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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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