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The phrase plane of the sky (POS) refers to a plane normal (i.e., perpendicular) to the line of sight and tangent to the celestial sphere. This makes sense when describing a small region of the sky and the concept is probably sometimes "stretched" to refer to a more significant portion of the sky. It can be used to describe the attitude or shape of something (e.g., a galaxy, cloud, or orbit), or for a description based upon what is viewable, i.e., the results of a blind survey at some wavelength.