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Introduction to Introduction to AstronomyAstronomy
Physics 103
Dr. Julie A RathbunDr. Tyler E Nordgren
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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 red star to the south?”
– News reports• “Scientists discover giant black hole.”
– Popular entertainment• “Armageddon”, “Mission to Mars”, etc
• Learn how science works and how we know what we know.
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Organization
• Lecture one night a week for 3 hours: – 2 lectures plus outdoor viewing
• Lab one night a week (Wed. or Thurs.)– Meet in lab room about EVERY OTHER
WEEK.
• Check the Syllabus for exact dates!
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How the Class Works
• Combine 3 hour Lecture with outdoor viewing.
• Homework assigned in Lecture.• Formal Labs about every other week.• Weeks without Lab we will assign small
Observing projects.• Record ALL observations in an Observing
Notebook.• 2 one-hour in-class exams.• One final.
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Grading
• Labs: 18%• Observing notebook: 18%• Homework assignments:
18%• 2 In-class exams: 24%• Final: 22%
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The Sky: What do YOU The Sky: What do YOU see?see?
• A sphere of stars.• Are they spaced uniformly?• Do they move?• Do they move relative to each other?• Some do: How do they move?• What are those two bright things (one
of which is always up during the day)?• How do they move and change?
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The Projected Sky• Stars are at different
distances.
• But we see them projected on to a plane.
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A Sphere of Stars
• We see the entire Universe projected on to a sphere.
• The stars are scattered randomly on this sphere (except for the Milky Way).
• In this randomness, we see pictures: Constellations.
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Constellations
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As the World Turns …• As the Earth turns
on its axis, the stars (and everything else) move from east to west.
• Only the North Star (Polaris) appears to stand still.
• There is no Southern Star.
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The Fixed Stars
• The stars are fixed relative to one another.
• The constellations you see today are the ones that ancient peoples saw long ago.
• Over thousands to millions of years even these constellations will change as stars move through the galaxy.
• On time scales of weeks to years, however, five ``stars’’ are seen to move relative to the others.– The Wanderers: the 5 naked-eye Planets.
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Venus
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The Sun and Zodiac• As the Earth
orbits the Sun, the Sun “appears” to move through some constellations.
• These constellations are the Zodiac.
• The path the Sun follows is called the Ecliptic.
• The Sun’s light always blots out the stars behind it.
• Result: Seasonal constellations.
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Seasons
• The North Pole of the Earth is not perpendicular to the plane in which it moves around the Sun.
• The result is Seasons.• Northern Summer: When the Northern
hemisphere is tilted towards the Sun (Southern winter).
• Northern Winter: When the Northern hemisphere is pointed away from the Sun (Southern Summer).
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Solstices, Equinoxes, Oh My….
• Summer Solstice – June 21: The longest day of the year. The Sun reaches its highest point in the sky.
• Winter Solstice – December 21: The shortest day of the year. The Sun is at its lowest.
• Spring Equinox – March 21: The days and nights are EQUAL.
• Fall Equinox – September 21: It’s the same then too.
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The Moon and Phases
• The Moon takes 27.3 days to orbit the Earth.
• The Moon takes 27.3 days to turn on its axis.
• Result: We always see the same face.
• There is no “dark” side of the Moon.
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Lunar Eclipse• Sometimes the Moon passes through
the Earth’s shadow as cast by the Sun.
1A1AComposite photo of the January 2001 lunar eclipse.
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Solar Eclipse
• Sometimes the Moon passes between the Sun and the Earth (and casts a shadow on the Earth).
August 1999 eclipse. Photo from crew of Mir.
1A1AComposite photo of August 1999 solar eclipse.
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Types of Solar
Eclipses• Partial• Total• Annular
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Angular Size
• The Sun is a million times larger than the Earth.
• The Moon is a fourth the size of the Earth.
• The distance from the Earth determines their ANGULAR SIZE.
• Angular size: How big does something look as viewed from the Earth.
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Angles
• The sky is 360 arc degrees around.• 60 arcminutes = 1 arc degree
– The Full Moon is about half an arc degree = 30 arcminutes.
• 60 arcseconds = 1 arcminute– Mars is about 2 arcminutes now.
• 1000 milliarcsecond = 1 arcsecond– Polaris is 46 milliarcseconds in diameter– An astronaut on the Moon is 2 milliarcseconds
tall!
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Angular vs. Linear (True) Size
• How big an object LOOKS from the Earth depends upon its TRUE size (Linear Size) and distance from Earth
• Angle = Linear Size / Distance
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