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19/01/1851 Dutch astronomer Jacobus Kapteyn is born

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Jacobus Kapteyn

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How Do We Know What the Milky Way Looks Like?

He is known for his observation of galactic rotation. Kapteyn was observing the motion of stars in the Milky Way on the background of background of the more distant objects. He noticed that this motion is not random and could be divided into two streams, moving in nearly opposite directions. This later led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.