ASTRO 101!
Principles of Astronomy!
Instructor: Jerome A. Orosz"
(rhymes with
boris )"
Contact:!
•  Telephone: 594-7118!
•  E-mail: jorosz@mail.sdsu.edu!
•  WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/!
•  Office: Physics 241, hours T TH 2:00-3:20!
Text: "
"
Discovering the Essential Universe,
Fifth Edition "
by "
Neil F. Comins"
!
Homework!
Course WWW Page!
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_spring2014.html!
Note the underline: … ast101_spring2014.html …!
!
Also check out Nick Strobel s Astronomy Notes:!
http://www.astronomynotes.com/!
!
Spring 2014
No appointment needed!
Just drop by!
Where: Room 215A, physics-astronomy building (PA-215A; it’s the
room right next to the main lab room).
When: All semester long, at the following days and times:
•  Monday:
1 – 3 PM
•  Tuesday:
11 AM – 3 PM; 5 – 6 PM
•  Wednesday: 12 – 2 PM; 5 – 6 PM
•  Thursday: 5 – 6 PM
Homework!
Lunar and Solar Eclipses!
•  Go to a planetarium show in PA 209:!
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The days and times of the shows will be (all shows last less than 1 hour):!
Tuesday February 4 2:00 PM!
Wednesday February 5 1:00 PM!
Thursday February 6 5:00 PM!
Friday February 7
1:00 PM!
Monday
February 10
1:00 PM!
Tuesday February 11 10:00 AM!
Wednesday February 12 11:00 AM!
Thursday
February 13
12:00 PM!
Friday
February 14
12:00 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 data and time of
the session.!
•  Homework due February 11: 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 February
11. !
Next:"
"
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!
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!
•  Trick photography often involves playing with
different distances to create the illusion of large
or small objects:!
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.!
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.!
http://www.tadbit.com/2008/03/06/top-10-holding-the-sun-pictures/!
http://www.stinkyjournalism.org/latest-journalism-news-updates-45.php!
!
•  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.!
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!
Lunar Eclipses!
•  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 .!
•  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.!
Annular Eclipses!
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.!
How often do we see an Eclipse?!
•  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. !
The Scientific Method!
Gravity and the motions
of the planets!
•  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.!
Lunar and Solar Eclipses!
•  The angular sizes of
the Sun and Moon
vary slightly, so
sometimes the Moon
isn’t big enough to
cover the Sun!
Next:!
Solar Eclipses!
•  Roughly every 18 months there is a total solar
eclipse visible somewhere on the Earth.!
Outline of the Scientific Method!
Outline of 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.!
•  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!
Stonehenge (c. 2000 B.C.)!
A Brief History of Astronomy!
Stonehenge was probably used to observe the sun and!
Moon. Image from FreeFoto.com!
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.!
The great pyramids of Egypt were aligned north-south.!
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.!
Geocentric Model!
•  The fixed stars were on the Celestial
Sphere whose rotation caused the rising
and setting of the stars.!
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.!
•  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!
Planetary Motion!
•  Here is a plot of the path
of Mars.!
•  Other planets show similar
behavior.!
•  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. !
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.!
Ptolomy s Epicycles!
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!
Ptolomy s Epicycles!
•  By adding epicycles, very complicated motion could be
explained.!
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.!
Image from Nick Strobel s Astronomy Notes (http://www.astronomynotes.com/).!
The Middle Ages!
•  Not much happened in Astronomy in the
Middle Ages (100-1500 A.D.).!
Next:!
The Copernican Revolution!