Transcript Review1(Feb8-2011)

Outline of lecture 1 (Ch 1)
I.
Our modern View of the Universe
Survey of the universe and powers of ten
II.
The scale of the Universe
Astronomical distances
III.
Spaceship Earth
Motions of Earth, Sun, Galaxies
Review today at 4:30 in HEC 119
by Tony and Zoe
DISTANCE SCALES
• ASTRONOMICAL UNIT (AU):
– The average distance between the Earth and the
Sun.
• Units used in our solar system
Sun.
1 AU
Earth
NOT TO SCALE!!!!
10
20
30
40
URANUS
NEPTUNE
PLUTO
KUIPER BELT AND OORT CLOUD
5
SATURN
.4 .7 1.0 1.5
JUPITER
ASTEROID BELT
MARS
EARTH
Distance
(AU)
MERCURY
VENUS
A TOUR OF THE SOLAR
SYSTEM
SCALE OF THE SOLAR
SYSTEM
• Let’s view it to scale
– say the Sun is the size of a large grapefruit
(13.9 cm) in UCF
– then the nearest star would be in California
DISTANCE SCALES
• LIGHT-YEAR (LY):
– The distance light can travel in one year.
– NOTE: this is a distance not a time!
“Celestial Bodies”
• Star - A large, glowing ball of gas that generates heat and light
through nuclear fusion
• Planets - A moderately large object which orbits a star; it
shines by reflected light. Planets may be rocky, icy, or gaseous
in composition.
• Moons - an object which orbits a planet
• Asteroids - A relatively small and rocky object which orbits a
star.
• Comets -A relatively small and icy object which orbits a star.
• Nebula - An interstellar cloud of gas and/or dust
• Galaxy - A great island of stars in space, all held together by
gravity and orbiting a common center
SOLAR (STAR) SYSTEM
A star and all the material which orbits it,
including its planets and moons
THE UNIVERSE
The sum total of all matter and energy;
that is, everything within and between
all galaxies
AGE OF UNIVERSE
Age of Universe: about 14 billion years
Age of Solar System: about 4.6 billion years
Where do we come from?
• The first (and simplest) atoms were created during
the Big Bang.
• More complex atoms were created in stars.
• When the star dies, they are expelled into space….
to form new stars and planets!
Most of the atoms in our
bodies were created in
the core of a star!
SPEED OF LIGHT
• The speed of light in the vacuum of space is
constant! All light travels the same speed!
c = speed of light
= 300,000 km/sec
= 3 x 106 km/sec
(no need to memorize)
Looking back in time
• Light, although fast, travels at a finite speed.
• It takes:
–
–
–
–
8 minutes to reach us from the Sun
4.3 years to reach us from our nearest star, Alpha Centauri
1,500 years to reach us from the Orion Nebula
Two million years to reach us from the Andromeda galaxy
• The farther out we look into the Universe, the farther
back in time we see!
1.1 A Modern View of the Universe
Our goals for learning:
• What is our physical place in the Universe?
We are on a planet, orbiting a star, in a galaxy
• Describe our cosmic origins and why we say that we
are “star stuff.”
The universe started with an explosion called the “Big Bang”
at that time hydrogen and helium were created, all the other
elements were “cooked” (created) inside stars
• Why does looking into space mean looking back in
time?
Because of the time it takes for light to travel from large
distances back toward us on Earth. For example, the
Andromeda galaxy is 2 million light years away
EXPANSION (of the Universe)
• We say that the universe is expanding
because the average distance between
galaxies is increasing with time.
– NOTE: individual galaxies and star systems are
not expanding within themselves!
EXPANSION
• Mostly all galaxies appear
to be moving away from us.
• The farther away they are,
the faster they are moving.
– Just like raisins in a raisin
cake; they all move apart
from each other as the dough
(space itself) expands.
Outline of Ch 2
1.

Patterns in The Sky: Stars and constellations
Celestial coordinates:
Celestial sphere, poles, equator, ecliptic, right ascension,
declination
2.
Seasons: Tilt in Earth’s axis (23.5 degrees)
Equinoxes and solstices, precession
The Moon and Eclipses
3.

4.
Lunar and Solar Eclipses
Ancient Mystery of the Planets:

Apparent Retrograde motion of planets
• What is a constellation?
• A constellation is a region of the sky. The sky is
divided into 88 official constellations.
Constellation:
Orion
• What is the celestial sphere?
• An imaginary sphere surrounding the Earth upon
which the stars, Sun, Moon, and planets appear to
reside.
The Celestial Sphere
North & South celestial poles
the points in the sky directly above the Earth’s
North and South poles
celestial equator
the extension of the Earth’s equator onto the
celestial sphere
ecliptic
the annual path of the Sun through the celestial
sphere, which is a projection of ecliptic plane
Measuring the Sky
We measure the sky in angles, not distances.
• Full circle = 360º
• 1º = 60 arcmin
• 1 arcmin = 60 arcsec
Measuring Angles in the Sky
Moon = 0.5°
Sun = 0.5°
The Local Sky
zenith
the point directly above you
horizon
all points 90° from the zenith
altitude
the angle above the horizon
meridian
due north horizon zenith due south horizon
Coordinates on the Celestial
Sphere (not in book)
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
• Declination: position north or south of celestial
equator (in degrees)
• Right Ascension: distance (in hours, 0 to 23h 59
min.) East of vernal equinox
(vernal equinox: where the sun crosses the celestial
equator going North)
The Daily Motion
• As the Earth rotates, the sky
appears to us to rotate in the
opposite direction.
• The sky appears to rotate around
the N (or S) celestial poles.
• If you are standing at the poles,
nothing rises or sets.
• If you are standing at the equator,
everything rises & sets 90 to the
horizon.
Annual Motion
• As the Earth orbits the Sun, the Sun appears to move
eastward with respect to the stars.
• The Sun circles the celestial sphere once every year.
Annual Motion
• The Earth’s axis is tilted 23.5° from being
perpendicular to the ecliptic plane.
• Therefore, the celestial equator is tilted 23.5°
to the ecliptic.
• As seen from Earth, the Sun spends 6 months
north of the celestial equator and 6 months
south of the celestial equator.
• Seasons are caused by the Earth’s axis tilt,
not the distance from the Earth to the Sun!
Annual Motion
ecliptic
the apparent path of the Sun through the sky
equinox
where the ecliptic intersects the celestial equator
solstice
where the ecliptic is farthest from the celestial equator
Annual Motion
Vernal Equinox (~March 21)
When Sun crosses celestial equator going North
Autumnal Equinox (~September 21)
When Sun crosses celestial equator going South
Summer Solstice (~June 21)
When Sun is farthest North (23.5 degrees) from celestial equator
Winter Solstice (~Dec. 21)
When Sun is farthest South (23.5 degrees) from celestial equator
2.4 Precession
• What is the Earth’s cycle of precession?
Precession of the Earth’s Axis
• The Earth’s axis precesses (wobbles) like a
top, once about every 26,000 years.
• Precession changes the positions in the sky of
the celestial poles and the equinoxes.
 Polaris won't always be the north star.
 However the tilt in the axis is the same
(23.5 degrees) as the Earth’s axis precesses
2.5 The Moon, Our Constant Companion
• Why do we see phases of the Moon?
• What conditions are necessary for an eclipse?
Lunar Motion
Phases of the Moon’s 29.5 day cycle
•
•
•
•
•
•
•
•
new
crescent
first quarter
gibbous
full
gibbous
last quarter
crescent
waxing
waning
Eclipses
• The Earth & Moon cast
shadows.
• When either passes
through the other’s
shadow, we have an
eclipse.
• Why don’t we have an
eclipse every full & new
Moon?
Eclipses
When the Moon’s orbit intersects the
ecliptic (node):
at new moon
solar eclipse
you must be in Moon’s shadow to see it
at full moon
lunar eclipse
• everyone on the nighttime side of Earth can see it
Solar Eclipse
Lunar Eclipse
What have we learned?
• Why do we see phases of the Moon?
• At any time, half the Moon is illuminated by the Sun
and half is in darkness. The face of the Moon that we
see is some combination of these two portions,
determined by the relative locations of the Sun, Earth,
and Moon.
• What conditions are necessary for an eclipse?
• An eclipse can occur only when the nodes of the
Moon’s orbit are nearly aligned with the Earth and
the Sun. When this condition is met, we can get a
solar eclipse at new moon and a lunar eclipse at full
moon.
2.6 The Ancient Mystery of the Planets
• Why do planets sometimes seem to move
backwards relative to the stars?
Retrograde Motion
• Planets usually appear to move
eastward relative to the stars.
• But as we pass them by in our
orbit, they move west relative to
the stars for a few weeks or
months.
 Noticeable over many nights
Explaining Apparent Retrograde
Motion
• Easy for us to explain: occurs when we
“lap” another planet (or when Mercury or
Venus lap us)
• But very difficult to explain if you think that
Earth is the center of the universe!
Why did the ancient Greeks reject
the notion that the Earth orbits the
sun?
• Mainly because ancient Greeks knew that
we should see stellar parallax if we orbited
the Sun – but they could not detect it.
Parallax Angle
Apparent shift of a star’s position due to the
Earth’s orbiting of the Sun
The nearest stars are
much farther away than
the Greeks thought.
So the parallax angles of
the star are so small, that
you need a telescope to
observe them.
Possible reasons why stellar
parallax was undetectable:
1. Stars are so far away that stellar parallax is
too small for naked eye to notice
2. Earth does not orbit Sun; it is the center of
the universe
Unfortunately, with notable exceptions like Aristarchus, the
Greeks did not think the stars could be that far away, and
therefore rejected the correct explanation (1)…
Thus setting the stage for the long, historical showdown
between Earth-centered and Sun-centered systems.
Ch. 3
Ch 3 (Histrory of Astronomy) “Does the World Turn?”
1.
2.
Archeoastronomy
Astronomy of Greece: good records & more rational
approach
a. Thales and Pithagoras: roots of science


Thales: Universe is rational
Pithagoras: nature is governed by musical (mathematical) principles
b. Plato and Aristotle: Geocentric universe
 Geocentric universe
 Earth is imperfect Heavens are perfect
 “uniform circular motion” is perfect. Hence, heavenly bodies
must follow uniform circular motion
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution “The Church Strikes back”
The Nature of Science
3.3 The Copernican Revolution
Histrory of Astronomy PART II: “The Church
Strikes Back”
1.
2.
3.
4.
5.
Problems with Ptolemy’s models, lots of errors
accumulated over the centuries
Copernicus: publishes heliocentric model and dies
Tycho observes planetary motions and dies
Kepler: uses Tycho’s observations & writes 3 laws (see
book):
a. Each planet moves in ellipse with the Sun at one focus.
b. The line between the Sun ……. (faster near Sun and vice versa)
c. p2 = a3
Galileo:
a.Uses telescope to discover moons of Jupiter, study
sunspots and phases of Venus
b.Supports Copernicus’ theory & gets in trouble with
Church
Copernicus’ Heliocentric Model
•Sun
is at center
•Earth orbits like any other planet
•Inferior planet orbits are smaller
•Retrograde motion occurs when we “lap”
Mars & the other superior planets
Tycho Brahe
• Greatest observer
of his day
• Charted accurate
positions of planets
• Hires Kepler
Johannes Kepler
• Greatest theorist of
his day
• Uses Tycho’s
observations to
come up with three
laws of planetary
motion
Kepler’s Laws
1 Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
Kepler’s Laws
2 A planet moves along its orbit with a speed that
changes in such a way that a line from the planet to
the Sun sweeps out equal areas in equal intervals of
time. (the closer to the Sun, the faster it moves)
Kepler’s Laws
3 The ratio of the cube of a planet’s average distance
from the Sun to the square of its orbital period is
the same for each planet.
3
2
a /P = 1
3
a =P
a in AU
P in years
2
Galileo Galilei
• First man to point a
telescope at the sky
• wanted to connect
physics on earth with
the heavens
• Dialogue Concerning
the Two Chief World
Systems
This book got him in trouble with the Church!
Galileo’s Observations
• Galileo discovered
that Jupiter had four
moons of its own.
• Jupiter was the
center of its own
system.
• Heavenly bodies
existed which did not
orbit the earth.
Galileo’s observation of the phases of Venus was the
final evidence which buried the geocentric model.
Geocentric
No gibbous or full phases!
Heliocentric
All phases are seen!
Galileo observed all phases!
3.5 The Nature of Science
Our goals for learning:
• How can we distinguish science from
nonscience?
Science and Pseudoscience
I.
Science:
1.
2.
3.
II.
Based on observations and theory
Open to criticism and constant challenge in light of
new evidence
Not perfect (human), not always right, but most
successful discipline at predicting the way nature
works.
Pseudoscience
1.
2.
3.
Generally holds “absolute truth”
Considers only part of evidence available
Tends to play on emotions and fears instead of
logic
Astrology




Empirical Discipline (no theoretical explanation)
Easily abused by charlatans
Big problem: astrologers do not agree on what
any given celestial configuration means
In other words, who will you believe if one
astrologer tells you this is a great time to travel
and another tells you to stay home?
Ch 4 Newton and Gravity
(soap opera’s final episode)
I.
Newton and Galileo
Galileo’s experiments with falling objects:
1.
2.
3.
II.
Newton’s Laws:
1.
2.
III.
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GMm/R2 (Inverse-square law)
Orbits: 1. Closed: circles (circular velocity) & ellipses (v > vc)
2. Open: parabolas and hyperbolas (escape velocity)
How does Newton’s law of gravity extend Kepler’s laws?
(some not in book)
• Ellipses are not the only
orbital paths. Orbits can be:
bound
• Circle (v = vc)
• Ellipse (v > vc)
unbound
• Parabola (v = ve)
• Hyperbola (v > ve)
• Circular and Escape
velocities (vc and ve)
 vc = GM/R
 ve = 2GM/R
circular and
Tides
• Gravitational force decreases with (distance)2
– The Moon’s pull on Earth is strongest on the side facing the Moon,
and weakest on the opposite side.
• The Earth gets stretched along the Earth-Moon line.
• The oceans rise relative to land at these points.
What have we learned?
• How do gravity and
energy together allow us
to understand orbits?
• Gravity determines orbits
• Orbiting object cannot
change orbit without
energy transfer
• Enough energy -> escape
velocity -> object leaves.
•How does gravity cause tides?
•Gravity stretches Earth along Earth-Moon line because
the near side is pulled harder than the far side.
5.1

Outline Ch 5 Light: The Cosmic Messenger
Basic Properties of Light and Matter
Light: electromagnetic waves
1. Velocity (c = speed of light), wavelength and frequency (colors),
energy.
2. Electromagnetic spectrum, visible spectrum, atmospheric windows

5.2
Matter: Atoms. How do light and matter interact?
Learning from Light: Origin of Starlight
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and
Emission
6. Radial Velocity: Doppler effect
5.3
Telescopes: reflecting and refracting, ground, airborne, space.
Kirchhoff’s Laws
1.
2.
3.
1
Continuous Spectrum (thermal radiation
spectrum)
Emission Spectrum
Absorption spectrum
3
2
Doppler Effect
•
•
•
Radial Velocity
Approaching stars: more energy,
Receding stars: less energy,
Radial Velocity
•
•
•
Approaching stars: more
energy, spectral lines undergo a
blue shift
Receding stars: less energy,
spectral lines undergo a red
shift
/ = v/c
Basic Telescope Design
• Refracting: lenses
Refracting telescope
Yerkes 1-m refractor
Basic Telescope Design
• Reflecting: mirrors
• Most research telescopes
today are reflecting
Reflecting telescope
Gemini North 8-m
3. Atmosphere absorbs most of EM spectrum, including
all UV and X-ray, most infrared