Unit1: The Physics of Astronomy

This Week & Next: Astronomy in Motion

•Kepler’s Laws•Newton’s Laws•Work & Energy

General Note: Please ASK if there are terms/math/physics that are new to you!

Today: Historical Background & Basic Refresher

Why is Astronomy Different?

Astronomy is the Science of Observation rather than Experiment

X

What are the Most Basic Observable Quantities?

Light

Motion

What were the first objects routinely observed?

When?

A Little History...Astronomy is an ancient science

•Maya•Aztec•Egyptians•Chinese•Greek

II. Early Scientific Astronomy

Plato (350 BC):Geocentric Model of SS

-planets & Sun all orbit a stationary Earth - orbits are perfect spheres

vs.

Aristarchus (270 BC):First Heliocentric Model -planets orbit the Sun

-spherical orbits

REJECTED!X

Ptolemy (100 AD): added epicycles to better fit observed motion of planets

Almagest“The Great Treatise”

II. Early Scientific Astronomy

Medieval Manuscript, Melk Abbey (Austria):Geocentric Solar System!(pic fr. 4/17/09)

15th Century!

15th Century!

BUT–Predicted

locations of planets, by late 1400s PROVED WRONG

Medieval Manuscript, Melk Abbey (Austria):Geocentric Solar System!(pic fr. 4/17/09)

Copernicus (1473 - 1543): On the Revolution of Heavenly SpheresNew Heliocentric Model

BUT...

-still Spherical-contained epicycles

III. Groundbreakers

Tycho Brahe (1546-1601):

Best set of astronomical positions to date

III. Groundbreakers

I sez Copernicus not gud yet - no cheezburgers

– * Incredible observer– naked-eye observations– Planetary & stellar positions

good to <1’– Duel: who is a better

mathematician????

Johannes Kepler (1571-1630)

• Recognized need for model to match Tycho’s observations

•New laws of Planetary Motion (1609)

What are Kepler’s 3 Laws of Planetary Motion?

III. Groundbreakers

Kepler’s Laws of Planetary MotionK1: Planets orbit the Sun in an ellipse, with the Sun at one focus of the ellipse

K2: A line connecting a planet to the Sun sweeps out equal areas in equal time intervals

K3 (Harmonic Law): P2∝a3

The squares of the sidereal periods is proportional to the orbital semi-major axis cubed

Isaac Newton (1642-1747)Principia (1687)

Galileo (1564-1642)

The Physics of Motion!

Newton’s Laws of Motion

N1(Law of Inertia): An object at rest will remain at rest and an object in motion will remain in motion in a straight line at constant speed unless acted upon by an outside force

N2: The net force on an object is proportional to the object’s mass & acceleration

N3: For every action there is an equal and opposite reaction

Newton

What is the physical explanation for Kepler’s Laws of planetary motion??

Derivation of Universal Law of Gravity

What is the Nature of this Force???