From AMS Glossary
One of the three commonly detectable points along the vertical circle through the sun at which the degree of polarization of skylight goes to zero; a neutral point.
The Arago point, so named for its discoverer, is customarily located at about 20° above the antisolar point, but it lies at higher elevations in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity. Measurements of the location of this neutral point are typically more easily carried out than measurements of the Babinet point and the Brewster point, both of which lie so close to the sun (about 20° above and below the sun, respectively) that glare problems become serious.
Neuberger, H. 1951. Introduction to Physical Meteorology. 196–204.