • Parallax is real, but the ancients couldn’t see it without the technology of the telescope.
Problems begging for solutions • After the time of Aristotle, there were several problems that the model of the cosmos couldn’t explain. • Retrograde motion of the planets. • The apparent speeding up and slowing down of the sun and planets at different times of the year. • Varying shapes and durations of the planetary retrograde motions.
Hipparchus of Rhodes • Hipparchus worked from about 160 to 130 B.C. • He was a mathematician who used geometry to try to solve the problem of retrograde motion. http://universe-review.ca/I08-18-Hipparchus.jpg
A Clockwork Cosmos • Hipparchus extended the idea of the crystalline spheres. The main path or orbit of the planet was termed the deferent. • Attached to and centered on the deferent was a second, smaller orbit called the epicycle. The planet revolved as the deferent and epicycle both revolved.
Real backward motion • As the deferent and epicycle both turned independently, the planet would actually move backward during the retrograde (westward) portion of its motion. • With a correctly sized deferent and epicycle, the predicted positions of the planets would match the actual positions within naked-eye accuracy limits!!!
Attacking problem #2 • To try to solve the problem of the sun and planets traveling faster at some times of the year than others, Hipparchus proposed the eccentric. • Despite the requirement that the earth be at the center of the cosmos, Hipparchus placed the earth off-center by a small distance.
The Eccentric • The off-center placement allowed the sun and planets to appear to speed up when they were closer to the earth and appear to slow down when they were farther away. (The angular velocity no longer appears to be uniform.)
The Eccentric • Imagine standing in the exact center of the infield of a race track. Walk towards the track’s inner edge and the cars appear to be moving faster on the side you’re closer to, and slower on the opposite side.
The 3rd Problem • The last problem to be solved was that of different shape & duration planetary retrograde motions from one year to the next. http://www.xtec.es/recursos/astronom/articulos/retro/indexe.htmhttp://jcboulay.free.fr/astro/sommaire/astronomie/univers/galaxie/etoile/systeme_solaire/mars/page_mars3.htm
Ptolemy • Hipparchus never solved this last problem. It had to wait for a Greek astronomer working in Alexandria, Egypt around 125 A.D. Claudius Ptolemy http://www.livius.org/a/1/greeks/ptolemy.jpg
The Equant • Ptolemy proposed a point in space opposite the eccentric point, called the equant, where the angular speeds of the sun and planets would appear to be uniform. http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit3/Images/equant.gif
The Equant (2) • While this helped solve the problem of differently shaped retrograde loops, it also violated the premise that the crystalline spheres turned with uniform speeds. Now they were required to actually speed up and slow down. • How does this happen when no force or engine drives the crystalline spheres?
A Special Problem - Epicycles of Venus & Mercury • Ptolemy also realized that Hipparchus’ model had another problem – with Mercury, Venus, and the Sun all revolving around the earth, Mercury and Venus should sometimes appear in opposition to the sun (180o from the sun in our sky). • However this never happened. Venus was never more than 46o from the sun, and Mercury never more than 28o.
The Solution for Mercury & Venus • Ptolemy proposed that the epicycles of Mercury & Venus be “pinned” to a line drawn between the Sun and the Earth. • In this way, those two planets could oscillate from one side of the sun to the other, yet continue orbiting the earth.
The Epicycles of Venus and Mercury, “pinned” to a line drawn fromthe Sun to the Earth.
A Prediction • Ptolemy’s setup for the epicycles of Mercury and Venus makes a prediction: each planet should be able to show crescent and new phases as seen from the earth, but never a full phase. • Later, we’ll see that we actually do see full phases for Mercury and nearly-full phases for Venus.
Ptolemy’s 2 other accomplishments • Ptolemy calculated what he believed to be the size of the cosmos: 20,000 earth radii or 134,000,000 kilometers (radius). • Ptolemy wrote the first astronomy textbook, the Almagest (the “Majestic Book”).
The Almagest http://www.er.uqam.ca/nobel/r14310/Ptolemy/Images/Regiomontanus/1496.g.jpg