Grazing lunar occultation
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.
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
occurs when the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, moving along its orbital path, passes in front of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
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.
See also
- OccultationOccultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
- Grazing occultationGrazing occultationAn 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,...
- Asteroid occultation
- TransitAstronomical transitThe term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
(occultations of planets by other planets)
External links and references
- International Occultation Timing Association Predicted paths for future grazing occultations
- Royal Astronomical Society of New Zealand More extensive description of grazing occultations
- Astronomy Picture of Day for March 16, 2007, showing grazing occultation of Saturn