Grazing Lunar Occultation

A lunar occultation occurs when the Moon, moving along its orbital path, passes in front of a star or other celestial object, as seen by an observer (normally on the Earth). A grazing lunar occultation (also lunar grazing occultation, lunar graze, or just graze) occurs when one of the two edges of the moon parallel to its orbital path appears to just touch or graze the object as the moon goes by. When this happens, a properly positioned observer will see the grazed object disappear and reappear, possibly several times, as mountains and valleys on the edge of the moon pass in front of it.

Grazes unfold over the course of a few minutes, and depending on the lunar terrain and the observer’s position, the object may disappear and reappear just once or more than 10 times. Observers deployed just a few hundred metres apart on a line perpendicular to the graze path may make radically different observations. For example, at one location a lunar mountain may pass in front of the object, causing it to disappear and reappear, while at a different position the mountain may simply pass below the star without obscuring it.

By carefully measuring the positions of many observers and timing the disappearance and reappearance events, it is possible to construct an extremely accurate profile of the lunar terrain. Since graze paths rarely pass over established observatories, amateur astronomers have become the primary recorders of graze data. They typically use portable GPS units to determine their positions and telescope mounted video cameras to record the disappearance and reappearance events. Accurate timing is provided by recording time signals (such as WWV) on the audio channel.