Dr. Henry Throop / University of Colorado
Astronomy 1110
June 20, 2000
Lecture 12: Cosmic Collisions (Planetarium)

Announcements
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Exams: will be back on wednesday.
HW #3: hand out today, due Thursday (not Wednesday)
Observing: Tomorrow, depending on weather

Today is summer solstice!  Day = 5:30 AM .. 8:30 PM = 15 hr day, 9 hr night 

So, if we were on south pole, we get no sun at all, and if we're
on north pole, the sun is as high as it gets, and is up all night
long.

Planetarium
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Retrograde motion
A lot of people seemed to have some misunderstanding as to what we mean
by retrograde motion.

  o Every day, stars rise & set, and so do planets
  o But, planets also move through their orbits: 
      Mercury takes 3 months to circle sun
      Mars takes 1.5 years
      Jupiter takes 10 years
      Pluto takes 243 years
      etc.
  o So, planets move across the sky.  
    If Jupiter takes 10 years to circle the sun, that means that for us,
    Jupiter takes 10 years to crawl across the sky, and 10 years later, it'll
    come back to where it started.

  o So, we can take a snapshot of it, night after night, and follow its motion

  o What happens is that once every year, the earth passes it -- just
    like passing a slow-moving car on the highway, and Jupiter appears to
    move backwards, for maybe a month.

  o Every year, as Earth goes through its orbit, we'll pass jupiter
    once.

  o And when we pass it, it looks like it goes backwards, and makes a
    little loop.  These are the `retrograde loops', which Ptolemy
    explained with the epicycles -- the planets were actually moving
    backwards in space -- and Copernicus said it was much more easy to
    talk about them as being if the Earth moves, and overtakes them in
    their orbit.

Zodiac
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We have several `special constellations' in the sky.  There's 88 in the
planetarium here total, and some of them are pink.  And we know that
the sun, & the planets, just go through one line on the sky, since all
the planets are in the same plane, like peas on a plate.

Used to be that, 2000 years ago, that if you were born with the sign
Gemini -- well, that meant that the sun was in Gemini on that day, or
that month, which is most of June.

The Greeks knew about motions of the sky, from one day to the next.
But what they didn't know about, was this slow precession of the Earth,
the wobble of its axis -- that the North pole, if you come back in a
thousand years, doesn't point North any more.  And if it doesn't, then
the Sun isn't in the same constellation any more.

So, the Greeks made up things, like if you're Gemini, you'll be
restless, intelligent, and talkative, or whatever.  And the situation
is that, well, if you're a Gemini these days, the sun isn't even _in_
Gemini when you're born -- the Sun is really in Cancer.  So it's a bit
silly to associate these things with your birth signs.

Nevertheless, it's not useless: we still have some nice constellations
from the Greeks, twelve of them, and we can go through them.

Capricorn
Aquarius
etc.