Announcements

First MIDTERM Exam: Mon, Sep. 22, covering chapters 1–3 + tutorials (review later today).

Interim grades online, coded by class ID. See course website “Grades” tab.

Another great night for observing! Meet in the lobby outside class around 9pm. Bring your blue ticket! Friends/relatives welcome.

APOD/2008–09–17

Last TimeSCIENCE: A human endeavor characterized by the hallmarks of 1) reliance on natural causes, 2) “Occam’s Razor”, 3) Verifiability, and 4) Falsifiability.

Planets move on the celestial sphere (as does the moon/sun). 5 “visible eye” planets: mercury/venus/mars/jupiter/saturn.

Planets normally “slip” W to E (“prograde”: lazy planets!). Sometimes move E to W (“Retrograde”). A mystery to the ancients (who assumed earth-centered universe!).

Important distinction: Diurnal motion: E to W (rise, transit, set), due to earth’s rotation. Motion on the celestial sphere (much slower, W to E “slipping”).

Last Time

Greeks took the celestial sphere “literally”. One sphere needed for each wandering object (sun, moon, + 5 planets), in addition to the “fixed stars”.

Problem: couldn’t explain retrograde motion.

Solution (Ptolemy): small circles upon large (offset) circles.

Last Time

Greeks knew the earth is round as early as 500BC, and estimated the circumference using angles of the sun from different cities separated by hundreds of km.

Rejected earth moving around the sun, since stars suffered no “parallax” (angular shift of foreground compared to background objects).

The Foundations

Ancient structure in many cultures marking progression of sun, moon, and planets.

Astronomy BeginningsAristotle (~400BC):

Earth is spherical (simple/perfect objects), and at center of the universe.

Shape of earth’s shadow during lunar eclipse.

Ships sailing over horizon.

RefinementsPtolemy (100-170 AD):

Greek astronomer in Alexandria, Egypt

Circular paths (some offset), add “epicycles” — circles upon circles — to produce retrograde motion.

Good within a few degrees,but very complex!

Model used for 1500 years!

Parallax (Again)

Earth moves around sun ➠ nearby stars “shift” compared to background stars due to “parallax”.

The lack of apparent parallax convinced greeks that earth must not move.

In reality, stars distances are so great, their parallax is too small to see with the eye.

Testing – the key to science

Greeks

Rational thought was sufficient

Inconsistencies blamed on faulty perception

Modern Science

Models make predictions

Verification provides support for model

A “Theory” is a generally accepted model that explains many different observations and has withstood extensive testing of its predictions

The Troubled Ages

Most astronomical records destroyed with loss of library of Alexandria.

Greek/Roman empires fall.

Islamic scientists kept record and further developed models during the dark ages.

Renaissance: Christian Church adopts geocentric cosmology.

Revolution

Copernicus (1473-1543):

Polish scientist.

Studied inaccuracies in ptolemaic model.

Better model, sun-centered, but kept “perfect circles”.

A true revolution, but not widely adopted.

InvestigationTycho Brahe (1546-1601):

Understood that better data were needed.

Three decades of naked eye observations (good to 1 arcmin): built an excellent data set.

Still could not see parallax.

Model of sun orbiting earth, other planets orbiting sun (not adopted).

Eureka

Johannes Kepler (1571–1630):

Hired by Tycho to make sense of the data.

Found a “new” ptolemaic model with circles, but 8 arcmin discrepancies remained.

Trusted the data... (but still clung to ideals of “perfection”).

Keplerian Model

Kepler’s First Law: The orbit of each planet around the Sun is an ellipse, with the sun at one focus.

Keplerian Model

Kepler’s First Law: The orbit of each planet around the Sun is an ellipse, with the sun at one focus.

Finite

Galileo Galelei (1564–1642):

Italian scientist, overturned final objections to Kepler’s models.

1. Earth moves, birds fall off.

2. Non-circular orbits not perfect.

3. No stellar parallax, so earth can’t move.

Galileo’s Impact

Experiments with rolling balls: Objects in motion stay in motion unless otherwise acted.

Universe is imperfect. Using one of the first telescopes, he observed spots on the sun, and crates and valleys on the moon.

Galileo’s Sunspots

Galileo’s ImpactUsing one of the first telescopes, observed Moons clearly orbiting jupiter (not the earth!)

Galileo’s ImpactSaw phases of Venus.

Geocentric: only crescent phases.

Heliocentric: All phases.

Galileo’s ImpactShowed that stars could be at great distances.

Milky way resolves into many stars in a telescope.

Galileo’s Price

Catholic church brought galileo before the roman inquisition.

Forced to recant his claim that earth orbit’s the sun.

Church formally apologized in 1992.

Mid-Term Exam #1 Review

Chapters 1–3 (skip Kepler’s 2nd and 3rd laws).

Workbook Exercises.

General Physical concepts stressed.

Applied understanding of motions of earth/moon/sun/stars.

Historical context, not date of birth/favorite breakfast cereal/etc.

Suggestions

Review “Big Picture”, “Key Concepts” and “Review Questions & Problems” at end of each chapter.

Better way: use the “Study Area” on mastering astronomy (nothing here is graded!).

Know how to apply concepts.

Concepts Covered

Relative scales of the Universe.

Things in the universe: planet, moons, comet, asteroid, star, solar system, galaxy, clusters.

Age of the universe.

Concepts CoveredConstellations

The celestial sphere.

Diurnal motions of the stars (rise/set/transit).

What stars we see based on our latitude on earth. “Circumpolar” stars.

Our system of Time, based on the sun’s position!

Concepts CoveredAngular Sizes: sun, moon, angles vs. distance.

Motions on the celestial sphere (why?).

Sun’s motion on the celestial sphere (as a consequence of earth’s orbit around the sun).

Sidereal vs. solar day.

Ecliptic: circular path of sun through

Concepts Covered

“Special” locations on the ecliptic = special times of year: solstices, equinox.

Tilt of the earth’s axis compared to plane of orbit, and the reason for seasons. Precession of the earth’s axis.

Explain the movement of the sun from S to N and back again through the year.

The Moon

Motion of the moon on the sky.

Orbit of the moon around earth.

Phases of the moon (and when you see them!).

Eclipses

Lunar vs. solar eclipses.

Different types of eclipses.

Why aren’t eclipses more common?

Near coincidence of moon’s and sun’s angular sizes

Planetary motion

Where planets are found on the sky (“the zodiac”).

The 7 “wandering” objects on the sky.

Retrograde motion: what causes?

Normal “slippage” motion of planets on the celestial sphere.

History

Greek explanation of planetary motions.

Geocentric vs. heliocentric models of the solar system.

How retrograde planetary motions were explained.

The hallmarks of modern science.

History

Earth is round. Size estimate size from sun’s position.

Parallax, and its importance in greek theories of the solar system.

Ptolemaic model, Copernican model, Tycho Brahe, Kepler, Galileo.

Reminders

First MIDTERM Exam: Mon, Sep. 22

Grades online now.

Observing open through Thursday: looking good all week!

Still need tickets? Come see me.