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Suppose all planes fly at 500 MPH = 733 ft/sec
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Observe the angle that a plane shifts in 1 second of time
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A plane that moves 10 in 1
sec (900 in
90 sec) is at 42,000 ft
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A plane that moves 20 in 1
sec (900 in
45 sec) is at 21,000 ft
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Works for stars too:
closer stars have faster “proper motion”
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Can only get average distances using average proper motion, since
any
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given star might be moving faster or slower than average
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Harlow Shapley found 11 Cepheids with proper
motion
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Used average proper motion, average distance, to
find average (M-m)
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Let him replace Leavitt’s relative “m” axis with
absolute “M”
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Now period M then M-m d
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