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A Brocken spectre (German Brockengespenst), also called Brocken bow or mountain spectre, is the apparently enormous and magnified shadow of an observer, cast upon the upper surfaces of clouds opposite the sun. The head of the figure is often surrounded by the glowing halo-like rings of a glory—rings of coloured light that appear directly opposite the sun when sunlight meets a cloud of uniformly-sized water droplets.
The phenomenon can appear on any misty mountainside or cloud bank, even when seen from an aeroplane, but the frequent fogs and low-altitude accessibility of the Brocken, a peak in the Harz Mountains in Germany, have created a local legend from which the phenomenon draws its name. The Brocken spectre was observed and described by Johann Silberschlag in 1780, and has since been recorded often in literature about the region.
Occurrence
The "spectre" appears when the sun shines from behind the observer, who is looking down from a ridge or peak into mist or fog. The light projects their shadow through the mist, often in a triangular shape due to perspective. The apparent magnification of size of the shadow is an optical illusion that occurs when the observer judges his or her shadow on relatively nearby clouds to be at the same distance as faraway land objects seen through gaps in the clouds, or when there are no reference points by which to judge its size. The shadow also falls on water droplets of varying distances from the eye, confusing depth perception. The ghost can appear to move (sometimes suddenly) because of the movement of the cloud layer and variations in density within the cloud.