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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)
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
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)
• accuracy of 1 arcminute (1)
1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
1 = 60 = 3600 
• 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
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)
Johannes Kepler (1571)
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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)
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)
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)
Mercury
3 mo.
Venus
7.4 mo
Earth
1 year
Mars
1.9 year
Jupiter
12 years
Saturn
29 years
• 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
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
Later Observations
 Sun has sunspots
 Saturn had structures attached
 Venus showed all phases
• Jupiter's moons and Venus' phases
evidence against geocentric
Geocentric
Heliocentric
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 - 1600's
• universe is heliocentric, infinite,
stars scattered randomly
• Earth is one of the planets
• Earth rotates on axis
• orbits described by Kepler's Laws