Download The Sky

Introduction to Astronomy
Physics 103
Dr. Julie A Rathbun
Dr. 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 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
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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
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Labs: 18%
Observing notebook: 18%
Homework assignments: 18%
2 In-class exams: 24%
Final: 22%
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The Sky: What do YOU see?
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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.
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Composite 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.
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Composite 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 =
arcminutes.
30
• 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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