Astro 201: Sept. 2, 2010 Do on-line practice quiz #2 (see d2l) Homework #2: posted on web page after class Today: – Ancient Greeks – Ptolemy V. Copernicus; retrograde motion – Galileo – Astronomical basis for calendars – Tycho, Kepler, Newton

Despite Aristarchus, for 2000 years, a GEOCENTRIC model of the Universe was favored EARTH at the center STARS affixed to the celestial sphere Moon, Planets and the Sun are between the EARTH and STARS

PTOLEMY AD Ptolemy’s ALMAGEST was translated into Latin in 1496 GEOCENTRIC: Earth at center

Copernicus ( ) Heliocentric Model: Sun at the center

In order to understand the motion of the planets in the sky Ptolemy’s model had to be modified In terms of the heliocentric model, the planets orbit the Sun in ellipses, not circles Retrograde motion of the planets with respect to the stars required EPICYCLES Ptolemy’s Epicycles

Ecliptic the fact that the planets orbit the Sun in a plane means that they always appear to lie on a great circle on the sky, called the ecliptic

Retrograde motion The planets sometimes appear to be traveling west to east, unlike the stars and Sun which always go east to west Next slide: animation Astronomy Picture of the Day for Dec. 20, Jupiter and Saturn showing retrograde motion. Combining 23 pictures taken at 2 week intervals from June May Planet = “wanderer”

Ptolemy’s explanation for retrograde motion of the planets

Earth Equant Epicycle Deferent The planet moves along its epicycle as the epicycle moves along the deferent around the Earth. To make the observations as accurate as possible, it was necessary to place the Earth slightly off center of the orbits, but to preserve symmetry that meant that there was an equal place (“Equant”) opposite the Earth from the center. The combined motion of the planet and the resulting retrograde motion are shown. Earth Off Center

Copernicus’ explanation for retrograde motion

Copernicus: “On the Revolution of the Heavenly Spheres” Published at the time of his death, in 1543 Major Conclusions: 1.The planets orbit the Sun 2. The apparent daily motion of the Sun and stars is the result of the Earth’s rotation 3.The stars are much farther away than the Sun

Heliocentric model: distance from Sun to stars must be much greater than distance from Sun to Earth. Since Earth orbits Sun, stars should show parallax (a shift in apparent position) over the course of half a year.

Implication: distance to stars is several thousand times Earth – Sun distance. OBSERVATION: Parallax of stars is TOO SMALL to be seen by the naked eye. Not observed until 1800s.

Parallax Unit of distance: the PARSEC A star which is one parsec from Earth shows a parallax of one arcsecond Remember: 360 degrees in a circle, 60 arcminutes per degree, 60 arcseconds per arcminute

Reactions to Copernicus : On March 5, 1616, Copernicus' work was banned from being taught and discussed by the Congregation of the Index "until corrected." It stayed on this list of prohibited books and teachings until Martin Luther ( ): [Copernicus] “is a fool who wishes to reverse the entire scheme of astronomy; but sacred scripture tells us that Joshua commanded the Earth to stand still, not the Sun.” Giordano Bruno ( ): burned at the stake for advocating that stars are suns in their own right, and that there is a plurality of worlds like the Earth.

v. 1.0 v. 2.0 v. 3.0 Cosmological Models: Version 1.0: “Superdome” model Version 2.0: Geocentric model Version 3.0: Heliocentric model

Which is “right”? Occam's Razor: Pluralitas non est ponenda sine neccesitate "plurality should not be posited without necessity." Given a set of otherwise equivalent models of a phenomenon, the simplest one is the best. Keep it simple, stupid.

English philosopher from Ockham Franciscan Monk Got into trouble with the Pope for advocating “apostolic poverty” Died of the Black Death while in exile Advocated Epistomological Parsimony c.f. Ontological parsimony Epistomology = theory of knowledge Ontology = what exists? William of Occam

1610 “Siderius Nuncius” (The Starry Messenger) “Spots” on the Sun; the Sun rotates The Moon has mountains, craters, rocky surface with imperfections The “planet” Jupiter is not a pinpoint star – but a disc in the sky WITH MOONS that orbit it Venus has “PHASES” like the MOON Galileo Galilei (1564 – 1642) Italian First to use a telescope to look at the sky

Spots on the Sun The Sun rotates Eventually went blind Galileo Galilei (1564 – 1642)

The Moon Has Mountains and Valleys Galileo Galilei (1564 – 1642)

Phases of Venus Galileo observed that Venus showed phases entirely like those of the moon from full to crescent, which it must do if the Copernican theory was correct. According to the Ptolemaic theory Venus would have to be a perpetual crescent. Galileo Galilei (1564 – 1642)

Observations consistent with Copernicus, inconsistent with Ptolemy.

Jupiter Has Moons Galileo Galilei (1564 – 1642) Galilean Moons – 4 Largest “moons” of Jupiter Io, Europa, Callisto, & Ganymede

1632: "Dialogo Dei Massimi Sistemi” Dialogue on the Two Chief World Systems In Italian, Not Latin -- For the common people Two people, one representing the view of Ptolemy and other the view of Copernicus, present their arguments before an intelligent layman. Galileo Galilei (1564 – 1642) "Eppur Si Muove“ "(And, yet it moves!") The Pope Urban II thought that Simplico, the character who upheld the views of Ptolemy in the book, was a deliberate and insulting caricature of himself 1633: Trial by Church (threatened with instruments of torture), forced to recant views

1633: Galileo is ordered to his house in Arcetri, under house arrest, where he remains for the rest of his life. The book remained on the index of forbidden books until 1822 Pope John Paul II finally gave an address that admitted that there had been errors in the treatment of Galileo by the church, in ASIDE: Contrast Galileo with Nicolas Steno ( ), who laid the groundwork for the interpretation of the fossil record. See The Seashell on the Mountaintop by Alan Cutler.

Modern Scientific Method 1. Observations and Experiments Data which describes the physical world 2. Theory, Hypotheses, Models: -> Organize Facts from experiment & observations Unifying principles Make testable predictions

Creationism and Intelligent Design vs. Evolution. The example of gravity as a scientific theory: Newton’s Law c AD Einstein’s theory of General Relativity c Photon versus wave description of light The most interesting scientific questions to work on for a professional scientist are those we don't know the answer to. Interesting theories to work on are those which fail to explain an observation or fact. Many modern astronomers are deeply religious people.