History of Astronomy - Part I Astronomy is a science that has truly taken shape only in the last couple centuries Many advances have been made in your lifetime However, astronomical concepts and ideas have been around since the birth of man The next two lectures will lead us through the development of astronomy from ancient times through the modern age Ancient Astronomy Astronomy held a special significance for ancient cultures for different reasons Very important navigation tool Mark the seasons Religious implications Stone Age England Many erected structures in order to track celestial events Ancient records are used to track supernovae and comets Native American Mayan Ancient Greece The Greeks were one of the first cultures to concentrate and debate the nature of the universe The matter was chiefly philosophical, but did seek to match the observed properties of the stars and planets with a suitable model The study of the universe as a whole, its structure, and its evolution is now known as cosmology For the Greeks, the universe was rather smaller than today, limited only to the Earth, Moon, Sun, planets and the celestial sphere 1
The Greek Frame of Mind Much of the Greek method of thinking revolved around philosophy instead of scientific reasoning Greeks valued perfection and therefore any model of the universe should involve the perfect shape, the circle Greek also had no reason to believe that the Earth was not the center of the universe. Egotistical, yes - but completely reasonable at the time Ptolemy ~140 AD The only 'scientific' data they had available to them was the motion of the Sun, Moon, and planets, which were monitored heavily at the time The Motion of the Planets Retrograde Motion A model of the universe would be very simple except for the fact that the planets undergo a looping motion in their orbits Retrograde Motion Remember, in one night, all planets still rise in the east and set in the west However, if you keep track of the planet's position versus the background stars night to night, you will see the planet 'move' The word 'planet' means wanderer in Greek Jupiter and Saturn (6/2000-5/2001) Ptolemaic Model In order to produce the retrograde motion of the planets, Ptolemy created a model with epicycles All the planets orbited the Earth in a perfect circle The planet itself made a smaller orbit centered upon the larger orbit around the Earth Deferent = larger circular orbit around Earth Epicycle = smaller circular orbit around the deferent With the right timing, this model can reproduce the retrograde motion seen from Earth 2
Ptolemaic Model In Ptolemy's complete model, each planet had its own orbit around the Earth with its own epicycle By changing the period of the orbit and the epicycle, the model could match observations relatively well The Sun and the Moon traveled around the Earth in perfect circles The entire model was composed of more than 80 circles and was very complicated Simplified Ptolemaic Model The Ptolemaic Model Survives Since the Ptolemaic model matched observations sufficiently and no contrary evidence was produced, it was supported for nearly 1,500 years! After all, if the Earth was moving, shouldn't we feel it? Also, the Greeks were smart enough to realize that if the Earth was orbiting the Sun, it would produce stellar parallax The Greeks didn't believe it existed because they didn't have telescopes to observe such small variations in a star's position On top of all this, the Dark Ages provided relatively little advance in any sciences for Europe Astronomy in the Dark Ages While Europe was suffering through the Dark Ages, Islamic nations were undergoing a golden age of astronomy Much of the knowledge of the Greeks was preserved and expanded upon during this time Many of the proper names for stars are relics of this era Rigel, Betelgeuse, Vega... 3
The Copernican Revolution At the end of the Dark Ages, a Polish cleric name Copernicus devised a new model of the universe where the Earth was no longer at the center The heliocentric (Sun centered) model placed the Earth out of its central position, yet still maintained many of the observations we see The beauty in his model was its simplicity over the Ptolemaic Occam's Razor The simplest solution is the best Nicolaus Copernicus (1473-1543) The Copernican Model In the Copernican model, retrograde motion is an apparent effect caused by the Earth 'overtaking' an outer planet in its orbit The Copernican Revolution Despite the fact that the Copernican model was a better representation of the solar system, it was not widely accepted While it did provide a much simpler description compared to Ptolemy, it did not necessaryly improve the predictive power of the model The religious dogma of the time insisted upon Earth being the center of the universe Copernicus published his works in Latin, which was unreadable by the common public 4
Galileo - The Observer A century after Copernicus' work, other scientists began to make strides toward popularizing the heliocentric model Galileo was the first to use a telescope to make detailed observations of the sky Though he did not invent the telescope, he made many working prototypes and trained them on a variety of celestial bodies Galileo Galilei (1564-1642) Galileo's Observations - I Galileo used his telescopes to make observations of many heavenly objects The sketch to the right shows Galileo's observations of the moons of Jupiter He noticed that the position of these four moons changed night to night, as if they were rotating around Jupiter These moons now bear his name The Galilean moons are: Io Europa Ganymede Callisto Galileo's Observations - II Galileo also noticed that Venus was not simply a point of light, but actually a disk He watched Venus go through complete phases, just like the Moon This cycle of phases can only be satisfied by the heliocentric model, not the geocentric The phases of Venus 5
Galileo's Observations - III Galileo also pointed his telescope toward the Sun NEVER DO THIS He discovered that the disk of the Sun was not perfect and was occasionally dotted with small black spots By making daily sketches of these spots, he was able to determine that the Sun itself was rotating Galileo's Conclusion All of Galileo's observations were pointing towards a heliocentric view of the universe Galileo published his observations and conclusions in multiple works, including some published in Italian to appeal to a wider audience Galileo was threatened with torture, forced to deny his beliefs in the heliocentric model, and sentenced to house arrest for the rest of his life The seeds of the Copernican Revolution had been planted Acceptance of the Copernican Model Despite the efforts of Galileo and many others, the acceptance of a Suncentered Solar System was a very slow process In 1728, James Bradley demonstrated the aberration of starlight, proving that the Earth is moving in its orbit around the Sun In 1838, Friedrich Bessel was the first to observe stellar parallax Aberration of starlight 6