AST101 - Lecture 4 Heliocentric solar System

Astronomy before 15th Century

• Sun, moon and stars rise and set every day 

• Set in different speeds 

• Each body going around at different speeds 

• The Earth is fixed 

• Stars don’t ever change 

• Move through the sky but don’t ever change 

• Things fall to Earth 

• Everything is being attracted to Earth (rock/rain) 

• Mostly philosophical 

• Heavens are perfect and incorruptible 

• Where the Gods live so they must live in a perfect place 

• Circles are perfect

Geocentric universe

• Earth is in the centre of the universe 

• All celestial objects (planets, moon, sun) revolve around the Earth 

• Sun’s tip changes as it revolves  

• No explanation God made it like that 

• Mars moves at different speeds of the stars, sometimes slower than the stars, sometimes faster 

• All planets do this 

• Planets move in circles within circles (epicircles) 

• Retrogade motion for planets

Astronomy before 15th Century

• Sun, moon and stars rise and set every day 

• Set in different speeds 

• Each body going around at different speeds 

• The Earth is fixed 

• Stars don’t ever change 

• Move through the sky but don’t ever change 

• Things fall to Earth 

• Everything is being attracted to Earth (rock/rain) 

• Mostly philosophical 

• Heavens are perfect and incorruptible 

• Where the Gods live so they must live in a perfect place 

• Circles are perfect

Geocentric universe

• Earth is in the centre of the universe 

• All celestial objects (planets, moon, sun) revolve around the Earth 

• Sun’s tip changes as it revolves  

• No explanation God made it like that 

• Mars moves at different speeds of the stars, sometimes slower than the stars, sometimes faster 

• All planets do this 

• Planets move in circles within circles (epicircles) 

• Retrogade motion for planets

Apparent Retrograde motions

optical illusion where a planet appears to move backward in its orbit. Sometimes seems to form a loop

Nicolas Copernicus

• It made sense but it looked goofy 

• Believed the sun is the center of the Universe (Heliocentric Model) 

• Planets orbit the sun in circle 

• The moon orbits the Earth 

• Planets orbits the sun in perfect circles (Heavens are perfect) 

• Explains retrograde motion 

• Didn’t match where the stars were

Tyco Brahe

• Observed a Super Nova 

  • Stars are not unchanging

• Used parallax to measure distances of things 

• The furthest a thing is the less it’ll move from an object (and vice versa) 

• From his data, current models had problems 

• Tracking where each planet and stars where every night 

• Made his own model 

• Geocentric 

• Sun went around the Earth 

• Planets went around the Sun

Kepler

• If God created the universe, God created the rules 

• Rules will be exact 

• Kepler’s Laws 

• Fits Brahe’s data and everybody else’s data 

Kepler’s Laws

1. The orbit of each planet about the Sun is an ellipse with the Sun at one focus with othing at the other focus. Planet follows an orbit with the sun at one of the two focuses 

2. A planet moves faster in the part of its orbit nearer the Sun and slower when farther from the Sun, sweeping out equal areas in equal times 

3. More distant planets orbit the Sun at the slower average speeds, oberying a precise mathematical relationship

Galileo

• Observed with telescopes 

• The moon has craters 

• Jupiter has moons that orbit it 

• Venus has phases

  •  Which shows that it orbits the sun, not the earth

• Moon has moutains and craters 

• Not everything in the heavens are perfect circles and spheres 

• Maybe ellpitical orbits are fine 

• Jovian moon 

  • Observed 4 moons 

• Not everything orbits the Earth