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# ls8 - class 8 tues feb 26 summary for lecture warning this...

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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 3-D 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 so-and-so much parallax, you immediately know the distance. the big deal with this is the satellite Hipparcos 1989-1992, 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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