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PHY134 Introductory Astronomy PowerPoint Presentation

PHY134 Introductory Astronomy

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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.

- 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

- 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

advance t

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