History of Astronomy

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 Stone Age England • Astronomy held a special significance for ancient cultures for different reasons • Very important navigation tool • Mark the seasons • Religious implications • 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

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 • 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 Ptolemy ~140 AD

The Motion of the PlanetsRetrograde Motion • A model of the universe would be very simple except for the fact that the planets undergo a looping motion in their orbits • 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 Retrograde Motion 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 • With the right timing, this model can reproduce the retrograde motion seen from Earth Deferent = larger circular orbit around Earth Epicycle = smaller circular orbit around the deferent

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 ...

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 necessarily 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

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

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 Sun-centered 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

History of Astronomy - Part I