Announcements 100 students have not yet registered and joined our class on Astronomy Place. There will be more assignments on Astronomy Place. You need to register and join our class if you want to get credit for those assignments. Remember - assignments, homework and in-class activities count for 25% of your grade. They are easy points. The assignment for Wednesday is to be done on Angel. It is due by the beginning of class.

What have we learned? Earth is a planet in a solar system, which is one of some 100 billion star systems in the Milky Way Galaxy, which is one of about 40 galaxies in the Local Group, which is part of the Local Supercluster, which is part of the Universe. The Universe began in the Big Bang, which produced only two chemical elements: Hydrogen & Helium. The rest have been produced by stars, which is why we are “star stuff.” On a cosmic calendar that compresses the history of the Universe into one year, human civilization is just a few seconds old. The stars are far apart. If the Sun were a grapefruit, the nearest stars to the Sun would be thousands of kilometers away. The galaxies are not far apart. If the Milky Way were the size of the Earth, the far edge of our local group would be the Andromeda galaxy, another Earth located at the Moon. The edge of the visible universe would be near the orbit of Saturn.

What have we learned? Motions of “spaceship Earth” Earth SPINS on its axis towards the EAST once a day. Earth ORBITS around the Sun once a year The Solar System orbits around the center of the Milky Way galaxy once in 2.3x10 8 years. Galaxies in the local group orbit around one another. All other galaxies are moving away from us with the expansion of the universe. The farther away they are the faster they are moving away.

What have we learned? Phases of the Moon

What have we Learned? Eclipses The Earth & Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. Why don’t we have an eclipse every full & new Moon?

What have we learned? Appearance of the sky All astronomical objects rise in the East and set in the West Sun –day and night Moon –29.5 day cycle of phases Planets –usually move toward East with respect to Stars –occasionally move westward retrograde motion Stars –see different stars in different seasons

Objectives: (from syllabus) Describe how scientific thinking is similar to everyday thinking Describe how the Ptolemaic, Copernican and Keplerian models of the solar system explain retrograde motion Describe the development of the models. Describe the observational evidence for and against the different models Compare and contrast the models in terms of geometry, physics, simplicity and prediction Evaluate the impact of the models on our concept of the universe

Science? Similar to everyday thinking Problem solving Organized trial and error

Scientific Process Have problem Make Model = Theory to explain problem –Goal: explain a wide variety of observations in terms of a few general principles Test Theory or Model –Make new predictions –Check predictions against new observations

Scientific Process

Model of Solar System - What is the problem? Problem: What produces observed motions 1)Daily rising and setting 2)Motion of planets, both eastward with respect to stars and westard -> “retrograde motion”

Geocentric Model Earth stationary at center of universe All heavenly bodies whirl around the Earth once a day -> daily motion Planets move with respect to the stars

How does one explain retrograde motion? Over a period of 10 weeks, Mars appears to stop, back up, then go forward again. Movie. Click to play.

Planetary Motion Usually planets appear to move toward the East re the stars. Occasionally, they appear to move backward - toward the West, called Retrograde Motion

Retrograde Motion View from Earth: explained by big circles + epicycles Epicycle, rolls on big circle. Explains retrograde loop. big circle=‘deferent’. Explains most of planets motion

Ptolemy’s Geocentric Model Earth is at center Sun orbits Earth Planets orbit on small circles whose centers orbit the Earth on larger circles – [the small circles are called epicycles]

Copernican Model ( ) Earth is not the center of the universe - the Sun is - HELIOCENTRIC Planets move on perfect circles New explanation of retrograde motion –Earth laps another planet Observations fix order of planets –always see Venus and Mercury close to Sun –speed of planets motion with respect to stars slower for more distant planets

Heliocentric Model

Retrograde Motion Like the Sun, planets usually appear to move eastward relative to the stars. But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months. Noticeable over many nights; on a single night, a planet rises in east and sets in west…

Apparent retrograde motion — try it yourself!

Predictions of Heliocentric Model (1) Parallax (2) Phases of Venus

Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun Try it yourself - look at your thumb at the end of your arm first with one eye and then the other, against the screen at front of the room.

Why did the ancient Greeks reject the notion that the Earth orbits the sun? It ran contrary to their senses. If the Earth rotated, then there should be a “great wind” as we moved through the air. Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

Possible reasons why stellar parallax was undetectable: 1.Stars are so far away that stellar parallax is too small for naked eye to notice 2.Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1). It takes a telescope to see the parallax of stars. First observed in 1838.

Phases of Venus Galileo ( ) observed Venus Geocentric Model - Venus always between Earth and Sun, never see it Full. Heliocentric Model - Venus Sometimes on far side of Sun From Earth - see it Full.

Heliocentric Model Problems –Why do objects fall straight down on spinning Earth –How is atmosphere carried along with moving Earth Appeal –Simple –More universal, Earth like other planets Revolutionary (word comes from title of Copernicus’ book - “On the Revolutions of the Heavenly Spheres”) –Earth not unique!

Galileo’s other Observations Moons of Jupiter orbiting around Jupiter - stay with Jupiter as it moves Heavenly objects not perfect spheres –Moon has mountains –Sun has dark sunspots

New Data Tycho Brahe ( ) observed the positions of the planets with much better accuracy than previously Hired Kepler ( ) as research assistant to prove his own geocentric model was correct Brahe died shortly after hiring Kepler. Kepler believed the heliocentric model and tried to fit it to Brahe’s excellent data.

Keplerian Model Planet orbits are ellipses Planets move with varying speed around orbit (faster when closer to the Sun) P 2 (years) = a 3 (AU) Kepler could not fit Brahe’s observations (within the accuracy of those observations) with circular orbits. Thought up new model: Kepler described planet motion, did not explain it. Need to know why!

Keplerian Model Tests –Derived for Mars, predicted positions of other planets, compared with their observed positions. Appeal –Accuracy much better –Universality: all orbiting bodies have same properties Philosophy –Not circular, varying speed -> not natural motion –Need a FORCE

Keplerian Model

Assignment for Wednesday Wednesday - Chapter 4 Matter & Energy Begin Chapter 5 Newton’s laws Make sure you get homework due Wednesday Jan 28. This homework is to be done on Angel. (If you miss a Wednesday class, homework is linked to the course web page