Origin of Modern Astronomy 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 .

Assigned reading: Chapter 4 When reading a chapter, make sure you study the Guidepost, the Connections, the Portfolio inserts and the Summary at the end

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. Some of this remains this very day (e.g. 2012 doomsday, astrology, UFO, etc.) 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

Guidepost The preceding chapters gave you a modern view of Earth. You can now imagine how Earth, the moon, and the sun move through space and how that produces the sights you see in the sky. But how did humanity first realize that we live on a planet moving through space? That required revolutionary overthrow of an ancient and honored theory of Earth’s place. By the 16th century, many astronomers were uncomfortable with the ancient theory that Earth sat at the center of a spherical universe. In this chapter, you will discover how a Polish astronomer named Nicolaus Copernicus changed the old theory, how a German astronomer named Johannes Kepler discovered the laws of planetary motion, and how the Italian Galileo Galilei changed what we know about nature.

Newgrange, Ireland, built around 3200 B.C.: The Roots of Astronomy Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky. Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. Those monuments were probably used as calendars or even to predict eclipses. Newgrange, Ireland, built around 3200 B.C.: Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice.

Probably used as calendar Stonehenge Summer solstice Heelstone Constructed: 3000 – 1800 B.C. Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices Probably used as calendar

Other Examples All Around the World Chaco Canyon, New Mexico Slit in the rock formation produces a sunlit “dagger” shape, indicating the day of summer solstice

Other Examples All Around the World (2) Mammoth tusk found at Gontzi, Ukraine: Inscriptions probably describing astronomical events

Ancient Greek Astronomers (1) Unfortunately, there are no written documents about the significance of stone and bronze age monuments. First preserved written documents about ancient astronomy are from ancient Greek philosophy. Greeks tried to understand the motions of the sky and describe them in terms of mathematical (not physical!) models.

Ancient Greek Astronomers (2) Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned: Geocentric Universe: Earth at the Center of the Universe “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles Greeks assumed the Earth was not moving because they did not observe parallaxes in the sky.

Ancient Greek Astronomers (3) Eudoxus (409 – 356 B.C.): Model of 27 nested spheres Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts: 1. Imperfect, changeable Earth, 2. Perfect Heavens (described by spheres) He expanded Eudoxus’ Model to use 55 spheres.

Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius Angular distance between Syene and Alexandria: ~ 70 Linear distance between Syene and Alexandria: ~ 5,000 stadia  Earth Radius ~ 40,000 stadia (~14 % too large) – better than any previous radius estimate

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 is due to not sufficiently sensitive observations Ultimately, Greek “science” still a blend of scientific method and “philosophical” believes, however. True scientific method had to wait for Galileo

The Old Astronomy: Geocentric Universe Aristotle (Greek, 384-322 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, because circular is the “perfect motion” The heavens were more perfect than the Earth, and objects in the heavens were eternal. There was a hierarchy of perfection in going from Earth to the Moon, the Sun, all the way up to the stars.

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

Later refinements (2nd century B.C.) Hipparchus: Placing the Earth away from the centers of the “perfect spheres” Ptolemy: Further refinements, including epicycles

Introduced to explain retrograde (westward) motion of planets Epicycles Introduced to explain retrograde (westward) motion of planets The Ptolemaic model was considered the “standard model” of the Universe until the Copernican Revolution.

The Copernican Revolution: The Heliocentric System Nicolai Copernicus (1473-1543). 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.

This made Ptolemy’s epicycles unnecessary. Copernicus’ New (and Correct) Explanation for the Retrograde Motion of the Planets Retrograde (westward) motion of a planet occurs when the Earth passes the planet. This made Ptolemy’s epicycles unnecessary.

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. Ultimately, however, although revolutionary, Copernicus model was flawed, because it could not explain the details of the motions. This had to wait until the first scientific explanation was offered by Kepler.

High precision observations of the positions of stars and planets Tycho Brahe (1546 – 1601) High precision observations of the positions of stars and planets Measurement of the nightly motion of a “new star” (supernova) showed no parallax Evidence against Aristotelian belief of “perfect”, unchangeable heavens

Key Observations of Tycho Brahe He made the most precise observations of planetary motion, particularly that of Mars. However, Tycho was a firm believer of the geocentric universe. Danish nobleman (1546-1601)

Tycho Brahe’s Legacy New World model New World model Still geocentric (Earth in the center of the sphere of stars) Sun and Moon orbit Earth; Planets orbit the sun.

The Laws of Planetary Motion Johannes Kepler: The Laws of Planetary Motion 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 (1571-1630), German

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

Kepler’s Laws of Planetary Motion The orbits of the planets are ellipses with the sun at one focus. c Eccentricity e = c/a

Eccentricities of Ellipses 1) 2) 3) e = 0.02 e = 0.1 e = 0.2 5) 4) e = 0.4 e = 0.6

Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles: Most extreme example: Pluto: e = 0.248 Earth: e = 0.0167

Planetary Orbits (2) 2. A line from a planet to the sun sweeps over equal areas in equal intervals of time.

(Py = period in years; aAU = distance in AU) Planetary Orbits (3) 3. A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: (Py = period in years; aAU = distance in AU) Py2 = aAU3

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=P2/3=(1.88)2/3=1.52 AU

Galileo Galilei 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. The Bible tells us how to go to Heaven, not how the heavens go (1564-1642), 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 phases of Venus Galileo explained the phases of Venus (including “full Venus”) as due to the fact that Venus orbits the sun, not the Earth!

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.

Using the telescope, Galileo was the first to resolve the Milky Way into individual stars, although he did not understand that the Milky Way is one galaxy among other galaxies (nor could have he understood that. Not even Einstein had that notion, initially. We had to wait 1929, when Huble discovered the cosmic expansion to understand that)

In summary: the three revolutionaries Tycho Brahe, Kepler and Galileo Galilei Tycho made fundamental contributions because of the accuracy of his observations He discovered a supernova and understood that it was a distant star, as distant as other fixed stars> Starry sphere no perfect Kepler discovered the laws of planetary motions thanks to Ticho observations, although he did not understood why

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

Kepler’s Three Laws of Orbits The orbit of each planet about the Sun is an ellipse with the Sun at one focus.

Kepler’s Three Laws of Orbits 2. As a planet moves around it’s orbit, it sweeps out equal areas in equal times. 1 month 1 month

Kepler’s Three Laws of Orbits A planet’s Period (the time it takes to complete one orbit) is related to its average distance to the sun. (orbital period in years)2 = (average distance in AU)3 P2 = a3 Notice that there is nothing stated about the planet’s or Sun’s mass here!

Galileo: 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. The Universe is a physical system to be explored empirically and described my mathematical models

What is truth to a scientist? Scientists explains the universe on terms of models whose predictions are confirmed by empirical observations. Truth is simply a theoretical framework capable to explain in quantitative ways what the empirical investigation finds. If new observations disagree with the model, the model is either improved or rejected and replaced with something else. Religion tells us a different truth that has nothing to do with the physical word. And tell us to believe it as is told. Galileo understood that the Universe is a physical system to be explored empirically and described by mathematical models. He was tried and found guilty not because he believed in the Copernicus cosmology, but because he he presented the universe as something that humans can explore and understand themselves instead of believing in absolute truths