1. The Solar System Copernicus (1473)
equant and doubling of Moon - disturbing
revived Aristarchus' heliocentric model
De Revolutionibus Orbium Coelestium (1543)
Sun at centre, all planets orbit Sun, Moon orbits Earth
stars are on CS, at rest
Earth rotates
Mercury, Venus, Earth, Mars, Jupiter, Saturn
same geometric tools as Ptolemy (no equant)

2. retrograde motion: result of relative motion of Earth and planets
no savings in complexity
same predictions as Ptolemy
predicts periodic shift of stars
- not observed
preface implied it was not to be taken seriously
banned 1616

3. Tycho Brahe (1546) Alfonsine and Prutenic tables disagreed
new star (nova) 1572, lack of motion meant it was on CS
more accurate predictions require better observations
prolific instrument maker
repetition of measurements
calibration
Uraniborg (1576)

4. accuracy of 1 arcminute (1)
1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
1 =  = 
1577 – comet path crosses other planets' orbits - solid crystal spheres do not exist
lack of parallax implies large distances to stars in heliocentric model
preferred geocentric model
hybrid model – Earth at centre, Moon and Sun orbit Earth, other planets orbit Sun
1599, Prague – hires Kepler to analyse data

5. Celestial Coordinates
Declination (dec.) – latitude
angle above the celestial equator (degrees)
Right Ascension (RA)– longitude
measured eastward from location of Sun at vernal equinox
measured in hours, minutes, seconds
(1 hr = 15 degrees, 1 minute RA = 15 arcminutes)

6. Johannes Kepler (1571) favoured Sun-centred solar system
analysed Brahe's data for Mars
best deferent/epicycle orbit disagreed with data
rejected deferents/epicycles
Kepler’s Laws of Planetary Motion
1st Law (1609): Planets revolve around the Sun in elliptical orbits with the Sun at one focus
Orbit specified by semi-major axis (size) and eccentricity (shape)

7. Planets move faster in their orbits when closer to Sun (speed)
2nd Law (1609): As a planet orbits the Sun, a line joining the Sun and the planet sweeps out equal areas in equal times
Planets move faster in their orbits when closer to Sun (speed)
Mercury
3 mo.
Venus
7.4 mo
Earth
1 year
Mars
1.9 year
Jupiter
12 years
Saturn
29 years
3rd Law (1619): The square of a planet's orbital period is proportional to the cube of its semi-major axis
The larger the orbit, the longer the period (time for one orbit)

8. absolute sizes of orbits unknown
tries to explain telative orbit sizes using geometric solids
suggests that physical force is responsible for orbits
produces Rudolphine Tables

9. Galileo Galilei (1564) supports Copernicus' model
1609 – constructs telescope (3-20)
1610 – reports observations in Siderius Nuncius
many stars not visible to naked eye
Milky Way composed of many stars
some nebulae also made of stars
Moon has craters, mountains
planets appear as discs
Jupiter has satellites

10. Later Observations
Sun has sunspots
Saturn had structures attached
Venus showed all phases
Jupiter's moons and Venus' phases evidence against geocentric
Geocentric Heliocentric

11. A Dialogue on the Two Great World Systems (1632)
"debate" between geocentric and heliocentric ideas
obviously favoured heliocentric
Roman Inquisition forces Galileo to recant, placed under house arrest
Dialogue banned until 1835
Mid 's
universe is heliocentric, infinite, stars scattered randomly
Earth is one of the planets
Earth rotates on axis
orbits described by Kepler's Laws