Occultations and Transits Special Interest Group

revised 2005 Feb 21

Grazing Occultations by the Moon

A special type of occultation occurs when the star appears to pass right on the edge of the Moon. The star grazes the limb of the Moon; hence, the name grazing occultation or "graze". Because of the mountains and craters on the Moon, the star may disappear and reappear several times during the few minutes of a grazing occultation. To see such an event, you must be positioned within a very specific path, normally no wider than 2 miles! And of course, the Moon cannot be blocked by trees or buildings, which usually makes finding a suitable observing site more difficult.

Low Power View of Grazing Occultation

Highly Exaggerated View of Grazing Occultation

But the rewards of locating a suitable observing site are worth the effort. A grazing occultation is one of few scientific endeavors in which an amateur, armed with nothing more sophisticated than a radio, tape recorder, telescope, map, and tape measure, can provide valuable scientific data. When a group of observers time when the star disappears and reappears (to the nearest fraction of a second!), the shape and height of the lunar mountains can be determined. Occasionally, a grazing occultation reveals that a star is a double star. See also Occultation Timings on the Home page for details on timing events.

Grazing occultations have the narrowest path of visibility of any occultation. The path is based on the height of the Moon's mountains (as projected onto the ground). Thus, the path can be anywhere from a fraction of a mile wide to a few miles wide that stretches across part of the Earth. In spite of the narrow path, knowing where to set up is relatively easy thanks to detailed predictions from the International Occultation Timing Association (IOTA).

Planned Grazing Occultation Expeditions for the year 2005

The list below is the grazing occultations visible from the Pittsburgh area. I have indicted the ones I intend to watch (yes and no in the View? column). Some grazes consists of two expeditions depending on the expected number of participants. Expedition #1, if it occurs, is a scouting trip to find suitable observing sites for each observer. This usually occurs the weekend before the graze. (There are few things worse than driving around in the dark, in the middle of the night, trying to read a map to find a suitable site! Fortunately, GPS units with mapping capabilities are making this much easier.) Expedition #2 is the actual grazing occultation itself. Please contact me (mailto:JWHoltz@aol.com) if you would like to be involved with either or both expeditions.

Date        Star     Time  Star  %    Dist Star  Star  Sun   Cusp   Go/   Approx
2005        Name     U.T.   mag  SNL  mile  Alt  Azim  Alt   Angle  No Go Location

Tue MAR 15  ZC  486   2:49  5.3  24+   74  17.4  283  -36.9   3.5D  Go Edinboro to Grand Valley
Fri MAR 18  X  8136   3:01  8.9  52+   76  46.8  268  -37.9   7.0D  No (too faint, too far)	
Tue APR 12  X  4972   1:06  8.1  11+  4.3  22.7  282  -12.9   6.7D  Go (Mars to New Kensington)
Thu APR 14  X  7525   4:13  8.8  27+   60  11.2  297  -37.0   8.9D  No (too faint, too low)	
Thu MAY 12  X  8734   2:36  7.7  13+   18  14.8  294  -20.2  12.4D  Go Slippery Rock to Kittanning
Tue MAY 17  X 15793   4:43  8.8  57+   77  24.3  265  -28.8  10.7D  No (too faint, too far)	
Tue MAY 31  X 31688   8:31  8.1  41-   27  20.4  118  -13.1   7.9D  Go Monongahela to Irwin
Mon JUL 18  ZC 2373   5:56  6.1  85+   80   5.2  228  -28.1   1.9D  No (6° in Columbus Ohio)	
Sun AUG 14  X 21938   2:17  7.8  60+   59  16.6  214  -20.7   3.3D  Go Ohiopyle to Meyersdale
Fri AUG 26  X  4920   8:07  8.9  53-   13  52.2  103  -26.5  15.5D  Go Washington Pa. to New Ken.	
Mon AUG 29  X  9379   9:42  7.8  24-   46  42.9   87  -12.1  13.0D  Go Mercer to Pleasantville
Thu SEP 22  ZC  493   6:36  6.9  79-   60  58.9  115  -45.2  14.1D  Go Confluence to Stoystown	
Sat OCT  8  ZC 2354   0:15  7.5  20+   98   6.5  226  -16.1   6.8D  No (too faint, too low)
Tue OCT 11  ZC 2855   0:11  7.6  52+   93  21.6  189  -16.9   7.5D  Go? Keyser W. Va.	
Thu OCT 13  ZC 3158   1:03  5.7  74+   68  28.5  172  -26.2   9.1D  Go? Edinboro
Thu OCT 13  ZC 3175   5:44  4.7  76+   11   7.1  237  -56.2   9.8D  Go Washington Pa. to New Ken.	
Mon OCT 24  X 11666   8:57  8.8  57-   99  62.1  109  -31.7   0.3T  No (bright limb)
Sat NOV  5  X 24797  23:52  7.7  17+   64   6.7  221  -18.6  13.4D  No (too faint, too low)	
Tue NOV  8  X 28252   0:50  9.0  37+   92  16.0  211  -29.2  14.2D  No (too faint)
Thu NOV 10  X 30504   1:01  7.9  60+  9.4  33.0  192  -33.4  15.1D  Go (Monongahela to New Kens.	
Thu NOV 10  X 30559   3:31  8.4  61+   44  19.2  229  -59.9  13.6D  No? (Uniontown to Johnstown)
Fri NOV 11  X 31275   0:14  7.1  71+   30  38.3  163  -24.0  14.1D  Go New Castle to Oil City	
Mon DEC  5  X 29148  22:35  8.6  22+   66  24.2  197   -7.6  16.8D  Go? Edinboro to Cambridge Sprn

where

Date: Date of graze in Universal Time (UT, Greenwich England).
Star Name: Either the Zodiacal Catalog number (ZC) or USNO number (X).
Time: Predicted time for the center of the graze in Universal Time (U.T.). The entire graze usually lasts two or three minutes. Subtract 5 hours from Universal Time (U.T.) to get Eastern Standard Time (E.S.T.); subtract 4 hours to get Eastern Daylight Time (E.D.T.). The time is expressed in 24-hour military time. Thus, 13:00 is 1 p.m., 14:00 is 2 p.m., and so on. After subtracting the 4 or 5 hours to get Eastern time, a negative time indicates the event occurs on the previous date (add 24 hours in this case to get the appropriate time).
Star mag is the magnitude (brightness) of the star being occulted. The larger the number, the fainter the star. A faint star close to the cusp (a few degrees) can be hard to follow along sunlit mountain peaks.
% SNL: Percentage of Moon sunlit; that is, the Moon's phase, either waxing (+) or waning (-). 0% is a New Moon, +50% First Quarter, 100% Full Moon, -50% Third Quarter.
Dist: Distance (in miles) from the Wagman Observatory to the closest point along the graze path. Note that the planned expedition may be to a spot that is more convenient to reach than the closest distance.
Star Alt is the altitude or angle of the star above (+) or below (-) the horizon at the time of the event. (Hopefully, the star is above the horizon. Otherwise, you are wasting your time!) 0° indicates the star is on the horizon, 45° is halfway up, and 90° is straight overhead.
Star Azim is the azimuth or angle of the star measured around horizon from due north. 90° is east, 180° is south, 270° is west, etc.
Sun Alt is the altitude of sun below (-) or above (+) the horizon. Events that occur with the Sun above the horizon are daytime events.
Cusp Angle is the angle in degrees measured from the Moon's cusp to the position of the occultation. at the midpoint. The entire event will generally span 4 to 6 degrees along the limb. A "D" after the angle indicates the graze occurs on the dark limb, "B" indicates a bright limb graze, and "T" indicates the graze occurs very near the terminator.
Go/No Go?: Indication if I am planning an expedition to view the graze. I can provide information if you want to go on your own expedition.
Notes: Most likely topographic map (quadrangle, named after a nearby city or landmark) from which the graze will be observed, or notes about why the graze will not be attempted.

Other Grazing Occultation Links:

John Holtz' Occultation Updates	maps and rendezvous information for the better grazing occultations in the Pittsburgh area.
IOTA's Lunar Occultations	Extensive information for total and grazing occultations

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