Transcript Today`s Powerpoint - Physics and Astronomy

Week 4 Day 3: Announcements
Clicker Review:
What time of day does the first quarter moon
set?
A: 6am
B: noon
C: 6pm
D: midnight
E: Never sets
Charting the Heavens:
Foundations of Astronomy
Learning Goals
• Describe the Celestial Sphere and how astronomers use
angular measurement to locate objects in the night sky.
• Account for the apparent motions of the Sun and the stars in
terms of the actual motion of the Earth. Explain why our
planet has seasons.
• Understand the changing appearance of the Moon and how
the relative motions of the Earth, the Sun, and the Moon
lead to eclipses.
Certain seasons are more likely to have eclipses. Solar “eclipse
season” lasts about 38 days. Likely to get at least a partial eclipse
somewhere. Animation
It's worse than this! The plane of the Moon's orbit precesses, so
that the eclipse season occurs about 19 days earlier each year.
Recent and upcoming total and annular solar eclipses
From Aristotle to Newton
The history of the Solar System (and the universe to
some extent) from ancient Greek times through to the
beginnings of modern physics.
Eratosthenes Determines the Size of the Earth in about 200 B.C.
Sun's rays
Syene
Alexandria
N
7.2o
S
Earth
He knows the distance between the two cities is 5000 "stadia”, where
1 stadia = 6.25 km
From geometry then,
7.2o
=
360o
5000 stadia
Earth's circumference
=> circumference is 250,000 stadia, or 40,000 km.
So radius is:
40,000 km
2p
=
6366 km
(very close to modern value, 6378 km!)
Clicker Question:
Who was the first person to use a telescope
to make astronomical discoveries?
A: Aristotle
B: Brahe
C: Kepler
D: Gallileo
E: Newton
Brainstorm: What is a model and how is it useful?
"Geocentric Model" of the Solar System
Ancient Greek astronomers knew of Sun, Moon, Mercury, Venus, Mars,
Jupiter and Saturn.
Aristotle vs. Aristarchus (3rd century B.C.):
Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.
Aristarchus: Used geometry of eclipses to show Sun bigger than Earth
(and Moon smaller), so guessed that Earth orbits the Sun. Also guessed
Earth spins on its axis once a day => apparent motion of stars.
Aristotle: But there's no wind or parallax (apparent movement of stars).
Difficulty with Aristotle's "Geocentric" model: "Retrograde motion of the
planets".
Planets generally move in one direction
relative to the stars, but sometimes they appear
to loop back. This is "retrograde motion".
But if you support geocentric model, you must attribute retrograde
motion to actual motions of planets, leading to loops called “epicycles”.
Ptolemy's geocentric model (A.D. 140)
"Heliocentric" Model
●
Rediscovered by Copernicus in 16th century.
●
Put Sun at the center of everything.
Much simpler. Almost got rid of retrograde
motion.
●
But orbits circular in his model. In reality,
they’re elliptical, so it didn’t fit the data well.
●
●
Not generally accepted then.
Copernicus 1473-1543
Illustration from
Copernicus' work
showing heliocentric
model.
Planets generally move in one direction relative
to the stars, but sometimes they appear to loop
back. This is "retrograde motion".
Planets generally move in one direction
relative to the stars, but sometimes they appear
to loop back. This is "retrograde motion".
Apparent motion
of Mars against
"fixed" stars
Mars
July
7
*
Earth
7
6
*
6
5
3
4
4
3
1
5
2
2
*
1
January
*
*
*
Clicker Question:
A flaw in Copernicus’s model for the
solar system was:
A: It didn’t explain retrograde motion.
B: He used circular orbits.
C: The Earth was still at the center.
D: He used the same mass for all the planets.
E: All of the above
Timelines of the Big Names
Galileo
Copernicus
1473-1543
1564-1642
Brahe
1546-1601
Kepler
1571-1630
Newton
1642-1727
Galileo (1564-1642)
Built his own telescope (1609).
Discovered four moons orbiting Jupiter =>
Earth is not center of all things!
Co-discovered sunspots. Deduced Sun
rotated on its axis.
Discovered phases of Venus, inconsistent
with geocentric model.
Kepler (1571-1630)
Used Tycho Brahe's precise data on
apparent planet motions and relative
distances.
Deduced three laws of planetary
motion.
Kepler's First Law
The orbits of the planets are elliptical (not circular)
with the Sun at one focus of the ellipse.
Ellipses
distance between foci
eccentricity =
major axis length
(flatness of ellipse)
Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas
in equal times.
slower
Translation: planets move faster
when closer to the Sun.
faster
Kepler's Third Law
The square of a planet's orbital period is proportional to the
cube of its semi-major axis.
P2
is proportional to
or
P2  a3
(for circular orbits, a=b=radius).
Translation: the larger a planet's orbit,
the longer the period.
a3
a
b
Solar System Orbits
Orbits of some planets (or dwarf planets):
Planet
a (AU)
Venus
Earth
Jupiter
Pluto
0.723
1.0
5.2
39.5
P (Earth years)
0.615
1.0
12
249
Copernican model was a triumph of the Scientific Method
Scientific Method:
a)
b)
c)
d)
e)
Make high quality observations of some natural phenomenon
Come up with a theory that explains the observations
Use the theory to predict future behavior
Make further observations to test the theory
Refine the theory, or if it no longer works, make a new one
- Occam’s Razor: Simpler Theories are better
-You can prove a theory WRONG but not
RIGHT
Prediction
Observation
Theory
At this time, actual distances of planets from Sun were
unknown, but were later measured. One technique is "parallax"
"Earth-baseline parallax" uses
telescopes on either side of Earth to
measure planet distances.
Newton (1642-1727)
Kepler's laws were basically playing with
mathematical shapes and equations and seeing
what worked.
Newton's work based on experiments of how
objects interact.
His three laws of motion and law of gravity
described how all objects interact with each other.
Newton's First Law of Motion
Every object continues in a state of rest or a state of uniform
motion in a straight line unless acted on by a force.
Newton's Second Law of Motion
When a force, F, acts on an object with a mass, m, it produces an
acceleration, a, equal to the force divided by the mass.
F
a=
m
or F = ma
acceleration is a change in velocity or a change in
direction of velocity.
Newton's First Law of Motion
DEMO - Smash the HAND
Newton's Second Law of Motion
Demo - Measuring Force and
Acceleration