PHYS Create a MODEL (metaphor) Geometry Physics Aesthetics Compare observations with predictions of model Revise to improve match with observations Model too complex and ungainly Throw out in favour of more acceptable model Observe the “real world”

PHYS Beginning of the Scientific Method a) Hipparcus “measures” apparent brightness of stars b) A model of the Universe is produced - geocentric c) Stars placed on the “celestial sphere” d) Sun traces out a path against the stars - the ecliptic e) Earth considered to be fixed at centre of Universe, now measure rotation using a Foucault pendulum f) No attempt to take into account the observed precession of the Earth’s axis

PHYS Observations to be explained by the model a) The path of the Sun on the sky throughout the year - the ecliptic Figs. Z1.14 & K1-17 b) Path of the planets and retrograde motion Figs. Z1.20 & K1-36 c) Daily rotation of the stars across sky d) Absence of stellar parallax Figs. Z2.4 & K2-2

PHYS Early Greek models of the Universe (Pythagoras, Aristotle and others) Based on: a) Observations of the Heavens b) Concepts of Beauty and Symmetry (Aesthetics) c) Little physics input, religion important d) Everything revolves around the Earth at a uniform speed GEOCENTRIC MODEL

PHYS Geocentric Model Explains: a) Daily motion of the Sun, stars and planets across the sky b) Annual motion of the Sun with respect to the stars c) Long term motion of the planets with respect to the stars d) Observed absence of parallax Does not explain: a) Retrograde motion b) Variation of retrograde motion Needs a fundamental change...

PHYS Later Greek Models of the Universe (Hipparchus, Ptolemy and others) a)Add the Eccentric - observed variable speeds of planets explained but planets still move with constant speed Fig. Z2.5 b)Add Deferent and Epicycle - retrograde motion explained Figs. Z2.6,2.7 & K2-5 c)Add the Equant - explained variation in retrograde motion Figs. Z2.9 & K2-19 Celestial motion no longer had to be uniform around the centres of circles - broke fundamental assumption of model! Model is becoming increasingly complex. Best available model - position predictions accurate to about 1 o - but flawed. Used for next 1400 years. Adopted by Christian Church.

PHYS Heliocentric Models Aristarchus 3rd century BC Proposed that Earth rotated on its axis and moved around the Sun once a year. Theory rejected as it suggested that the Earth moved and it required stellar parallax. Copernicus ( ) - recaptures simplicity a)Earth and planets move around the Sun in a circular orbit at constant speed b)Earth spins on its axis c)Retrograde motion explained Figs. Z3.4 & K2-16 d) Explains Sun’s motion against the stars Figs. Z3.3 & K2-14

PHYS Galileo’s Contribution ( ) First to use the telescope for astronomy. Discovered mountains and craters on the Moon, therefore Moon is Earth-like and not a perfect celestial body. Discovered the moons of Jupiter, suggesting that Earth, like Jupiter, was just a planet. Saw all of the phases of Venus - not possible with a geocentric model. Figs. Z4.5 & K3-6 Observed sunspots - defects on the surface of the supposedly perfect Sun. Placed under house-arrest for his ideas.