The Solar SystemCopernicus (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)

• repetition of measurements

• calibration

• Uraniborg (1576)

• 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

• 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

1 degree = 60 arcminutes

1 arcminute = 60 arcseconds

1 = 60 = 3600

• accuracy of 1 arcminute (1)

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)

• favoured Sun-centred solar system • analysed Brahe's data for Mars• best deferent/epicycle orbit disagreed with data • rejected deferents/epicycles

1st Law (1609): Planets revolve around the Sun in elliptical orbits with the Sun at one focus

Kepler’s Laws of Planetary Motion

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

Sun has sunspots

Later Observations

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