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Unit VI: AstronomyThe night skyWhat is astrology?Carl Sagan video clipBill Nye video clip

2Ancient and Pre-Modern Theories of the Universe/Solar SystemAristotles theory of four elementsAstronomy and Astrology existBut Heavens are considered to be a separate realm from earthly objects3Our Planet and Solar System:Two Competing Theories

4Geocentric vs. Heliocentric ModelsGeocentric ModelPlaces Earth at the center of the universeObjects in Heavens (planets, Sun, stars) are fixed crystalline spheres, nested around EarthBig achievement: it explained the motion of the Sun, moon, planets, and stars.Established c. 500 B.C.

Based on:ObservationsBelief that the heavens must be geometrically perfect.Ptolemy c. A.D. 100-170He was a Roman citizen that lived in Egypt and wrote in Greek.Combined others work and created a quantitative modelModel was fairly accurate in making predictions (within ~5-10 degrees of arc)

Did you record how to estimate angular sizes in your notes? However, some planets exhibited a really strange thing. Sometimes, they turned around a went backwards

Ptolemy used circles within circles to account for this retrograde motion: when planets appear to go backwards.Use the bike wheel and a laster pointer to demonstrate this. 12Problems with the Geocentric ModelCircles within circles inelegant (or imperfect)Didnt explain phases of the moonPredictions werent perfectProblems were not considered big enough to switch to an alternative theory

Nicholas Copernicus (1473-1543)Used a heliocentric model of the solar systemSurprisingly, not new. Proposed by a Greek astronomer Aristarchus in 260 B.C.

Nicholas Copernicus (1473-1543)Copernicus applied math to the data (the data wasnt all that good)Even though more correct, still assumed perfect circlesPredictions werent much better than the geocentric model.

Tycho Brahe (1546-1601)Took really good data and tons of it.

Tycho was convinced other planets orbited the Sun, but concluded (b/c he couldnt detect stellar parallax) that Earth must remain stationary.

Parallax: the apparent movement of an object due to your point of view moving, rather than the object itself moving.Tycho Brahe (1546-1601)Was funded by the king of Denmark and then the German emperor.Died without knowing whether he had made any breakthroughs.

Johannes Kepler (1571-1630)Worked for TychoApproached the data mathematically; assumed circular orbits based on the idea the heavens are perfect.

Johannes Kepler (1571-1630)After 8 years, found a mathematical model for a circular orbit of Mars that almost matched Tychos observations. There was a difference of 8 arcminutes.

Johannes Kepler (1571-1630)A minute of arc is 1/60 of a degree of arc. This is like the thickness of your fingernail when held at arms length.Remember, Ptolemys geocentric was off by ~5-10 of arc

How Science WorksGuess Compare to Experiment/ExperienceCompute ConsequencesJohannes Kepler (1571-1630)How do they determine if this is an error in the model or in the data?How do you decide if an error is significant?

Johannes Kepler (1571-1630)How would they know if this is correct???Remember, no one at this point in time knows the answer.

His aha moment: If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy. - Johannes KeplerKeplers decision to trust the data over his preconceived beliefs marked an important transition point in the history of science.Johannes Kepler (1571-1630)Even though model was really close, he wasnt willing to accept it.Tried ellipses instead of circles and the new model made better predictions.

Many scientists continued with the geocentric model for the following reasons:

Earth couldnt be moving, otherwise birds and thrown rocks would be left behind.

Non-circular orbits meant heavens werent perfect

Stellar parallax still wasnt detectable, so the Earth cant orbit the Sun.

Galileo Galilei (1564-1642)Demonstrated Newtons 1st law before Newton was even born (in 1642)Firm believer in scientific inquiryRemember our pendulum experiment?

Galileo Galilei (1564-1642)Used a telescope to look at the heavens. Galileo made observations that refuted the geocentric model.Sunspots

Mountains & craters on the moon

Moons of Jupiter

Galileo Galilei (1564-1642)Saw sunspots on the sun so now the Sun isnt perfect

Galileo Galilei (1564-1642)Saw mountains and craters on the moonSo the Moon isnt perfect either

36Galileo Galilei (1564-1642)Discovered that Jupiter has moons that orbit itClearly dont revolve around the Earth

The imperfections just pile up. . .

Galileo Galilei (1564-1642)Aristotle and Ptolemys doctrines considered part of Catholic Church doctrine, so not open to question.Galileo persecuted by church for publishing his results late in life

Summary of Scientists & IdeasPtolemyCopernicusTychoKeplerGalileoRetrograde motionParallax