" Earth's place in the cosmos " The nature of planet motion " How we understand “the truth” " Addressing these three key issues led to the birth of science. Origin of Modern Astronomy

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Archeoastronomy There is evidence that Human kind has paid very closed attention to the sky, its motions and periodicity. Humans always tried to explain them Archeoastronomy a blend of superstition, astrology, religion, but also common sense Greeks were the first to try to understand (logically) the universe. Thales of Miletus: first, true scientific attitude, humans can understand –Very different from previous “mysteric” attitude: humans cannot comprehend mysteries –Unknown vs. Unknowable Pythagoras: universe ruled by geometrical, mathematical relationship –This notions still fundamental today

The birth of modern science Thales: Universe can be understood Pythagoras: Observations show that rules are mathematical Plato: observations imperfect, pure thought can achieve the truth All ingredients there for the methods of modern science: have a theory and test it against reality –Science: test theory with observations –A theory is good only if it (or its consequences) explain the observations/experiments Often, the survival of a (wrong) theory due to not sufficiently sensitive observations Ultimately, Greek “science” still a blend of scientific method and “philosophical” believes. True scientific method had to wait for Galileo

The Old Astronomy: Geocentric Universe Aristotle (Greek, BC)

Not to scale

Three Basic Assumptions The Earth was at the center of the Universe. The only motion in the heavens was uniform circular motion The heavens were more perfect than the Earth, and objects in the heavens were eternal.

Very different from methods of modern science Build a theory on some hypothesis Test the predictions of the theory against phenomenological reality Modify, update, generalize or discard and replace the theory if *any* of its predictions or consequences fail to reproduce reality

"Common Sense" If the Earth actually spun on an axis, why didn't objects fly off the spinning Earth? If the Earth was revolving around the sun, why didn't it leave behind the birds flying in the air? If the Earth were actually on an orbit around the sun, why wasn't a parallax effect observed?

Two problems for the Geocentric model: (1) Change of brightness (2) Retrograde Motion Ptolemy Solution: add a special fix: epicycle or circle on circle Brightness changes because of distance change

Ptolemic Model

The Copernican Revolution: The Heliocentric System Nicolai Copernicus ( ). The Sun, not the Earth, was the center of the Solar System. The Earth is just another planet (the third outward from the Sun), and the Moon is in orbit around the Earth, not the Sun. The stars are distant objects that do not revolve around the Sun.

Retrograde Motion and Varying Brightness of the Planets " The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth. " The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits.

Copernicus’ ideas remained rather obscure for about 100 years after his death. Later work of Kepler, Galileo, and Newton would build on the heliocentric Universe, leading to the birth of modern astronomy and natural science. This sequence is commonly called the Copernican Revolution.

He made the most precise observations of planetary motion, particularly that of Mars. However, Tycho was a firm believer of the geocentric universe. Key Observations of Tycho Brahe Danish nobleman ( )

Unlike Brahe, Kepler believed firmly in the Copernican system. Based on Tycho's data on Mars, Kepler concluded that the orbits of the planets were not circles, but were instead ellipses Johannes Kepler: The Laws of Planetary Motion ( ), German

1 The orbits of the planets are ellipses, with the Sun at one focus of the ellipse. 2 Planets move proportionally faster in their orbits when they are nearer the Sun. 3 More distant planets take proportionally longer to orbit the Sun Kepler’s Laws of Planetary Motion

Calculations Using Kepler's Third Law The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes. R(AU) 3 =P(years) 2 As an example, the "radius" of the orbit of Mars (the length of the semimajor axis of the orbit) is: R=P 2/3 =(1.88) 2/3 =1.52 AU

Galileo discovered that our Moon has craters, that Jupiter has it's own moons, that the Sun has spots, that Venus has phases like our Moon, and many more discoveries. These discoveries confirmed the Copernican hypothesis that the Earth was just another planet. Galileo Galilei ( ), Italian

The Phases of Venus Venus went through a complete set of phases, just like the Moon. This was the first empirical evidence that allowed a definitive test of the geocentric and heliocentric models.

The Moons of Jupiter Galileo observed 4 points of light that changed their positions with time around the planet Jupiter. He concluded that these were objects in orbit around Jupiter. Therefore, a planet could have moons circling it that would not be left behind as the planet moved around its orbit.

Imperfect and Changing Universe Sun had dark patches on Sun. The motion of such sunspots indicated that the Sun was rotating on an axis. The Milky Way was composed of enormous numbers of stars that had not been seen before. The planet Saturn had "ears". The Moon was not smooth, but was covered by mountains and craters.