Last time: 2.1….. celestial coordinates2.2 Seasons,…Precession2.2 (cont.). 2.3. The Moon and Eclipses

Lunar Phases and Eclipses

Today:2.4 Ancient Mystery of the Planets:

Apparent Retrograde motion of planets

ParallaxCh 3 (Histrory of Astronomy)

Class 4 (Sept 2, 2010): End of Ch 2 and Ch. 3

2.4 The Ancient Mystery of the Planets

• Why do planets sometimes seem to move backwards relative to the stars?

Apparent retrograde motion — try it yourself!

Retrograde Motion

• 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

Explaining Apparent Retrograde Motion

• Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us)

• But very difficult to explain if you think that Earth is the center of the universe!

• In fact, ancients considered but rejected the correct explanation…

Why did the ancient Greeks reject the notion that the Earth orbits the

Sun?

• It ran contrary to their senses.

• Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

Parallax AngleApparent shift of a star’s position due to the Earth’s orbiting of the Sun

The nearest stars are much farther away than the Greeks thought.

So the parallax angles of the star are so small, that you need a telescope to observe them.

Possible reasons why ancient Greeks could not detect stellar parallax :

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 ancient Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)…

Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

Chapter 3The Science of Astronomy

(I prefer to call it History of Astronomy)

Ch 3 (Histrory of Astronomy)1. Ancient Roots of Science (Archeoastronomy)2. Astronomy of Greece: good records & more

rational approacha. Thales and Pithagoras: roots of scienceb. Plato and Aristotle: Geocentric universec. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution4. The Nature of Science

Ch 3 (Histrory of Astronomy)1. Ancient Roots of Science (Archeoastronomy)2. Astronomy of Greece: good records & more

rational approacha. Thales and Pithagoras: roots of scienceb. Plato and Aristotle: Geocentric universec. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution4. The Nature of Science

Histrory of Astronomy is more like a Soap Opera than a Sitcom

Ch 3 (Histrory of Astronomy) “Does the World Turn?”1. Ancient Roots of Science (Archeoastronomy)2. Astronomy of Greece: good records & more

rational approacha. Thales and Pithagoras: roots of scienceb. Plato and Aristotle: Geocentric universec. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution 4. The Nature of Science

Ch 3 (Histrory of Astronomy) “Does the World Turn?”1. Ancient Roots of Science (Archeoastronomy) 2. Astronomy of Greece: good records & more

rational approacha. Thales and Pithagoras: roots of scienceb. Plato and Aristotle: Geocentric universec. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution “The Church Strikes back”

4. The Nature of Science

How did astronomical observations benefit ancient societies?

• Keeping track of time and seasons– for practical purposes, including agriculture– for religious and ceremonial purposes

• Aid to navigation

3.1 The Ancient Roots of Science(Archeoastronomy)

What did ancient civilizations achieve in astronomy?

• daily timekeeping • tracking the seasons and calendar• predicting eclipses• and more…

3.1 The Ancient Roots of Science(Archeoastronomy)

Days of week were named for Sun, Moon, and visible planets

England: Stonehenge (completed around 1550 B.C.) was used as an observatory

Yucatan, Mexico: Mayan Observatory at Chichen Itza

3.2 Ancient Greek Science

• Why does modern science trace its roots to the Greeks?

• How did the Greeks explain planetary motion?

• How did Islamic scientists preserve and extend Greek science?

Our goals for learning:

• Greeks: good records and more rational approach than previous civilizations

• were the first people known to make models of nature.

• They tried to explain patterns in nature without resorting to myth or the supernatural.

Greek geocentric model (c. 400 BC)

Why does modern science trace its roots to the Greeks? (some not in book)

• Thales and Pithagoras: roots of sciencea. Thales: Universe is rational (our mind can

understan it)b. Pithagoras: nature is governed by musical

(mathematical) principles • Plato and Aristotle: Geocentric universe

⁻ Earth at center of universe⁻ Earth is imperfect Heavens are perfect⁻ “uniform circular motion” is perfect. Hence, heavenly

bodies must follow uniform circular motion• Ptolemy: mathematical models of geocentric views

(sufficiently accurate to remain in use for 1,500 years)

Prominent Ancient Greeks (some not in book):

PlatoAristotle

Ptolemy

Ptolemy: mathematical models of geocentric views, but this made it difficult to explain apparent

retrograde motion of planets…

So how does the Ptolemaic model explain retrograde motion?Planets do go backward in this model…

Ptolemy’s Model

What have we learned?•Why does modern science trace its roots to the Greeks?

Greeks: good records and more rational approach than previous civilizations.

•How did the Greeks explain planetary motion?

The Ptolemaic model had each planet move on a small circle whose center moves around Earth on a larger circle.•How did Islamic scientists preserve and extend Greek science?While Europe was in its Dark Ages, Islamic scientists preserved and extended Greek science, later helping to ignite the European Renaissance.

Much of our mathematical and scientific heritage originated with the civilizations of the Middle East

Histrory of Astronomy PART II: “The Church Strikes Back”

1. Problems with Ptolemy’s models2. Copernicus: publishes heliocentric model and

dies3. Tycho observes planetary motions and dies4. Kepler: uses Tycho’s observations & writes 3

laws (on pages 67-69 of book):a. Each planet moves in ellipse with the Sun at one focus.b. The line between the Sun ……. (faster near Sun and vice versa)c. p2 = a3

5. Galileo:a.Uses telescope to discover moons of Jupiter, study sunspots and phases of Venus b.Supports Copernicus’ theory & gets in trouble with Church (bad boy! go to your room and stay there till you die)

3.3 The Copernican Revolution

Copernicus’ Heliocentric Model

•With better observations more problems with the Ptolemaic model became apparent…..and created more complex fixes • Copernucus’ solution was to put the Sun at the center. •The math was much simpler in Copernucus’ solution

Copernicus’ Model

Ptolemy’s Model

Copernicus’ Heliocentric Model

•Sun is at center•Earth orbits like any other planet•Inferior planet orbits are smaller•Retrograde motion occurs when we “lap” Mars & the other planets

Occam’s Razor• Among competing theories

(otherwise equal), the best theory is the simplest theory-that is, the one with the fewest assumptions.

• Heliocentric model is much simpler than Ptolemy’s model

Tycho Brahe • Father of observational astronomy.

• Danish nobleman and Scientist. Lost his nose in a duel…. (ask me how he died)

• He had a young assistant, Johannes Kepler

Johannes Kepler • German astronomer and

mathematician• Used Tycho’s observations to

write 3 laws of planetary motion

• He was deeply religious and actually strongly believed in the predictive power of astrology (interesting characters in our soap opera, don’t you think so?)

Kepler’s Laws (on page 67-69 of book)

1 Each planet’s orbit around the Sun is an ellipse, with the Sun at one focus.

Kepler’s Laws2 A planet moves along its orbit with a speed that

changes in such a way that a line from the planet to the Sun sweeps out equal areas in equal intervals of time. (the closer to the Sun, the faster it moves)

Kepler’s Laws 3 The ratio of the cube of a planet’s average

distance from the Sun to the square of its orbital period is the same for each planet.

a3 / P2 = 1

a3 = P2

a in AU

P in years

Johannes Kepler(1571-1630)

• Kepler first tried to match Tycho’s observations with circular orbits

• But an 8 arcminute discrepancy led him eventually to ellipses…

“If I had believed that we could ignore these eight minutes [of arc], I wouldhave 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.

Galileo Galilei (1564-1642)• First man to point a

telescope at the sky• wanted to connect

physics on Earth with the heavens

• Wrote the book “Dialogue Concerning the Two Chief World Systems” .

This book got him in trouble with the Church!Go to your room!

Galileo’s Observations• Galileo discovered

that Jupiter had four moons of its own.

• Jupiter was the center of its own system.

• Heavenly bodies existed which did not orbit the earth.

Galileo’s observation of the phases of Venus was the final evidence which buried the geocentric model.

Geocentric Heliocentric

No gibbous or full phases! All phases are seen!

Galileo observed all phases!

Galileo Galilei

The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in 1633

His book on the subject was removed from the Church’s index of banned books in 1824

Galileo was formally vindicated (pardoned) by the Church in 1992 (ask me if you want to know more juicy details)

What have we learned?• How did Copernicus, Tycho and Kepler challenge

the Earth-centered idea?• Copernicus created a sun-centered model; Tycho

provided the data needed to improve this model; Kepler found a model that fit Tycho’s data.

• What was Galileo’s role in the Copernican revolution?

• His experiments and observations overcame the remaining objections to the Sun-centered solar system

Ch 3 (Histrory of Astronomy) “Does the World Turn?”1. Archeoastronomy2. Astronomy of Greece: good records & more rational approach

a. Thales and Pithagoras: roots of scienceb. Plato and Aristotle: Geocentric universec. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution “The Church Strikes back”

4. The Nature of Science

3.4 The Nature of Science

Our goals for learning:How can we distinguish science from

nonscience?

I. Science:1. Based on observations and theory2. Open to criticism and constant challenge in

light of new evidence3. Not perfect (human), not always right, but is

the most successful discipline at predicting the way nature works.

II. Pseudoscience1. Generally holds “absolute truth”2. Considers only part of evidence available 3. Tends to play on emotions and fears instead of

logic

Science and Pseudoscience (not exactly like the book)

• Science seeks explanations that rely solely on natural causes.

• Science progresses by creating and testing models of nature that explain observations as simply as possible: Occam’s Razor

• A scientific model must make testable predictions that could force us to revise or abandon the model.

• Scientific Theory: a model which survives repeated testing

Hallmarks of Good Science

Ancient Astronomers were also Astrologers (e.g., Kepler)

Astrology is not a science Empirical Discipline (no theoretical

explanation) Easily abused by charlatans Big problem: astrologers do not agree on

what any given celestial configuration means

In other words, who will you believe if one astrologer tells you this is a great time to travel and another tells you to stay home?

Astrology (not exactly like book)