UNIT ESECTION 3.3 THE COPERNICAN REVOLUTIONCopernicus

By the time he was a teen, tables of planetary motion were inaccurate but no one fixed them

Believed there was a much simpler explanation of retrograde motion in a Sun-centered model

Was able to calculate each planet’s orbital period around Sun and its distance from Sun However, still believed heavenly motion occurred in perfect circles

o No one followed his new idea because his system was just as complex as Ptolemy’s

Tycho Brahe Wanted to improve astronomical prediction after a Jupiter-Saturn alignment happened 2

days later than Copernicus predicted it Used parallax to show a supernova and comet were much further away than the moon Made many accurate observations within 1 arcminute His model of the solar system put Earth stationary, the Sun orbits the Earth, and all other

planets orbit the SunJohannes Kepler

Believed understanding geometry of the sky would bring him closer to God He was unable to find solutions to the planets’ orbits using circular orbits Discovered orbits are actually ellipses

o Has 2 foci on major axis (the one that runs completely across the lengthwise) Semimajor axis: ½ of the major axis

o Vertical axis is called minor axiso Eccentricity: how much the ellipse is stretched out compared to a perfect circle,

goes from 0-1Kepler’s 3 Laws of Planetary Motion

#1: The orbit of each planet around the Sun is an ellipse with the Sun at 1 focuso Nothing at other focuso A planet’s distance from the Sun varies during orbito Perihelion: point closest to Suno Aphelion: point farthest from the Suno Semimajor axis is avg. of perihelion and aphelion, or the avg. distance from the

Sun #2: As a planet moves around its orbit, it sweeps out equal areas in equal times

o Planet travels faster the closer it is to the Sun #3: The more distant planets orbit the Sun at slower avg. speeds, obeying the law,

o P2 = a3, P is planet’s orbital period in years, a is its avg distance from the sun in AU

Three objections to the heliocentric model: Aristotle said if Earth was moving, then objects in sky would be left behind as Earth

moved Noncircular orbits contradicted Aristotle’s idea that the heavens must be perfect and

unchanging No one detected stellar parallax that should occur if Earth orbits Sun

Galileo answered all these questionsGalileo

Contradicted Aristotle’s physics by saying an object in motion stays in motion unless a force acts on it (why clouds stay in motion with the Earth)

First to really use the telescopeo Saw sunspots and terrain of the Moon (showed there were imperfections in the

heavens)o Saw 4 moons orbiting Jupiter and not the Eartho Venus went through phases that could only happen if it orbited the Sun