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Parallax and Proper Motion

Parallax and Proper Motion. AST 112. The Geocentric Model. IT IS WRONG!. Fine. So what is it?. The Geocentric Model places Earth at the center of the Universe. Everything (Sun, Moon, stars, etc.) revolves around the Earth. The Geocentric Model.

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Parallax and Proper Motion

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  1. Parallax and Proper Motion AST 112

  2. The Geocentric Model IT IS WRONG!

  3. Fine. So what is it? The Geocentric Model places Earth atthe center of the Universe. Everything(Sun, Moon, stars, etc.) revolves aroundthe Earth.

  4. The Geocentric Model But it’s wrong. So why did it survivefor 2000 years?

  5. Parallax • Things at different distances line updifferently when you look at them fromdifferent angles.Hold your two index fingers at different distances and move your head around to see this.

  6. Parallax

  7. Opposition to the Geocentric Model • Sun-centered model proposed by Aristarchus • All stars thought to be on surface of a sphere • At equal distances! • Assertions: • If Earth orbits sun, angular separations of stars change • If they don’t, stars must be unrealistically far away • Observation: • The angular separations of stars don’t seem to change

  8. Parallax and the North Star What if the North Star were really close?

  9. Stellar Parallax • Stellar parallax was finally observed in 1838 • Measured in arcseconds • 1/60 of the thickness of a fingernail at arm’s length • Can be used to find the distance to nearby stars • This marked the definitive end of the Earth-centered model.

  10. Parallax • Earth moves to opposite sides of the Sun every 6 months • This creates a very small parallax • Closer stars show more parallax than farther stars

  11. Stellar Parallax • Stellar parallax is usually measured in arcseconds • The thickness of your fingernail at arm’s length is an arcminute. • 1/60 of an arcminute is an arcsecond. • We can use this to measure the distance to stars out to 1000 LY

  12. Measuring Distances • Can measure parallax angle • We have a triangle where we know: • One angle • One side (which one?) • So we can calculate distance

  13. The Parsec • “Parallax of one arcsecond” • How far away something must be to generate a parallax angle of 1 arcsecond at opposite sides of Earth’s orbit • 3.26 light years

  14. Hipparcos • High Precision Parallax Collecting Satellite • Precision parallax measurements of 100,000+ stars • Good to 0.002 arcseconds! • The planetarium uses the Hipparcos catalog!

  15. Proper Motion Stars orbit around the center of a galaxy. In addition to this motion, they have their own extra “random motion”. Shouldn’tthe stars in the skymove relative to each other over time?

  16. Proper Motion Yes. But it takes tens of thousands of years for the motion to “add up”.

  17. Proper Motion • A typical star speed is 20 miles per second relative to Earth. • That’s fast, right? • Do why does it take thousands of years to see this motion?

  18. Proper Motion • A star’s changing position depends on its motion relative to Earth: • Directly toward or away: no apparent motion • Moving at right angles to our line of sight: moves across the sky

  19. Barnard’s Star This star is close enough and moves quickly enough across the sky that its movement is noticeable over tens of years.

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