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) 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)

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)

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