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ay45c2-page11 - λ0 v c λ0 position one crest ago...

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Unformatted text preview: λ λ0 v c λ0 position one crest ago Redshift  z > 0  object receding Blueshift  z < 0  object approaching Lab Observed position now ∆λ λ0 λ blue λ red ii) proper motion Here we observe motion of object in plane of the sky over time (sometimes a very long time). We get vt = v sin  from the rate of change of position on the sky if we know the distance to the object. We generally do know the distance (from parallax measurements q.v.) because we can only see proper motions for nearby stars. E.g., to see appreciable motion, say  100/century, given that the typical velocity of a star is  10 km s , we can only measure vt for stars within distances < 2 kpc, about one- fth the distance to  the center of the galaxy. 1 20 ...
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