TODAY: From The Sun’s Perspective: Aristarchus

Occam’s RazorThe Copernican Theory

Giordano BrunoBrahe and Kepler

Kepler’s 3 Laws of Planetary Motion

THE HELIOCENTRIC SYSTEM OF ARISTARCHUS We have no written record of this system by Aristarchus himself.--all his works (except the one mentioned above) were lost when the library in Alexandria was burned.

Written accounts of this system are found in "The Sand Reckoner" by Archimedes (287-212 BC) and "On the Face of the Moon Disc" by Plutarch (46-120 AD).

Features: 1. Planets orbit the sun, explaining retrograde motion 2. stellar parallax not seen because stars are at great distance.

William of Occam ~1200 AD

OCCAM’S RAZOR—a simpler workable theory is a better one.

Nicolas Copernicus--De Revolutionibus--1543

Features of his heliocentric model:

1. Planets still on spheres, but centered on sun.2. Stars still on a sphere, but more distant than

in Ptolemy's model.3. Planets in proper order from sun.4. Planetary distances calculated--close to

modern values.

Planetary Distance (in AU):Copernicus vs Modern

Mercury 0.38 0.387Venus 0.72 0.723Earth 1.00 1.000Mars 1.52 1.524Jupiter 5.2 5.203Saturn 9.2 9.539

Giordano Bruno--burned at stake in 1590--Universe is infinite!--Stars are suns with planets and beings on them.

Tycho Brahe and Johannes Kepler

Tycho Brahe--1546-1601--Observed planetsvery accurately--found orbits--especially Mars--were not circular as Copernicus still held.

Brahe’s Observatory at Uraniborg, Denmark

Johannes Kepler (1571-1630)--Found the trueshape and motions of planets about the sun fromBrahe's data.

Kepler's 3 Laws of Planetary Motion:1. Planets move in elliptical orbits with the sunas a focus.2. A line from the planet to the sun sweeps outequal areas in equal times. Thus planets andcomets move more slowly farther from the sun.3. P2= a3. P = planetary period--time for 1revolution. a = avg. distance from sun =semimajor axis (in AU)

Kepler’s First Law: Planets move in ellipses.

Kepler’s 3rd LawP = period, a = avg dist. to sun = semi-major axis.

Eccentricity: distance between foci/widest dist.

Planetary orbit eccentricities—how stretched is orbit?

Mercury .206

Venus .0068

Earth .0167

Mars .0934

Jupiter .0485

Saturn .0556

Uranus .0472

Neptune .0086

Pluto .25