Before you start. One idea carries this lesson: light takes time to travel, so when you look far away you see the past. The Night Sky Is a Time Machine builds that idea from Rømer’s late moon; it is enough to know that starlight from far away left its star long ago. Why the Siren Drops mentions that light from receding galaxies is stretched redder; this lesson uses that idea once, and explains it again where it does.
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