PHY134 Introductory Astronomy

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# PHY134 Introductory Astronomy - PowerPoint PPT Presentation

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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### PHY134Introductory 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.
• 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
• 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.
• Planet moves faster near perihelion, slower near aphelion
• Comets in highly eccentric orbits – dramatic effect
Kepler’s Third
• 3. Square of sidereal period proportional to cube of semimajor axis

Same for all planets!

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

what is magnitude of ?

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
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
• . 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