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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.

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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