PHYS1142 1

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”

PHYS1142 2

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

PHYS1142 3

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

PHYS1142 4

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

PHYS1142 5

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...

PHYS1142 6

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 1o - but flawed. Used for next 1400 years. Adopted by Christian Church.

PHYS1142 7

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 (1473 - 1543) - 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

PHYS1142 8

Galileo’s Contribution (1564 - 1642)

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.