http://www.plavchan.com/ast/113/
ASTRONOMY 113
Modern Astronomy
Lecture: Tuesday, Thursday 9:30-10:45am
Temple 0002
Dr. Peter Plavchan
626-234-1628
PeterPlavchan@missouristate.edu
Office Hours: Tues 11am-1pm; Thurs 11am-2pm
Textbook: Sapling Learning + Discovering the Essential Universe, 6th edition
Introductions
http://www.plavchan.com
You
Please take out a piece of paper, or compose an email to me on your
phone/computer, and answer:
1. Name & Student ID# (also for attendance)
2. How many semesters have you completed at MSU?
3. What do you know or have you heard about astronomy?
4. What is your dream career?
5. Why are you taking this class?
6. What do you dread doing? What do you love doing?
7. Can you give me tips on how you learn best?
8. What would you like to know about me?
9. What would you like me to know about you?
10. What can I teach you?
11. What can you teach me?
Syllabus & Exams
http://www.plavchan.com/ast/113/
The internet will be used extensively in
this class.
Lecture notes will be posted after class.
Please let me know if you need help
accessing the web, or if the web pages
are not working.
Class Blog
http://www.blogger.com
You will be invited to be an author at:
http://ast113.blogspot.com
Group Presentations
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Sign up for a group presentation
Presentations are 5 minutes each. You will be expected to prepare a presentation
with Powerpoint, Prezi, Google Sheets or similar, and add an audio narrative with
free software like Jing, Screencast, etc., and post the final combined video to the
blog as a single video on youtube or with other video software (like iMovie).
From each lectures topic, pick a subject to go into detail on. Consult with the
professor on the topics covered in class well before you prepare your presentation.
Do not present a broad overview of the weeks lecture materials.
Will count towards 10% of final grade.
Students in the class will post questions as comments on the blog, and you are
required to respond to them the two weeks following your presentation, which
will count towards your group project grade.
The week after your presentation, you will be required to ask at least one question
on the blog (posted as a comment) for the next week’s presenters. They will have
to respond to your questions.
Extra Credit Options
Extra credit is available and can contribute up to 5-10%. The purpose of extra credit is to find a
balance between your interests and the subject material of astronomy. Possible extra credit
options must be approved by the instructor, but can include for example:
•
Visit Baker Observatory. Bring back proof - photographic - of your visit. To qualify for the extra
credit, please write a one page summary of your visit. The public Baker Observatory night is TBD
•
Astronomy as art. Many of you are non-science majors, and excel in other areas of specialization - art,
writing, music, etc. There is a vast history of art inspired by astronomy. For example, consider constructing
a scale model of our galaxy or local group of galaxies.
In order to qualify for the extra credit, please create an original work of art, music or writing inspired by
astronomy and the material you have learned in class. You may team with up to two other people.
ALL EXTRA CREDIT REPORTS ARE DUE BY THE FINAL EXAM, DIRECTLY TO ME.
Homeworks - Sapling Demo
Our Place in the Universe
Our Modern View of the
Universe
A Modern View of the Universe
Our goals for learning:
•
•
•
•
What is our place in the universe?
How did we come to be?
Can we know what the universe was like in the past?
Can we see the entire universe?
Astronomy
The branch of science dedicated to the study
of everything in the Universe that lies above
Earth’s atmosphere
Scale of our Universe
• Video
Universe
80 yr ago
What is our place in the universe?
400 yr
2500 yr
Star
A large, glowing ball of mostly hydrogen gas
that generates heat and light through nuclear
fusion at its center
Our star – the Sun
Context: “gas” in
astronomy usually
means H, or H2, but
could include other
atoms and molecules in
the gas state.
Planet
Mars
Neptune
A moderately large object that orbits (goes around) a star;
it shines by reflected light. Planets may be rocky, icy,
or gaseous in composition.
Moon (or satellite)
An object that orbits
a planet.
Ganymede (orbits Jupiter)
Asteroid
A relatively
small and
rocky object
which orbits a
star.
Comet
A relatively
small and icy
object that
orbits a star.
Solar (Star) System
A star and all
the material
that orbits it,
including its
planets and
moons.
Note: planets and orbits
are not to scale;
planets are tiny
compared to their
orbits.
Nebula
An interstellar cloud
of hydrogen gas and/or tiny smoke-like particles
called “dust”
Galaxy
A great island of stars in space, all held together by
gravity and orbiting a common center
M31, The Great Galaxy in Andromeda
Universe
The sum total of all matter and energy;
that is, everything within and between
all galaxies
How can we know what the universe was
like in the past?
• The key: light travels at a finite speed
– 300,000 km/s, 186,000 miles/s, 670 million miles per hour
• You can circle Earth 8 times in 1 second
• More on the Speed of Light when we discuss Relativity
Destination
Light travel time
Moon
1 second
Sun
8 minutes
Sirius
8 years
Andromeda Galaxy 2.5 million years
Thus, we see objects as they were in the past:
The farther away we look in distance,
the further back we look in time.
Light-year
•
•
•
•
•
The distance light can travel in one year.
About 10 trillion km (6 trillion miles).
A light-year is NOT a unit of time!
Can also talk about light-seconds.
At great distances we see objects as they were
when the universe was much younger.
What have we learned?
• What is our physical place in the universe?
– Earth is part of the Solar System, which is in the Milky Way galaxy, which is a
member of the Local Group of galaxies in the Local Supercluster
• How can we know that the universe was like in the
past?
– When we look to great distances we are seeing events that
happened long ago because light travels at a finite speed
• Can we see the entire universe?
– No, the observable portion of the universe is about 13.7
billion light-years in radius because the universe is about
13.7 billion years old. (We may round this number to 14
billion for convenience but the best modern measurements
give 13.7 billion years since the time from the Big Bang.)
The Scale of the Universe
Our goals for learning:
•
•
•
•
•
How big is Earth compared to our solar system?
How far away are the stars?
How big is the Milky Way Galaxy?
How big is the universe?
How do our lifetimes compare to the age of the
universe?
It is very important to grasp the huge distances and
enormous time spans that we deal with in astronomy. The
way to do this is to create a SCALE MODEL.
The easy questions
• How far away are the Sun and
Moon?
• The Moon is 384 million m
from Earth (~250,000 miles).
11
• The Sun is 1.5x10 m from
Earth (150 billion m, or ~93
million miles). This distance is
called 1 astronomical unit (AU).
How big is Earth compared to our Solar System?
Let’s reduce the size of the solar system by a factor of
10 billion; the Sun is now the size of a large grapefruit
(14 cm diameter or about 5.6 inches; 2.54 cm = 1 inch).
How big is Earth on this scale?
A.
B.
C.
D.
an atom
a ball point
a marble
a golf ball
Radius of the Sun = 700,000 km
Diameter Sun = 1.4 x 1011 cm
Divide by 1010 to get 14 cm
Let’s reduce the size of the solar system by a factor of 10
billion; the Sun is now the size of a large grapefruit (14 cm
diameter).
How big is Earth on this scale?
A.
B.
C.
D.
an atom
a ball point
a marble
a golf ball
How far apart would our model Sun and Earth be on this scale?
The scale of the solar system
• On a 1-to-10 billion scale:
– Sun is size of a large grapefruit (14 cm)
– Earth is size of a ball point, 15 meters away
– 15 meters is about 107 grapefruits
– Which means in the real solar system you could
fit about107 Suns into the Earth –Sun distance
How far away are the stars?
On our 1-to-10 billion scale, it’s just a few minutes walk to
Pluto. How far would you have to walk to reach the nearest
star to the Sun - Alpha Centauri?
A.
B.
C.
D.
1 mile
10 miles
100 miles
the distance across the U.S. (2500 miles)
How far away are the stars?
On our 1-to-10 billion scale, it’s just a few minutes walk to
Pluto. How far would you have to walk to reach the nearest
star to the Sun - Alpha Centauri?
A.
B.
C.
D.
1 mile
10 miles
100 miles
the distance across the U.S. (2500 miles)
Answer: D, the distance across the U.S.
Thought Question
Suppose you tried to count the more than 100 billion (1011)
stars in our galaxy, at a rate of one per second…
How long would it take you?
A.
B.
C.
D.
a few weeks
a few months
a few years
a few thousand years
Suppose you tried to count the more than 100 billion stars in
our galaxy, at a rate of one per second…
How long would it take you?
A.
B.
C.
D.
a few weeks
a few months
a few years
a few thousand years (100 billion seconds is nearly
3,200 years. Why? Because there are about 30 million
seconds in a year, so 1011/3x107 = 0.33x104 = 3.3 x 103)
How big is the (observable) Universe?
• The Milky Way is one of about 100 billion galaxies.
• (1011 stars/galaxy) x (1011 galaxies) = 1022 stars
As many stars as grains of (dry) sand on all Earth’s beaches…
How do human lifetimes compare to the
AGE of the Universe?
• The Cosmic Calendar: a scale on which we
compress the history of the universe into 1 year!
How do human lifetimes compare to the
age of the Universe?
• The Cosmic Calendar: a scale on which we compress the
history of the universe into 1 year. 1 day represents about
40 million years; 1 second represents about 440 years.
What have we learned?
• How big is Earth compared to our solar system?
– The distances between planets are huge compared to their
sizes—on a scale of 1-to-10 billion, Earth is the size of a
ball point and the Sun is 15 meters away
• How far away are the stars?
– On the same scale, the stars are thousands of km away
• How big is the Milky Way galaxy?
– It would take more than 3,000 years to count the stars in
the Milky Way Galaxy at a rate of one per second, and
they are spread across 100,000 light-years
What have we learned?
• How big is the universe?
– The observable universe is almost 14 billion
light-years in radius and contains over 100
billion galaxies with a total number of stars
comparable to the number of grains of sand
on all of Earth’s beaches
• How do our lifetimes compare to the age of
the universe?
– On a cosmic calendar that compresses the
history of the Universe into one year, human
civilization is just a few seconds old, and a
human lifetime is a fraction of a second
Spaceship Earth
Our goals for learning:
•
•
•
•
How is Earth moving in our solar system?
How is our solar system moving in the Galaxy?
How do galaxies move within the Universe?
Are we ever sitting still?
How is Earth moving in our solar system?
• Contrary to our perception, we are not “sitting still.”
• We are moving with the Earth in several ways, and at
surprisingly fast speeds…
The Earth rotates
around its axis once
every day.
The spin rate at the Equator is ~1000 mph,
twice as fast as a commercial airliner.
Earth orbits the Sun (revolves) once every year:
• at an average distance of 1 AU ≈ 150 million km
• with Earth’s axis tilted by 23.5º (pointing to Polaris)
• and rotating in the same direction it orbits, counterclockwise as viewed from above the North Pole.
Our Sun moves relative to the other stars in the local
Solar neighborhood…
• typical relative speeds of more than 70,000 km/hr
• but stars are so far away that we cannot easily notice their motion
… And orbits the center of the Milky Way galaxy
every 230 million years.
More detailed study of the Milky Way’s rotation reveals
one of the greatest mysteries in modern astronomy
Most of Milky Way’s
light comes from disk
and bulge …
…. but most of the
mass is in a huge
and DARK halo.
How do galaxies move within the universe?
The Universe is
expanding.
In the 1920s Edwin
Hubble discovered
that galaxies are
carried along with
the expansion of
the Universe.
But how did
Hubble figure out
that the universe is
expanding?
Hubble discovered that:
• All galaxies outside our Local Group are
moving away from us.
• The more distant the galaxy, the faster it is
racing away.
Conclusion: We live in an expanding universe.
What have we learned?
• How is Earth moving in our solar system?
– It rotates on its axis once a day and orbits the
Sun at a distance of 1 A.U. = 150 million km
• How is our solar system moving in the Milky
Way galaxy?
– Stars in the Local Neighborhood move
randomly relative to one another and orbit the
center of the Milky Way in about 230 million
years
What have we learned?
• How do galaxies move within the universe?
– All galaxies beyond the Local Group are
moving away from us with expansion of the
Universe: the more distant they are, the faster
they’re moving
• Are we ever sitting still?
– No!
OKAY, NOW WE HAVE A GOOD OVERALL PERSPECTIVE.
NEXT WE NEED SOME MORE DETAILS
Discovering the Universe for
Yourself
Patterns in the Night Sky
Our goals for learning:
• What does the universe look like from Earth?
• Why do stars rise and set?
• Why do the constellations we see depend on
latitude and time of year?
The Celestial Sphere
Stars at different
distances all appear
to lie on the Celestial
Sphere.
Ecliptic is Sun’s
apparent path
through the celestial
sphere.
The Celestial Sphere
The 88 official
constellations
cover the celestial
sphere.
It is important to realize that
these named patterns have no
relation to life on Earth and
the stars in a given
constellation are often not
connected with each other
physically.
The Milky Way
Fish Eye lens view
A band of faint light
making a circle
around the celestial
sphere.
What is it?
Our view into the
“plane” of our
spiral galaxy.
The Local Sky
Altitude (above horizon) Azimuth (along horizon) specifies location
Zenith: The point directly overhead
Horizon: All points 90° away from zenith
Meridian: Line passing through zenith from N to S points
We measure the sky using angles
• Full circle = 360º
• 1º = 60
(arcminutes)
• 1 = 60
(arcseconds)
Angular Size
360 degrees
angular size = physical size ´
2p ´ distance
An object’s angular
size appears smaller
if it is farther away
Aside: We can define a new unit of angular
measure called a radian such that
1 radian = 360/2π = 57.3 degrees
Why do stars rise and set?
Earth rotates west to east, so
stars appear to circle from
east to west.
The sky varies with latitude but not longitude
The sky varies as Earth orbits the Sun
• As the Earth orbits the Sun, the Sun appears to move eastward
along the ecliptic.
• At midnight, the stars on our meridian are opposite the Sun in
the sky.
The Reason for Seasons
Our goals for learning:
• What causes the seasons?
• How do we mark the progression of the
seasons?
• How does the orientation of Earth’s axis
change with time?
What causes the seasons?
Seasons depend on how Earth’s axis affects the directness of sunlight
Axis tilt changes directness of sunlight
during the year
Sun’s altitude in the sky also changes
with seasons
Sun’s position at noon in summer:
higher altitude means more direct
sunlight.
Sun’s position at noon in winter:
lower altitude means less direct
sunlight.
How do we mark the progression of the seasons?
• We define four special points:
summer solstice
winter solstice
spring (vernal) equinox
fall (autumnal) equinox
We can recognize solstices and equinoxes by
Sun’s path across sky
Summer solstice: Highest path,
rise and set at most extreme
north of due east.
Winter solstice: Lowest path, rise
and set at most extreme south of
due east.
Equinoxes: Sun rises precisely
due east and sets precisely due
west.
How does the orientation of Earth’s axis
change with time?
•Although the axis seems fixed on human time
scales, it actually precesses over about 26,000 years.
 Polaris won’t always be the North Star.
 Positions of equinoxes shift around orbit; e.g.,
spring equinox, once in Aries, is now in Pisces!
Earth’s axis
precesses like the
axis of a spinning
top
The Moon,
Our Constant Companion
Our goals for learning:
• Why do we see phases of the Moon?
• What causes eclipses?
Phases of Moon
• Half of Moon is
illuminated by Sun
and half is dark.
• NOT caused by
Earth’s shadow!
• We see a changing
combination of
the bright and
dark faces as
Moon orbits
We see only one side of Moon
Synchronous rotation:
the Moon rotates
exactly once with each
orbit of ~28 days
That is why only one
side is visible from Earth
What causes eclipses?
• The Earth and Moon cast shadows.
• When either passes through the other’s shadow,
we have an eclipse.
When can eclipses occur?
• Lunar eclipses
can occur only
at full moon.
• Lunar eclipses
can be
penumbral,
partial, or total.
• August 21st,
2017
When can eclipses occur?
• Solar eclipses can
occur only at new
moon.
• Solar eclipses can
be partial, total,
or annular.
Why don’t we have an eclipse at every new and full moon?
– The Moon’s orbit is tilted 5° to ecliptic plane…
– So we have about two eclipse seasons each year, with a lunar
eclipse at new moon and solar eclipse at full moon.
Length of a Day
• Sidereal day:
Earth rotates once
on its axis relative
to the distant stars
in 23 hrs, 56 min,
and 4.07 sec.
Length of a Day
• Solar day: The Sun
makes one circuit
around the sky in
24 hours (by
definition)
Why are the two different?
• Solar day is longer than a sidereal day by about 1/360
because Earth moves about 1° in orbit each day
Length of a Month
• Sidereal month: Moon
orbits Earth in 27.3 days.
• Earth & Moon travel 30°
around Sun during that
time (30°/360° = 1/12)
• Synodic month: A cycle
of lunar phases;
therefore takes about
29.5 days, 1/12 longer
than a sidereal month
• Synodic means “meeting”
Length of a Year
• Sidereal year: Time for
Earth to complete one
orbit of Sun
• Tropical year: Time for
Earth to complete one
cycle of seasons
• Tropical year is about 20
minutes (1/26,000)
shorter than a sidereal
year because of Earth’s
precession.
How do we tell the time of day?
• Apparent solar time
depends on the
position of the Sun in
the local sky
• A sundial gives
apparent solar time
Universal Time
• Universal time (UT) is defined to be the mean
solar time at 0° longitude.
• It is also known as Greenwich Mean Time (GMT)
because 0° longitude is defined to pass through
Greenwich, England
• It is the standard time used for astronomy and
navigation around the world
Standard Time & Time Zones
• Rapid train travel required time to be standardized
into time zones (time no longer local)
When and why do we have leap
years?
• The length of a tropical year is about 365.25 days.
• In order to keep the calendar year synchronized
with the seasons, we must add one day every four
years (February 29).
• For precise synchronization, years divisible by 100
(e.g., 1900) are not leap years unless they are
divisible by 400 (e.g., 2000).
Celestial Coordinates
• Right ascension: Like longitude on celestial sphere
(measured in hours with respect to spring equinox).
• Declination: Like latitude on celestial sphere (measured
in degrees above celestial equator)
Time by the Stars
• Sidereal time is equal to right ascension that is
passing through the meridian
• Thus, the local siderial time is 0h0m when the
spring equinox passes through the meridian
• A star’s hour angle is the time since it last
passed through the meridian
Local Sidereal Time = RA + Hour Angle
How can you determine your latitude?
• Latitude equals altitude of celestial pole
• Altitude and declination of star crossing
meridian also gives latitude.
• You can determine Sun’s declination
from the day of the year
• Measuring the Sun’s altitude when it
crosses meridian can tell you latitude
How can you determine your
longitude?
• In order to determine your longitude
from the sky, you need to know time of
day because of Earth’s rotation
• You also need to know day of year
because of Earth’s orbit
• Accurate measurement of longitude
requires an accurate clock.
GPS Navigation
• The Global Positioning System (GPS) uses a set of
satellites in Earth orbit as artificial stars
• GPS devices use radio signals to determine your
position relative to those satellites
• GPS satellites correct for General Relativity!
• Ok, on to the terrestrial planets!
Planets Known in Ancient Times
• Mercury
– difficult to see; always close
to Sun in sky
• Venus
– very bright when visible;
morning or evening “star”
• Mars
– noticeably red
• Jupiter
– very bright
• Saturn
– moderately bright
What was once so mysterious
about planetary motion in our sky?
• Planets usually move slightly eastward from night to
night relative to the stars.
• But sometimes they go westward relative to the stars
for a few weeks: apparent retrograde motion
We see apparent retrograde motion when
we pass by a planet in its orbit.
Explaining Apparent Retrograde
Motion
• Easy for us to explain: occurs when we “lap”
another planet (or when Mercury or Venus
laps us)
• But very difficult to explain if you think that
Earth is the center of the universe!
• In fact, ancients considered but rejected the
correct explanation
Why did the ancient Greeks reject the real
explanation for planetary motion?
Their inability to observe stellar parallax was a major factor.
p
tan p = 1AU/d (AU)
For small angles:
d
p = 1/d
If the angle p = 1 second
of arc then d is defined as
1 parsec (=206265 AU).
1 AU
Not to scale
The Greeks knew that the lack of observable
parallax could mean one of two things:
1. Stars are so far away that stellar parallax is
too small to notice with the naked eye
2. Earth does not orbit the Sun; the Earth is the
center of the universe
With rare exceptions such as Aristarchus, the Greeks
rejected the correct explanation (1) because they
did not think the stars could be that far away
Thus setting the stage for the long, historical showdown between
Earth-centered and Sun-centered systems.