class 8. tues feb 26
summary for lecture.
warning: this is not supposed to be a substitute for
reading the textbook.
in textbook: chapter 10/11 end, chapter 15 start

Lect. Part. A.
next chapters 10 and 11.
various effects that influence the coordinates of objects you see.
precession.  like in handout #11. you probably should see an example
based on earth's axis's motion.
nutation.  based on earth's axis's motion, an extra wobble, an
extra component. magnitude is a few arcseconds. At worst
your off by 9.2 arcsec.
aberration of starlight  just like aberration of rainfall.
at worst can be off by about 21 arcsec.
stellar parallax.
our view of space changes a bit, the 3D aspect comes into place.
parallax on Earth.
refraction by the atmosphere.
we covered prec, nut, and aberration last time
stellar parallax 
earth moves in its orbit, so position of object will change.
also depends on eclip coordinates. As we move in a ~circle,
the star will move in a little ellipse  if the star's near
the ecliptic pole, it will move in a circle too. if the star's
near the ecliptic, it will move in a thin ellipse  a line.
the magnitude depends on the distance, not on the coords.
e.g. if something is 1 l.y. away, how much shift is this?
set up the triangle. tan pi = 1 AU/ 1 l.y.
1 l.y. = 9.46e12 km. so answer is 3.26 arcsec.
in fact this defines a 'parsec'. 1 arcsec of parallax.
greatest parallax is for alpha Cen, 4.4 ly = 1.3 pc away.
Usually the way this is used is to tie down the distance
scale. If you soandso much parallax, you immediately know
the distance. the big deal with this is the satellite
Hipparcos 19891992, measured parallax for >10^5 stars
to milliarcsec precision. Next step is Gaia, launch in 2011,
measure many more stars to precision of 20 microarcsec.
Getting the parallax to more distant stars will give us
a great 3D view of the Milky Way.
parallax on earth 
important for solar system, not so much for other stuff.
why? farthest you can be from center of earth is 6378 km.
in stellar terms, that's nothing.
but for solar system, it can be important.
amount depends on distance. e.g.
an asteroid transits for you and for somebody at NP.
0.1 AU from Earth, declination is 45 deg.
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you observe the asteroid's declination to be
45 deg 01' 02.2". 
so >1' difference!
refraction by atmos  draw.
snell's law. n of vacuum is 1. sin i/sin r = n.
note that this is a simplification, in fact the atmosphere
has layers and each layer has a slightly different n.
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 Spring '09
 Fernandez
 Astronomy

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