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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• 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.
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.
Next:
Lunar and Solar Eclipses
Lunar and Solar Eclipses
• But first, let’s discuss “angular size” and
“linear size”…
Angular Size
• The physical size is
measured in meters,
light-years, etc.
• The distance is
measured in the same
units.
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
Angular Size
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
• As you move the
same object further,
its angular size gets
smaller.
Angular Size
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
• If two objects are at
the same distance, the
larger one has the
larger angular size.
Angular Size
• Trick photography often involves playing with
different distances to create the illusion of large
or small objects:
http://www.tadbit.com/2008/03/06/top-10-holding-the-sun-pictures/
http://www.stinkyjournalism.org/latest-journalism-news-updates-45.php
Angular Size
• This figure illustrates how objects of very different
sizes can appear to have the same angular sizes. The
Sun is 400 times larger than the Moon, and 390 times
more distant.
Lunar and Solar Eclipses
• A solar eclipse is seen when the Moon
passes in front of the Sun, as seen from a
particular spot on the Earth.
• A lunar eclipse is seen then the Moon
passes into the Earth’s shadow.
Shadows
• If the light source is extended, then the shadow of an
object has two parts: the umbra is the “complete”
shadow, and the penumbra is the “partial shadow”.
Shadows
• If the light source is
extended, then the
shadow of an object has
two parts: the umbra is
the “complete” shadow,
and the penumbra is the
“partial shadow”.
Lunar Eclipses
• During a total lunar eclipse, the Moon passes through
Earth’s shadow.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
• During totality the
faint outer atmosphere
of the Sun can be seen.
Annular Eclipses
• The angular sizes of
the Sun and Moon
vary slightly, so
sometimes the Moon
isn’t “big enough” to
cover the Sun
Lunar and Solar Eclipses
• Why isn’t there an eclipse every month? Because the orbit of the
Moon is inclined with respect to the orbital plane of the Earth
around the Sun.
How often do we see an Eclipse?
• Roughly every 18 months there is a total solar
eclipse visible somewhere on the Earth.
Next:
The Scientific Method
Gravity and the motions
of the planets
Outline of the Scientific Method
• Gather data, make observations, etc.
• Form a hypothesis on how the object of
interest works.
• Determine the observable consequences of
your idea, using reasonable assumptions
and well-established “laws.”
• Formulate experiments to see if the
predicted consequences happen.
Outline of Scientific Method
• If the new observations agree with the
predictions: great, keep going.
• If the new observations don’t agree with
the predictions: start over!
Outline of Scientific Method
A Good Recap From Nick Strobel
http://www.astronomynotes.com/scimethd/s1.htm
Next:
The motion of the planets
A Brief History of Astronomy
Stonehenge (c. 2000 B.C.)
Stonehenge was probably used to observe the sun and
Moon. Image from FreeFoto.com
The great pyramids of Egypt were aligned north-south.
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.
• This is the constellation of
Orion
• 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