10

•**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 1****0**** in 1 sec (90****0**** in 90 sec) is at 42,000 ft
**

–**A
plane that moves 2****0**** in 1 sec (90****0**** in 45 sec) is at 21,000 ft
**

•**Works
for stars too: closer stars have
faster “proper motion”
**

–**Can
only get average distances using average proper motion, since any ****given star might be moving faster or
slower than average
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•**Harlow
Shapley found 11 Cepheids with proper motion
**

–**Used
average proper motion, and average distance, to find average (M-****m)
**

–**Let
him replace Leavitt’s relative “m” axis with absolute
“M”
**

–**Now
period ****Þ**** M then M-m ****Þ**** d**