GEGENSCHEIN or counterglow, an extremely faint lumi nescence of the sky, seen opposite the direction of the sun. Its faintness is such that it can be seen only by a practised eye under favourable conditions. It is invisible during the greater part of June, July, December and January, owing to its being then blotted out by the superior light of the Milky Way. It is also invisible dur ing moonlight and near the horizon, and the neighbourhood of a bright star or planet may interfere with its recognition. When none of these unfavourable conditions supervenes it may be seen at nearly any time when the air is clear and the depression of the sun below the horizon more than 20 degrees. (See ZODIACAL LIGHT. )