Unformatted text preview: Î» Î»0 v
c Î»0 position
ago Redshift z > 0
Blueshift z < 0
object approaching Lab
blue Î» 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
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.
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- Fall '09
- proper motion, object receding, proper motions, parallax measurements, object approaching