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Serendipitous Lunar Graze

Robert A. Garfinkle, F. R. A. S.


It is not very often that you get the opportunity to observe a graze occultation of a star by the lunar south polar region from the vantage of your own front yard. I observed this graze on the night of September 6, 2000 through my 10-inch Meade Schmidt-Cassegrain from the front yard of my home in Union City, California. As part of my research for my lunar observers' handbook, I was studying and taking notes on the southern highlands crater Manginus and several of its neighbor craters as the terminator crept past first quarter. Illuminated by low-angle sunlight of the 8.6 day-old Moon, the rugged eastern rim of Clavius was within a few degrees of the terminator while the floor was in darkness.

As I observed the Moon through my 12mm Nagler II eyepiece, I glanced over toward the dark western limb and noticed a triangle consisting of a 7th and two 8th magnitude stars. The asterism was fast approaching the dark southwestern limb. Some quick calculations of speed and direction indicated that there was the possibility of observing a dark limb immersion occultation or even a possible graze occultation within a matter of minutes.

The Moon was passing through the open star cluster M21 (NGC 6531) in the constellation of Sagittarius. The brightness of the Moon overpowered most of the faint cluster stars. The Moon was 64.13 percent illuminated at the start of the first graze with a phase angle of 73.58 degrees. The total libration was L=3.138 and b=-2.476. The region from the lunar south pole up to about 78 degrees south latitude was in darkness. The crater Short was bisected with its eastern wall and central peak being illuminated and the rest of the crater totally dark, giving this crater the appearance of being the southern limb.

I turned on my short wave radio and tuned in station WWV in order to time the event. Unfortunately, I did not have my cassette tape recorder handy, but got ready to count the seconds along with the radio pings as the first star, 8.68-magnitude GSC 6263:104, approached the limb. At 4:03.02 Universal Time (UT), the first star disappeared into a total occultation. I was still not sure if the brighter star, 7.21-magnitude SAO 186215 (GSC 6842:246), would graze or be a total occultation, because I could not see the southwestern limb. At 4:06.01 UT, the blue-white subgiant star began to flicker, flash, dim, and flash again as it skimmed the rugged mountains along the limb. I timed the twinkles and flashes and the moment it blinked out entirely. Two more flickers then it appeared to clear the limb at 4:06.09. I noticed that as the brightness of the star dimmed to about 9th magnitude as it flickered from blue-white to golden then back to blue-white. This change in color was repeated several times in quick succession.

I was concentrating on watching the bright star and did not notice the instant the third star, 8.10-magnitude GSC 6263:525, went into occultation, but it happened during the middle of the graze. I noticed that all three stars were occulted before the first graze ended.

SAO 186215 hung close to the dark limb and I decided to continue watching until it was obviously clear of the Moon. To my good fortune, the star began a second graze at 4:09.10 UT. In between the time of the two grazes, the south polar Aitkin Basin, which is a 1500-foot deep depression, which gives the Moon's south polar region the appearance of being flat, had been passing by the star. Once again, the star began to flicker as the limb mountains intermittently blocked its light. At 4:11.06 UT, the star disappeared completely. It reappeared from the illuminated limb at 4:19.25 UT at about 75 degrees south latitude near the crater Legentil. The star did not emerge directly from the limb, but reappeared from a black band created by the intense irradiation that blocked the relatively weaker light from the 7th magnitude star. Due to the brightness of the illuminated east limb, I did not see the fainter two stars even though I knew that they were both no longer being occulted.

This occultation demonstrated several things including that the Aitkin Basin does in fact form a noticeable depression along the south polar limb and the zone of irradiation that surrounds the brightly illuminated portion of the Moon can block the light of a faint star. The best part of this occultation was the fact that it was totally unexpected by me until moments before it occurred. You never know what to expect when you get out to observe the night sky.


Mail to: Robert A. Garfinkle
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Last updated: July 19, 2007

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