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PHY134 Introductory Astronomy

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

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  1. PHY134Introductory Astronomy Galileo – and Newton!!

  2. 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

  3. 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.

  4. 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

  5. Kepler’s Third • 3. Square of sidereal period proportional to cube of semimajor axis Same for all planets!

  6. 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

  7. 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

  8. Motion • 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

  9. Circular motion • We found that directed to center and of constant magnitude. If radius is and speed what is magnitude of ?

  10. Mechanics • 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

  11. 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

  12. 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 forces accelerations advance t

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