ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: orosz@sciences.sdsu.edu
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework
• Homework due February 5: Question 11 from
Chapter 2 (In what ways did the astronomical
observations of Galileo support a heliocentric
cosmology?)
• Write down the answer on a sheet of paper and
hand it in before the end of class on Febuary 5.
Dispatches from the Deparment
of Anal Retention
• The Earth’s axis is tilted…
• What is the tilt relative to?
• The Northern Hemisphere is more exposed
to the Sun…
• Why is it warmer in the summer than in the
winter?
Homework
• Go to a planetarium show in PA 209:
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Tuesday, January 29
Wednesday, January 30
Friday, February 1
Monday, February 4
Tuesday, February 5
Wednesday, February 6
Thursday, February 7
Friday, February 8
1:00 -- 2:00 PM
2:00 -- 3:00 PM
1:00 -- 2:00 PM
11:00 AM -- 12:00 PM
5:30 PM -- 6:30 PM
2:00 -- 3:00 PM
5:30 -- 6:30 PM
2:30 -- 3:30 PM
• Get 10 points extra credit for homework part of grade.
• Sign up for a session outside PA 209.
• Hand in a sheet of paper with your name and the date and time of
the session.
Watch Your Head!
• http://www.cnn.com/2013/02/05/world/spaceasteroid/index.html?hpt=hp_c4
A Brief History of Astronomy
A Brief History of Astronomy
• An early view of the skies:
 The Sun: it rises and sets, rises and sets…
 The Moon: it has a monthly cycle of phases.
 The “fixed stars”: the patterns stay fixed, and
the appearance of different constellations marks
the different seasons.
• Keep in mind there were no telescopes, no
cameras, no computers, etc.
A Brief History of Astronomy
• But then there were the 5 “planets”:
 These are star-like objects that move through the
constellations.
 Mercury: the “fastest” planet, always near the Sun.
 Venus: the brightest planet, always near the Sun.
 Mars: the red planet, “slower” than Venus.
 Jupiter: the second brightest planet, “slower” than
Mars.
 Saturn: the “slowest” planet.
A Brief History of Astronomy
• By the time of the ancient Greeks (around
500 B.C.), extensive observations of the
planetary positions existed. Note, however,
the accuracy of these data were limited.
• An important philosophical issue of the
time was how to explain the motion of the
Sun, Moon, and planets.
What is a model?
• A model is an idea about how something
works.
• It contains assumptions about certain things,
and rules on how certain things behave.
• Ideally, a model will explain existing
observations and be able to predict the
outcome of future experiments.
Aristotle (385-322 B.C.)
• Aristotle was perhaps the most influential
Greek philosopher. He favored a
geocentric model for the Universe:
 The Earth is at the center of the Universe.
 The heavens are ordered, harmonious, and
perfect. The perfect shape is a sphere, and the
natural motion was rotation.
Geocentric Model
• The motion of the Sun around the Earth
accounts for the rising and setting of the
Sun.
• The motion of the Moon around the Earth
accounts for the rising and setting of the
Moon.
• You have to fiddle a bit to get the Moon
phases.
Geocentric Model
• The fixed stars were on the “Celestial
Sphere” whose rotation caused the rising
and setting of the stars.
• The constellations rise and set each night, and individual
stars make a curved path across the sky.
• The curvature of the tracks depend on where you look.
Geocentric Model
• The fixed stars were on the “Celestial
Sphere” whose rotation caused the rising
and setting of the stars.
• However, the detailed motions of the
planets were much harder to explain…
Planetary Motion
• The motion of a planet with respect to the background
stars is not a simple curve. This shows the motion of
Mars.
• Sometimes a planet will go “backwards”, which is
called “retrograde motion.”
Planetary Motion
• Here is a plot of the path
of Mars.
• Other planets show similar
behavior.
Image from Nick Strobel Astronomy Notes (http://www.astronomynotes.com/)
Aristotle’s Model
• Aristotle’s model
had 55 nested
spheres.
• Although it did
not work well in
detail, this model
was widely
adopted for nearly
1800 years.
Better Predictions
• Although Aristotle’s ideas were commonly
accepted, there was a need for a more
accurate way to predict planetary motions.
• Claudius Ptolomy (85-165) presented a
detailed model of the Universe that
explained retrograde motion by using
complicated placement of circles.
Ptolomy’s Epicycles
• By adding epicycles, very complicated motion could be
explained.
Ptolomy’s Epicycles
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/).
Ptolomy’s Epicycles
Ptolomy’s Epicycles
• Ptolomy’s model was considered a
computational tool only.
• Aristotle’s ideas were “true”. They
eventually became a part of Church dogma
in the Middle Ages.
The Middle Ages
• Not much happened in Astronomy in the
Middle Ages (100-1500 A.D.).
Next:
The Copernican Revolution
The Sun-Centered Model
• Nicolaus Copernicus
(1473-1543) proposed
a heliocentric model of
the Universe.
• The Sun was at the
center, and the planets
moved around it in
perfect circles.
The Sun-Centered Model
• The Sun was at the
center. Each planet
moved on a circle,
and the speed of the
planet’s motion
decreased with
increasing distance
from the Sun.
The Sun-Centered Model
• Retrograde motion
of the planets could
be explained as a
projection effect.
The Sun-Centered Model
• Retrograde motion
of the planets could
be explained as a
projection effect.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/)
Copernican Model
• The model of Copernicus did not any better
than Ptolomy’s model in explaining the
planetary motions in detail.
• He did work out the relative distances of the
planets from the Sun.
• The philosophical shift was important (i.e.
the Earth is not at the center of the
Universe).
Tycho Brahe (1546-1601)
• Tycho was born in a very
wealthy family.
• From an early age, he devoted
himself to making accurate
astronomical observations.
• He received a great deal of
support from the king of
Denmark, including the use of
his own island.
Tycho
• Tycho lived before the invention of the telescope.
• His observations of Mars were about 10 times more
accurate than what had been done before.
Johannes Kepler (1571-1630)
• Kepler was a mathematician by training.
• He believed in the Copernican view with the
Sun at the center and the motions of the
planets on perfect circles.
• Tycho hired Kepler to analyize his
observational data.
Johannes Kepler (1571-1630)
• Kepler was a mathematician by training.
• He believed in the Copernican view with
the Sun at the center and the motions of the
planets on perfect circles.
• Tycho hired Kepler to analyize his
observational data.
• After years of failure, Kepler dropped the
notion of motion on perfect circles.
Kepler’s Three Laws of
Planetary Motion
• Starting in 1609, Kepler published three
“laws” of planetary motion:
Kepler’s Three Laws of
Planetary Motion
•
Starting in 1609, Kepler published three
“laws” of planetary motion:
1. Planets orbit the Sun in ellipses, with the Sun
at one focus.
Ellipses
• An ellipse is a “flattened circle” described by a
particular mathematical equation.
• The eccentricity tells you how flat the ellipse is:
e=0 for circular, and e=1 for infinitely flat.
Ellipses
• You can draw an
ellipsed with a loop of
string and two tacks.
Kepler’s Three Laws of
Planetary Motion
•
Starting in 1609, Kepler published three
“laws” of planetary motion:
1. Planets orbit the Sun in ellipses, with the Sun
at one focus.
Kepler’s Three Laws of
Planetary Motion
•
Starting in 1609, Kepler published three
“laws” of planetary motion:
1. Planets orbit the Sun in ellipses, with the Sun
at one focus.
2. The planets sweep out equal areas in equal
times. That is, a planet moves faster when it
is closer to the Sun, and slower when it is
further away.
Kepler’s Second Law
• The time it takes for the
planet to move through
the green sector is the
same as it is to move
through the blue sector.
• Both sectors have the
same area.
Kepler’s Three Laws of
Planetary Motion
•
Starting in 1609, Kepler published three
“laws” of planetary motion:
1. Planets orbit the Sun in ellipses, with the Sun
at one focus.
2. The planets sweep out equal areas in equal
times. That is, a planet moves faster when it
is closer to the Sun, and slower when it is
further away.
Kepler’s Three Laws of
Planetary Motion
•
Starting in 1609, Kepler published three
“laws” of planetary motion:
1. Planets orbit the Sun in ellipses, with the Sun
at one focus.
2. The planets sweep out equal areas in equal
times. That is, a planet moves faster when it
is closer to the Sun, and slower when it is
further away.
3. (Period)2 = (semimajor axis)3
Kepler’s Third Law
P2
distance d3
Mercury 0.241
0.058
0.387
0.058
Venus
0.615
0.378
0.723
0.378
Earth
1.000
1.000
1.000
1.000
Mars
1.881
3.538
1.524
3.540
Jupiter
11.857
140.588 5.203
140.852
Saturn
29.424
865.772 9.537
867.432
Period
The Kepler’s Law Simulator
• There are some animations on the web
illustrating Kepler’s Laws:
– http://www.astro.utoronto.ca/~zhu/ast210/kepler.html
Heliocentric or Geocentric?
• The year is around 1610. The “old” school
is Aristotle and a geocentric view. The
“new” school is the heliocentric view
(Copernicus and Kepler).
• Which one is correct?
Heliocentric or Geocentric?
• The year is around 1610. The “old” school is
Aristotle and a geocentric view. The “new”
school is the heliocentric view (Copernicus and
Kepler).
• Which one is correct?
• Observational support for the heliocentric
model would come from Galileo.
Heliocentric or Geocentric?
• The year is around 1610. The “old” school is
Aristotle and a geocentric view. The “new”
school is the heliocentric view (Copernicus and
Kepler).
• Which one is correct?
• Observational support for the heliocentric
model would come from Galileo.
• Theoretical support for the heliocentric model
would come from Isaac Newton.
Next:
Who Wins?
Galileo Galilei (1564-1642)
• Galileo was one of the
first to use a telescope
to study astronomical
objects, starting in
about 1609.
• http://www.pacifier.c
om/~tpope/index.htm
Galileo Galilei (1564-1642)
• Galileo was one of the first to use a telescope
to study astronomical objects, starting in about
1609.
• His observations of the moons of Jupiter and
the phases of Venus provided strong support
for the heliocentric model.
Jupiter’s Moons
• The 4 objects circled Jupiter, and not the Earth!
Jupiter’s Moons
• You can watch Jupiter’s moons move from one
side of Jupiter to the other in a few days.
Jupiter’s Moons
• Not all bodies go around the Earth!
Venus
• Venus, the brightest planet, is never far
from the Sun: it sets at most a few hours
after sunset, or rises at most a few hours
before sunrise.
Venus
• Venus, the brightest planet, is never far
from the Sun: it sets at most a few hours
after sunset, or rises at most a few hours
before sunrise.
• It is never out in the middle of the night.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
• Furthermore, the crescent Venus was always larger
than the full Venus.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
• Furthermore, the crescent Venus was always larger
than the full Venus.
• Conclusion: Venus shines by reflected sunlight, and it
is closer to Earth when it is a crescent.
Venus in the Geocentric View
• Venus is always
close to the Sun on
the sky, so its
epicycle restricts its
position.
• In this view, Venus
always appears as a
crescent.
Venus in the Heliocentric View
• In the heliocentric
view, Venus orbits
the Sun closer than
the Earth does.
• We on Earth can see
a fully lit Venus
when it is on the far
side of its orbit.
Venus in the Heliocentric View
• The correlation between
the phases and the size
is accounted for in the
heliocentric view.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo was put before the Inquisition and
forced to recant his views.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo was put before the Inquisition and
forced to recant his views.
• Pope John Paul II admitted in 1992 that the
Church was wrong to denounce Galileo.
Isaac Newton (1642-1727)
http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Newton.html
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
• He was professor of mathematics at
Cambridge University in England. (Steven
Hawking currently hold’s Newton’s Chair
at Cambridge).
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
• He was professor of mathematics at
Cambridge University in England. (Steven
Hawking currently hold’s Newton’s Chair
at Cambridge).
• He was later the Master of the Mint in
London, where first proposed the use of
grooved edges on coins to prevent shaving.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
• His laws of motion and of gravity could
explain Kepler’s Laws of planetary motion.
Newton’s Laws of Motion
Newton’s Laws of Motion
1.
2.
3.
A body in motion tends to stay in motion in a straight
line unless acted upon by an external force.
The force on an object is the mass times the acceleration
(F=ma).
For every action, there is an equal and opposite reaction.
(For example, a rocket is propelled by expelling hot gas
from its thrusters).
What is Gravity?
What is Gravity?
• Gravity is a force between all matter in the
Universe.
What is Gravity?
• Gravity is a force between all matter in the
Universe.
• It is difficult to say what gravity is.
However, we can describe how it works.
What is Gravity?
• Gravity is a force between all matter in the
Universe.
• It is difficult to say what gravity is.
However, we can describe how it works.
What is Gravity?
• The gravitational force between larger
bodies is greater than it is between smaller
bodies, for a fixed distance.
What is Gravity?
• As two bodies move further apart, the
gravitational force decreases. The range of
the force is infinite, although it is very small at
very large distances.
Newton’s Laws
• Using Newton’s Laws, we can…
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
 Measure the mass of the Sun, the Moon, and
the Planets.
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
 Measure the mass of the Sun, the Moon, and
the Planets.
 Measure the masses of distant stars in binary
systems.
Laws of Physics
• The models of Aristotle and Ptolomy were
based mainly on beliefs (i.e. that motion should
be on perfect circles, etc.).
• Starting with Newton, we had a physical
model of how the planets moved: the laws of
motion and gravity as observed on Earth give a
model for how the planets move.
• All modern models in Astronomy are based on
the laws of Physics.