Download 2. Revolution of the Spheres

Astronomy 110: SURVEY OF ASTRONOMY
2. Revolution of the Spheres
1. Earth, Sun and Moon
2. The Puzzle of the Planets
This “revolution” is a revolution of ideas. Copernicus
started it by describing the Earth as one of many
planets. Newton completed the revolution by showing
that the same laws of motion work everywhere — on
Earth and in space. In between them, Kepler found the
first precise laws of planetary motion, and Galileo
studied motion on Earth as well as celestial objects.
1. EARTH, SUN AND MOON
a. Reasons for seasons
b. Phases of the Moon
c. Eclipses of Sun and Moon
Reasons for Seasons: Summer and Winter
March equinox
June solstice
December solstice
September equinox
North: SUMMER
South: WINTER
North: WINTER
South: SUMMER
Reasons for Seasons: One Year
From one Dec.
solstice to next,
Sun moves north
and returns south.
Reasons for Seasons: Hot and Cold
Not true in tropics!
Not true in tropics!
Summer: Sun higher, days longer
Winter: Sun lower, days shorter
HOT
COLD
Reasons for Seasons: In the Tropics
Why are seasons different in the Tropics?
1. Length of daylight does not change much.
(At equator, does not change at all.)
2. Sun is not highest at solstice.
(e.g., Lahaina noons in May & July in Hawaii.)
1. What kind of seasons would we have if the Earth’s
orbit was a perfect circle centered on the Sun?
A. None at all — constant temperature everywhere.
B. Normal seasons much like we have now.
C. North would always be cold, south always hot.
D. Days and nights would be 12 hours long.
E. North and south hemispheres would have
winter and summer at the same time of year.
1. What kind of seasons would we have if the Earth’s
orbit was a perfect circle centered on the Sun?
A. None at all — constant temperature everywhere.
B. Normal seasons much like we have now.
C. North would always be cold, south always hot.
D. Days and nights would be 12 hours long.
E. North and south hemispheres would have
winter and summer at the same time of year.
Phases of the Moon
The Moon is a ball of
rock lit by the Sun.
Starts With A Bang: A Moon-stravaganza
We see phases because the angle between the Moon
and the Sun varies as the Moon orbits every 28 days.
Phases of
the Moon
How to Simulate Phases of the Moon
Phases of the Moon
Astr 110L: Shape of the Moon’s Orbit
Note that (1) size varies and (2) Moon “wobbles”.
The Moon’s Elliptical Orbit
2. What is wrong with
this picture?
A. The cell phone
is bugged.
B. It’s night, but the
Moon’s phase shows
that the Sun is up.
C. The water is only
3 feet deep.
New Yorker: 07/24/06
2. What is wrong with
this picture?
A. The cell phone
is bugged.
B. It’s night, but the
Moon’s phase shows
that the Sun is up.
C. The water is only
3 feet deep.
New Yorker: 07/24/06
Eclipses: Geometry
and Shadows
Solar Eclipse
Totality only
visible here!
Penumbra
Umbra
Penumbra
Eclipses: Geometry
and Shadows
Lunar Eclipse
Penumbra
Umbra
Penumbra
Visible everywhere
on night side
Total Solar Eclipse
Solar Eclipse Simulation
Note: Penumbra should be about twice Moon’s diameter;
Earth should be shown rotating; Sun should not move with
respect to background stars.
Astronomy Picture of the Day: A Darkened Sky
Solar Eclipses: Three Types
Total Eclipse
Partial Eclipse
Annular Eclipse
3. During a solar eclipse, which way does the Moon’s
shadow typically move?
A. North to south
B. East to west
C. South to north
D. West to east
E. No motion at all
3. During a solar eclipse, which way does the Moon’s
shadow typically move?
A. North to south
B. East to west
C. South to north
D. West to east
E. No motion at all
Total Lunar Eclipse
Astronomy Picture of the Day: Kalamalka Lake Eclipse
Astronomy Picture of the Day: Shadow Play
Astronomy Picture of the Day: Eclipsed Moon and Stars
Lunar Eclipses: Three Types
EARTH, SUN AND MOON : SUMMARY
a. Reasons for seasons
Earth spins at an angle to its orbit, exposing us to
more or less sunlight as we go around the Sun.
b. Phases of the Moon
The Moon is a ball of rock lit by the Sun; we see
only part of the illuminated side.
c. Eclipses of Sun and Moon
The Earth and Moon cast shadows, and each can
travel through the other’s shadow.
2. THE PUZZLE OF THE PLANETS
a. Planetary motion
b. Models of the Solar System
c. Kepler’s laws of planetary motion
Planetary Motion
The Sun & Moon
always move westto-east relative to
the stars.
The planets usually
move west-to-east
as well, but sometimes they move
the other way!
Planetary Motion: Mars Goes Retrograde
Mars at Opposition
All planets undergo retrograde motion.
Models of the Solar System
Pythagorean
model: rotating
Earth at center.
Could not explain
retrograde
motion.
Physics for the Inquiring Mind
Models of the Solar System
Aristarchus’
model: Sun at
center. Explained
retrograde
motion, predicted
stellar parallax.
Physics for the Inquiring Mind
Parallax
If the stars are scattered
through space, nearby ones
should appear to shift as
the Earth orbits the Sun.
(They do, but it’s
hard to detect.)
If all stars are at the same distance, constellations
should grow and shrink as the Earth moves around.
Neither effect was seen by ancient astronomers, so
they rejected Aristarchus’ model.
Ptolemaic model
Earth at center, circled
by everything else. Each
planet combines two
uniform circular motions.
This worked well enough
for “practical” purposes.
Copernican model
1. Heavenly motions are uniform, eternal, and
circular or compounded of several circles.
2. The center of the universe is near the Sun.
3. Around the Sun, in order, are Mercury,Venus,
Earth and Moon, Mars, Jupiter, Saturn, and the
fixed stars.
4. The Earth has three motions: daily rotation,
annual revolution, and annual tilting of its axis.
5. Retrograde motion of the planets is explained
by the Earth's motion.
6. The distance from the Earth to the Sun is small
compared to the distance to the stars.
Tycho’s Data
The Copernican model was
no more accurate than the
Ptolemaic model.
Tycho’s very precise
observations showed that
both models were flawed.
Galileo’s Discoveries
1. The Moons of Jupiter
Physics for the Inquiring Mind
2. The Phases of Venus
Conclusive proof that
Venus orbits the Sun!
Museum of Science, Florence
Phases of Venus
Ptolemaic model:
Venus never more
than a crescent.
Copernican model:
Venus shows full
cycle of phases.
Kepler’s Laws of Planetary Motion
Law 1: Planets move in elliptical
orbits with the Sun at one focus.
Wikipedia: Kepler’s Laws
Drawing an Ellipse
Kepler’s Laws of Planetary Motion
Law 1: Planets move in elliptical
orbits with the Sun at one focus.
Closest to Sun at perihelion, furthest from Sun at aphelion.
Wikipedia: Kepler’s Laws
Width of ellipse is 2a, where a is the semi-major axis.
Law II: A line from the Sun to a planet
sweeps out equal areas in equal times.
In other words, (area)/(time) is a constant, now known as the
angular momentum.
Wikipedia: Kepler’s Laws
Law III: The quantity P2 / a3 , where P is orbital period
and a is semi-major axis, is the same for all planets.
If P is measured in years and a is measured in AU, then P2 = a3.
Scientific Method
Observe Nature
Suggest Explanation
(a.k.a. Hypothesis)
False?
Make Predictions
True?
Trust Hypothesis
(a little, anyway)
Scientific Method: Examples
Retrograde Motion:
Falsified the Pythagorean model
Motivated all Sun-centered models
Motivated the Ptolemaic (2 circle) model
Scientific Method: Examples
Stellar Parallax:
Falsified all Sun-centered model
Verified all Sun-centered models
Falsified all Earth-centered models
Scientific Method: Examples
Tycho’s data:
Falsified Kepler’s initial model
Verified all of Kepler’s laws
Scientific Method: Examples
Phases of Venus:
Falsified all Earth-centered models
Verified all Sun-centered models
Position 3
None - it is always above the horizon.
E
N
S
W
Imaginary plane
No; the Earth blocks the view.
Star A at position 4 is hidden by the Earth.
Agree with student 2; star B never crosses
horizon plane, so it can’t rise or set.
south, high
west, low
not visible
northeast, high
north, low
Rising
Setting
No - it is always above the horizon from this location.
An observer further south would see it set.