Models of the Solar System, Gravitation and the motion of the Planets A.K.A DEAD WHITE GUYS WEEK! 1/28/14

Cosmogony A cosmogony is theory about ones place in the universe. A geocentric cosmogony is a theory that proposes Earth to be at the center of the universe. A heliocentric cosmogony is a theory that proposes the Sun to be at the center of the universe.

Which is the geocentric cosmogony and which is the heliocentric cosmogony? geocentric (Earth-centered) heliocentric (Sun-centered)

Pythagorean Universe time: about 550 bc first recorded attempt at rational explanation of heavens formed foundation for Greek astronomy

Philosophical Ideas The heavens represent perfection. The heavens are immutable. The circle is the perfect shape. All heavenly motions must be circular.

Pythagorean Model stars reside on stellar sphere stellar sphere rotates about axis once a day Sun follows path called ecliptic going around Earth once in one year accounts for most observations!!...but not all

BUT - THERE WAS A COMPLICATION. There were stars that appeared to move across the sky without staying in their respective constellations WANDERING STARS (Greek word for wanderer is planete`)

Prograde and Retrograde Motion Prograde Motion (normal motion) The apparent West to East motion of objects (over many nights) as compared to the stationary background stars. Retrograde Motion - The apparent East to West motion of objects (over many nights) as compared to the stationary background stars.

Planets were often called wandering stars because they seem to slowly move from one constellation to the next. East South West

The astronomer must try his utmost to explain celestial motions by the simplest possible hypothesis; but if he fails to do so, he must choose whatever other hypotheses meet the case. -Ptolemy of Alexandria (140 A.D.)

Ptolemy tried to create a model that would account for retrograde motion.

Ptolemy s Epicycles Epicycle Deferent Earth

Ptolemy s Epicycles

Where is retrograde motion occurring? Planet s Path Ptolemy s Epicycles

Ptolemy s Epicycles Where is retrograde motion? Planet s Path

Mars retrograde Motion (Ptolemic View)

On the Ptolemaic system: A system of this sort seemed neither sufficiently absolute nor sufficiently pleasing to the mind. --Copernicus Copernicus developed the first Sun centered model of the Solar System or Universe. He was a priest and a lawyer.

Mars retrograde motion (Copernican View) 53

Mars retrograde motion (Copernican View)

To demonstrate that the appearances are saved by assuming the Sun at the center and the Earth in the heavens is not the same thing as to demonstrate that in fact the Sun is in the center and the Earth in the heavens. I believe that the first demonstration may exist, but I have grave doubts about the second. --Cardinal Bellarmine (1615)

Tycho Brahe (1546-1601) Had artificial wooden and silver noses (lost his nose in a duel) Rumored to have died when his bladder burst The world s best naked eye astronomer - he did not use a telescope!!

Brahe s observations of Supernova Brahe observed SN 1572 Proved that this object was in the realm of the stars -- the heavens do change!

Tycho showed the Supernova was in the realm of the heavens using something called PARALLAX

Scientists use parallax to measure distances.

Parallax Brahe proved the SN was not in Earth s atmosphere by parallax When viewing an object from different positions, one sees the object in different positions relative to things behind it The variation in angle when viewing an object from two different positions is called parallax The further the object, less parallax

Parallax Angle 62

Parallax If distance is measured in parsecs then d = 1 PA Where PA is the parallax angle, in arcsec NOTE: The distance from the Sun to the Earth is called the Astronomical Unit (AU) A parsec=206265 AU 63

Lecture Tutorial(s): The Parsec (p. 37) Parallax & Distance (p. 41) 64