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PHY134 Introductory Astronomy . Galileo – and Newton!!. So Which is it?. Both models produce predictions matching observations of apparent motion Culturally difference is huge: does Earth move ? Is it central ? Scientifically difference is in motion of planets in 3d .

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phy134 introductory astronomy

PHY134Introductory Astronomy

Galileo – and Newton!!

so which is it
So Which is it?
  • Both models produce predictions matching observations of apparent motion
  • Culturally difference is huge: does Earth move? Is it central?
  • Scientifically difference is in motion of planets in 3d.
  • Other things change in the heavens: Comets come and go and their motion does not fit well with either model
  • Real determination: better observations with new technology
  • Leads to new understanding of fundamental laws that are universal. That is the scientific revolution
kepler s laws
Kepler’s Laws
  • 1. Orbit of a planet is an ellipse with Sun at one focus
  • What’s at other focus? Nothing, not even the same for all planets
  • Eccentricity small: 0.017 for Earth, 0.2 for Mercury.

2. Line from Sun to planet sweeps out equal areas in equal times

  • Planet moves faster near perihelion, slower near aphelion
  • Comets in highly eccentric orbits – dramatic effect
kepler s third
Kepler’s Third
  • 3. Square of sidereal period proportional to cube of semimajor axis

Same for all planets!

galileo s smoking scope
Galileo’s Smoking Scope
  • New technology: Galileo (1610) turns telescope up
  • Finds
    • phases of Venus showing it orbits Sun
    • Galilean moons orbiting Jupiter
    • Mountains on Moon, spots on Sun, ears on Saturn
this is progress
This is Progress
  • Galileo studies heavens as a physical system to be observed
  • Kepler’s laws predict planetary motion with unprecedented accuracy from simple model
  • They are universal. In fact, they govern orbiting systems from Solar System to Saturn’s Moons to…electrons in an Atom (with different K)
  • Such universality is a hint of underlying fundamental laws
  • Galileo also studied mechanics (science of motion) and formulated principle of inertia: An object will retain its state of motion unless disturbed externally
  • It took Newton and new math to find them
  • State of motion is velocity - speed and direction in
  • Rate of change of is acceleration in
  • Acceleration can be speeding, slowing, or turning and is directed in direction of change
circular motion
Circular motion
  • We found that directed to center and of constant magnitude. If radius is and speed

what is magnitude of ?

  • Acceleration due to a force applied by another object:
  • is a property of object mass - in
  • is measured – in
  • When object A applies a force to B, then B applies a force to A
weight and mass
Weight and Mass
  • Weight of an object is the force gravity applies to it
  • We know objects fall with acceleration so force of gravity is
  • is property of Earth
  • My mass is . My weight on Earth is
this is everything
This is Everything
  • . The rest is details
  • If we can figure out forces this is a way to predict from where things are today where they will be in future (or were in past):

positions, velocities



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