Grazing occultation
From Wikipedia, the free encyclopedia
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.
Grazing lunar occultation
From Wikipedia, the free encyclopedia
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. |
置顶卡
变色卡
千斤顶
显身卡