Cosmic Rulers
Planetary Planarity
Radial Velocity Technique
Target: Gamma Cephei
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Extrasolar Planets, Radial Velocity, and Astrometry

Two Adults on a Teeter-Totter

Here's what you would see if two adults were playing on the teeter-totter:

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Notice that both participants are now moving up and down at the same speed. The adult on the left had to move farther from the fulcrum to balance the adult on the other end.

Now suppose the only information you had about the two people on the teeter-totter was how fast the adult on the left was going?

How could you measure the speed of the adult? Wrap him in tinfoil and use a radar gun!

photo of police officer holding a radar gun

If you stood beside the teeter-totter, say 10 feet from the fulcrum, your radar gun would detect no speed for the adult in tinfoil because all of the adult’s motion is up and down.

So how can you measure motion towards or away from you? Climb a tree!