The antisolar point is the imaginary point on the celestial sphere exactly opposite the sun. It is where anticrepuscular rays appear to converge, and on a moonless night away from city lights, it is often possible to see the gegenschein there. In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of gigantic radius, concentric with the Earth. ... A sun is the star at the center of a planetary system. ... Anticrepuscular rays are similar to crepuscular rays, but seen opposite the sun in the sky. ... Gegenschein (German for counterglow) is a faint brightening of the night sky in the region of the zodiac directly opposite the Sun caused by reflection of sunlight by small dust particles that lie in the plane of the Solar system. ...
The Babinet point is one of the three commonly detectable points of zero polarization of diffuse sky radiation, neutral points, lying along the vertical circle through the sun; the other two are the Arago point and the Brewster point.
The Arago point, so named for its discoverer, is customarily located at about 20° above the antisolarpoint; but it lies at higher altitudes in turbid air.
The Brewster point, discovered by Brewster in 1840, is located about 15° to 20° directly below the sun; hence it is difficult to observe because of the glare of the sun.
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