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all but the brightest stars difficult; even at large angular distances, scattered moonlight usually precludes direct observation of objects fainter than about 4th magnitude, and if there is any haze or high cloud, the situation is worse. Because the moonlight is widely scattered, a cloud temporarily hiding the Moon is insufficient to reveal the stars (Minnaert 1954, pp. 103-104). The mechanism that renders starlight invisible to the naked eye in the daytime and dimmed in moonlight is almost certainly what Minnaert (1954, pp. 102-103) refers to as the "veil effect." The eye's sensitivity adjusts to the brightest object it sees, whether direct or scattered, and the loss of sensitivity renders the much fainter stars invisible.

In addition to twilight, zodiacal light, airglow and aurorae (all described in ยง5), starlight, and the Milky Way, all contribute to sky brightness on moonless nights. Given otherwise dark conditions, the intrinsic background light can be surprisingly revealing. William Keel (1992) describes seeing from Mt. Pastukhov the snow-capped peaks of the Caucasus illuminated solely by starlight.

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