Used by James Bradley to in 1728 to measure the speed of light.

Because of the Earth's orbit around the Sun, we are moving at about 18.5 miles per second. That means that our motion makes the stars appear in slightly the wrong position, because we are moving through the starlight, which therefore appears "slanted" in much the same way as when moving through rain, you need to hold the umbrella in front of you, not above.

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