PSC101 Introduction to Astronomy

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PSC101- Introduction to Astronomy
You know Orion always comes up sideways.
Throwing a leg up over our fence of
mountains, And rising on his hands, he looks in
on me Busy outdoors by lantern-light with
something I should have done by daylight, and
indeed, After the ground is frozen, I should have
done Before it froze, and a gust flings a
handful Of waste leaves at my smoky lantern
chimney To make fun of my way of doing things, Or
else fun of Orion's having caught me. Has a man,
I should like to ask, no rights These forces are
obliged to pay respect to?" So Brad McLaughlin
mingled reckless talk Of heavenly stars with
hugger-mugger farming, Till having failed at
hugger-mugger farming, He burned his house down
for the fire insurance And spent the proceeds on
a telescope To satisfy a life-long
curiosity About our place among the
infinities. The Star-Splitter Robert Frost
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What is Astronomy?

• A branch of science.
• Studies the motions, origins, and properties of
  celestial objects using the scientific method.
• Has its origins in ancient history.

Crescent Jupiter and Io from Cassini Spacecraft
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What is Astronomy?

• Almost all knowledge is gained through study of
  light.
• Almost always impossible to perform experiments
  (too far)
• Astronomy has made great advances with new
  technologies.

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Origins of Astronomy - Time

• The Sun, Moon and stars served humanity for
  thousands of years as a clock and a calendar.
  Their motions were predictable.
• Today we have other means of measuring time but
  long ago an accurate understanding of the skies
  could mean life or death.
• Studying the skies told people when to plant and
  harvest, when to go out to sea, when to expect
  good or bad weather, etc.

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Time - Day Night

• How do you know what time it is?
• How high is the sun in the sky?
• Morning - sunrise
• Evening - sunset
• Does the Sun really rise and set?

Sunrise behind the Space Shuttle
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Time Day Night- Earths Spin

• In reality because the Earth rotates on its axis
  from West to East the Sun, Moon and stars all
  appear to move from East to West.
• The Sun appears to rise in the East and set in
  the West because the Earth spins.
• The Moon however also orbits the Earth traveling
  from West to East but it takes much longer than
  24 hours to orbit the Earth. However from night
  to night you can see the Moon change position.

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Time - The Moon and Months

• It takes the Moon about one month to orbit the
  Earth
• Our calendar of months was originally based on
  the Moon
• The word Month is derived from the word Moon
• Jewish and Islamic calendars are still based on
  the Moon. The New Moon starts a new month.

Photograph of crescent Moon and crescent Venus
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Time - Year and Seasons

• Cultures around the world marked the Suns yearly
  motion through the stars.
• Summer - Sun is farthest North.
• Winter - Sun is farthest South.
• Knowing when these dates occurred was very
  important for agricultural societies

First day of Summer at Stonehenge, England
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Earths Orbital Motion

• As the Earth orbits the Sun it appears that the
  Sun is actually moving.
• The Sun appears to move through a set of 12
  constellations over one year.
• Some constellations can only be seen during
  certain times of year.

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Earths Orbital Motion

• The path through the sky along which the Sun
  appears to move is called the ecliptic

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Earths Orbital Motion
The other planets appear to follow similar paths
near the ecliptic. The band through which the Sun
and the planets appear to move is called the
Zodiac
But the planets really do move! As seen from
Earth their motions appear more complicated than
they actually are. In fact they sometimes appear
to move backwards! This is called retrograde
motion.