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). Light from the occulted object is blocked by the moon and a shadow of the moon is cast onto the ground. The moon's shadow moves west-to-east due to the orbit of the moon and the rotation of the earth, as shown on the image at left.
The left (western) ellipse represents moonrise for that region of the globe. The right (eastern) ellipse represents moon set for that region. The northern and southern path limits are shown. Sites between the white lines will see the event occur at night, while sites between the blue lines will see the event occur during twilight, and sites between the red dotted lines may witness the event during the day-time, for stars brighter than mag.2. Source - Occult (Software).
A grazing lunar occultation (also lunar grazing occultation, lunar graze, or just graze) is seen at locations along the north and south limits, and the observer will see the object disappear as the shadow of mountains pass by, and reappear as the light passes down the valleys on the edge of the moon.
A plot of the results from a single observer graze expedition, where eight events were observed. The path of the star is shown curved, when in reality it is the moon moving past the star. The first disappearance is shown on the left (red) and the last reappearance on the right (green). The olive coloured dots are altitude soundings from the data points from the Kaguya lunar orbiter.
A team of many observers can combine observed events and construct an extremely accurate profile of the lunar terrain. Since graze paths rarely pass over established observatories, amateur astronomers use portable observing equipment and travel to sites along the shadow path limits. The goal is to report the UTC of each event as accurately as possible, and GPS disciplined devices are frequently used as the time-base.
Two methods are used to observe
- visual - the observer has an audible UTC beeping device (e.g. a shortwave radio tuned to WWV) and an audio recorder (e.g. a tape recorder) and watches the target through the telescope, and calls 'Gone' when the star disappears, and 'Back' when the star reappears. The audio recording is later analysed to extract the event times.
- video - the observer uses a small video camera, usually mounted in the focuser of the telescope. A Video Time Inserter (VTI) is commonly used to insert a UTC time-stamp onto each frame of the recording. Either a camcorder, DVR or a laptop computer is used to record the video stream. The video recording is later analysed to extract event times.
Such observations are useful for;
- refining knowledge of the positions and motions of stars
- examining limb profile around the lunar polar region.
- Measurement of the Earth's rotation
- International Occultation Timing Association Predicted paths for future grazing occultations
- Royal Astronomical Society of New Zealand More extensive description of grazing occultations
- Zodiacal Catalogue - XZ80Q - of stars that can be occulted by the moon.
- Wood, Chuck (March 7, 2005). "One Good Occultation Deserves Another". Lunar Photo of the Day., showing grazing occultation of Jupiter as seen from Melbourne, Australia
- Wood, Chuck (March 5, 2007). "Ring Eclipse". Lunar Photo of the Day. Archived from the original on August 15, 2011., showing grazing occultation of Saturn, later appeared on APOD, the link below
- Astronomy Picture of Day for March 16, 2007, showing grazing occultation of Saturn
- Predictions and analysis of Lunar Occultations and Grazing Lunar Occultations. Occult (Software), Lunar Occultation Workbench (LOW) and GRAZEPREP
- Herald, David. "VizieR Lunar Occultation Archive". vizier.u-strasbg.fr. Retrieved 2018-06-16.
- Soma, Mitsuru. "Examination of the Hipparcos Proper Motion System from Lunar Occultation Analysis". adsabs.harvard.edu. Retrieved 2018-06-16.
- Sôma, Mitsuru; Kato, Yuji (2002-01-01). "Limb profiles of the Moon from grazing occultation observations collected at RGO". Publications of the National Astronomical Observatory of Japan.
- Stephenson, F. R.; Morrison, L. V.; Hohenkerk, C. Y. (2016-12-01). "Measurement of the Earth's rotation: 720 BC to AD 2015". Proc. R. Soc. A. 472 (2196): 20160404. doi:10.1098/rspa.2016.0404. ISSN 1364-5021. PMC 5247521.
- Herald, David. "VizieR - Catalogue XZ80Q". vizier.u-strasbg.fr. Retrieved 2018-06-16.
- "Draft:Occult (software)", Wikipedia, 2018-06-04, retrieved 2018-06-16
- Eric, Limburg, (2002-01-10). "Lunar Occultation Workbench 3.1: stellar occultations tailor-made to your needs". Occultation Newsletter, International Occultation Timing Association (IOTA). 8 (4). ISSN 0737-6766.