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General Astronomy
Physics 103
Dr. Tyler E. Nordgren
Dr. Julie A. Rathbun
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Topics: The Night Sky …
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… from the Solar System…
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… to the Stars…
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… to the Milky Way …
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… and to the Universe Beyond.
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Our Goal
• Understand:
– The sky above
• “What’s that bright yellow star over head?”
• How can I tell what time it is by where the Sun is?
– News reports
• “Scientists discover giant black hole. Should I be afraid?”
– Popular entertainment
• “Armageddon”, “Enterprise”, etc
• Learn how science works and how we know what
we know.
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Organization
• Lecture two days a week (Tues. and Thurs.):
– 1 hour and 20 minutes
• Lab one day a week (Mon. or Wed.):
– 2 hours and 50 minutes in Hornby 2
• Evening observing (4 times): 7:30 pm – 9:30 pm
• Check the syllabus and web site for exact dates!
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Grading
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Homework: 20%
Quizzes: 10%
Labs: 20%
Exams: 30% (15% each)
– 3 exams, lowest grade dropped
• Final: 20%
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Naked-Eye Astronomy
The second oldest profession
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Goals
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How does astronomy affect your daily life?
The Sun: Time without clocks
The Seasons: Dates without calendars
The ancients did it, so can you!
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The Sun
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The Sun is a star.
We live by its warmth.
We die by its extreme heat or frigid absence.
It is the source of nearly all our energy.
– How?
• It is the source of all our food.
– How?
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The Day
• The Earth is a sphere that turns
on its axis.
– How long does it take to turn once
on its axis?
• Light from the sun lights only
one half of Earth.
• As the Earth turns the sun
“rises” and “sets.”
• So does the moon.
• So do the stars.
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The Year
• The Earth orbits the Sun.
– How long does it take to go once around the Sun?
• The orbit is nearly circular, but is actually an
ellipse.
An exaggeration.
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Group Question
• Why are there seasons? Hint: It has nothing to do
with the distance from the Sun.
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The Seasons
• The Earth is tilted on its axis.
• During half the year, the northern hemisphere is
tilted more towards the Sun.
– Summer in California.
– Winter in Australia
• During the other half of the year the southern
hemisphere is tilted towards the Sun.
– Winter in California
– Summer in Australia
http://kids.msfc.nasa.gov/News/2000/News-VernalEquinox.asp
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Summer
• During summer:
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Hemisphere is tilted towards Sun.
Sun is high overhead.
Sunlight is more direct.
Daytime is longer. Why?
S
N
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Winter
• During winter:
N
S
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Hemisphere is tilted away from Sun.
Sun is low on horizon.
Sunlight is less direct.
Daytime is shorter. Why?
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Time
• Q: So, how can we use the Sun to tell time?
• A: Sundials
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Time
• Sun rises in the East.
– If the Sun is towards the East, it is
before noon.
• Sun sets in the West.
– If the Sun is towards the West, it is
after noon.
• At noon?
– For most of northern hemisphere: Sun is due south.
– For most of northern hemisphere: Sun is due north.
– Near equator, Sun is overhead.
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Date
• During northern summer, Sun high over head.
– When Sun casts the smallest shadow at noon 
Summer solstice.
• During northern winter, Sun low to the south.
– When Sun casts the longest shadow at noon  Winter
solstice.
• During northern spring and fall, Sun in between.
• Sun also rises due east, sets due west.
– When Sun casts shadow due west at sunrise  spring
or fall equinox.
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• In Summer, the sun is high.
• In winter, the sun is low.
• The height of the sun (at a given time) can tell you
the date.
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The Constellations
• We may also use the stars to tel
time.
• We see stars projected on the
sphere of the sky.
• They make patterns:
constellations.
– No physical association
• We see those constellations
opposite the Sun from Earth.
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Seasonal Constellations
• As the Earth orbits
the Sun, different
stars are up at night.
– There are summer
and winter
constellations!
• “The Dog Days of
Summer”
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Ancient Astronomy
• Stonehenge (~3000 BC):
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Ancient Astronomy
• Chaco Canyon (“Anasazi” ~1200 AD):
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Fajada Butte
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