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General AstronomyGeneral Astronomy

Physics 103

Dr. Tyler E. NordgrenDr. 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 GoalOur 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

• Homework: 20%• Quizzes: 10%• Labs: 20%• Exams: 30% (15% each)

– 3 exams, lowest grade dropped

• Final: 20%

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Naked-Eye AstronomyNaked-Eye Astronomy

The second oldest profession

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GoalsGoals

• 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

• 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 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.

The Year

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:– Hemisphere is tilted towards Sun.– Sun is high overhead.– Sunlight is more direct.– Daytime is longer. Why?

N

S

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Winter

• During winter:– Hemisphere is tilted away

from Sun.– Sun is low on horizon.– Sunlight is less direct.– Daytime is shorter. Why?

S

N

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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 tell 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