Diffuse Sky Radiation - Neutral Points

Neutral Points

There are three commonly detectable points of zero polarization of diffuse sky radiation (known as neutral points) lying along the vertical circle through the sun.

  • The Arago point, named after its discoverer, is customarily located at about 20° above the antisolar point; but it lies at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity.
  • The Babinet point, discovered by Babinet in 1840, is located about 15° to 20° above the sun, hence it is difficult to observe because of solar glare.
  • The Brewster point, discovered by Brewster in 1840, is located about 15° to 20° below the sun; hence it is difficult to observe because of solar glare.

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    The seashore is a sort of neutral ground, a most advantageous point from which to contemplate this world. It is even a trivial place. The waves forever rolling to the land are too far-traveled and untamable to be familiar. Creeping along the endless beach amid the sun-squall and the foam, it occurs to us that we, too, are the product of sea-slime.
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