Grazing occultation

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An occultation is an astronomical event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer. A grazing occultation is a special type of lunar occultation that occurs within a mile or two of the edge of an occultation's predicted path, referred to as its northern or southern limit. Within that narrow zone, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the moon moves past the star, creating what is known as a grazing occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations.

The accurate timing of lunar occultations is performed regularly by (mostly amateur) astronomers. If several observers with telescopes and timing equipment are positioned at intervals within this zone, they can each time the sequence of disappearances and reappearances as seen from their observing location. If the positions of the observing locations are measured, the timings can be analyzed afterwards to obtain a very accurate fix of the position of the Moon relative to the star and to determine the details of the lunar mountains and valleys seen in profile along the moon's edge.

Such observations are useful for refining knowledge of the positions and motions of stars. Also, the star's disappearances or reappearances may occur in steps, indicating a previously undiscovered close double star that cannot be resolved by direct observations.