Lunar graze occultation occurs when the team of observers is near the northern or southern edge of the Moon's shadow, such that the star behind the Moon is seen to graze the lunar limb. The star blinks on and off as it goes behind mountains and then pops back into view. A team is setup in a line of a mile or two to achieve a good lunar profile. A team leader does the planning and plotting and site selection.
Lunar graze occultations are very exciting and can be done with simple equipment: a 6" scope, a tape recorder and a small portable WWV radio.
Lunar grazes are used to refine the long term orbit of the Moon, more accurately describe the lunar profile (accurate lunar profiles are important in studying the Sun's size during solar eclipses), and refine star postions. Occasionally, a double star is discovered.
Lunar graze of a Geminorum --- September 4, 1991
graph by Larry Dunn showing observers' results (I'm MB)
The following is a transcript of the tape recording made by Mel Bartels during the lunar graze of a Gem, September 4, 1991, with additional notes supplied after the observations.
UT
h m s
11 20 04 Alright, this is a test at 4:20 in the morning. Seeing is
getting better. I have the Moon and the star in sight,
11 22 02
It's test number two at 4:22.
11 24 03 Test number 3, and we're
trying to adjust the sound levels on WWV and my voice to hear both
clearly on the tape,
11 42 11 Alright, it's 4:42 and 12 seconds
after. We're set up with the 6 inch scope and about 67 power. It's
really a beautiful night; been able to see 15 members of the Pleiades
unaided-eye. The star is just a little brighter than the bright limb
of the Moon.
11 43 08 And one final check to make sure it's
recording. And the star is getting pretty close to the Moon now.
11
44 00 OK, we're getting pretty close here now. OK, it's almost in
contact here.
11 45 17 Boy, that star is just skimming right
along the edge there.
11 46 10.38 OFF!
11 46 15.73 ON!
11
46 16.28 OFF!
11 46 26.77 ON!
11 46 27.28 OFF!
11 46
27.93 ON!
11 46 28.34 OFF!
11 46 42.31 ON!
11 46 43.77
OFF!
11 46 47 We're on the dark side now.
11 46 49.64 ON!
It's just barely... Er, no, no, I'll scratch that one.
11 46
54.72 ON!
11 47 10 Boy, that was great; that was about four
'offs' and 'ons' there.
Really spectac...
11 47 16.72 OFF!
11 47 36.96 ON!
11 47 37.48 OFF!
11 47 38.04 ON!
11
47 39.26 OFF!
11 47 40.50 ON!
11 47 42.73 O..
11 47 43.15
ON!
11 47 43.50 OFF!
11 47 45.57 ON! Almost seems to have two
brightnesses there; the second dip just before was about half bright.
Well this is just spectacular!
11 48 31 Well that one spot where
I retracted an 'on' there was a little speck of bright Moon sitting
by itself and it looked like it brightened just a tiny bit. I almost
wonder if this is some kind of double star with a fainter companion
and that somehow brightened it, but I'm not real confident of that.
(Note.1)
11 49 12 Well, I can see a distinct separation between
the star and the dark side of the Moon now, but we'll keep watching.
11 50 34 Well, I think that's about it, there's quite a
separation between the star and the Moon now, Boy, that was really
spectacular. So, we'll sign off now.
NOTES
1. Yes, it is a double star, mag. 3.5 and 8.2. I (LGD) neglected to emphasize that during the briefing because I assumed the secondary would be invisible under these conditions. In retrospect, it apparently was separately visible to some observers. it is possible (though unlikely due to the geometry) that this secondary was the cause of a 'brightening' of a point on the limb, but probably not the cause of a half-dimming effect.
Narrative transcription by Larry Dunn, November 4, 1991
The original recording consisted of WWV and observer voice comments recorded by a single microphone on a monaural recorder. The WWV reception was generally good. The event callouts were clean and clear. Still, there were some difficulties in measuring precise timings of the event callouts. Some events occurred during WWV voice announcements, obscuring the WWV time pulses. Other sources of noise also obscured the time pulses at some points.
The timings were obtained from the field-recorded tape using an audio editor on a NeXT computer, Segments of the recording several seconds long were digitized and their waveforms displayed; through the use of additional utility programs, these displayed segments were printed by a laser printer, Portions of these segments were selected on the display and played back for identification, and the callouts were marked on the printed waveforms. The distance between the beginning of each callout and a nearby identified WWV time pulse was measured on the printouts. The measured time between WWV pulses was used as the time scale.
Timings digitized/printed by Larry Dunn, November 9, 1991
Measured by Larry Dunn, November 17, 1991
For these calculations I have used a reaction time value of .4 ± .2 second. This is a compromise between the IOTA-suggested .3 sec and my experience measuring my own reaction time on a graze simulation as .5 sec.
In the following table, DIFF is the difference between the time of the observed event and the predicted time of central graze, in seconds.
Observer location: 0.25 mi. N of predicted limit, corrected for
altitude.
Predicted central graze time: 11 47 03.2 UT. FINAL DIFF NOTE OUT 11 46 10.38 - .4 = 11 46 10.0 ± .2 -53.2 IN 11 46 15.78 - .4 = 11 46 15.4 ± .2 -47.8 OUT 11 46 16.28 - .4 = 11 46 15.9 ± .2 -47.3 IN 11 46 26.77 - .4 = 11 46 26.4 ± .2 -36.8 OUT 11 46 27.28 - .4 = 11 46 26.9 ± .2 -36.3 IN 11 46 27.93 - .4 = 11 46 27.5 ± .2 -35.7 OUT 11 46 28.34 - .4 = 11 46 27.9 ± .2 -35.3 IN 11 46 42.31 - .4 = 11 46 41.9 ± .2 -21.3 OUT 11 46 43.77 - .4 = 11 46 43.4 ± .2 -19.8 IN 11 46 54.72 - .4 = 11 46 54.3 ± .2 -8.9 OUT 11 47 16.72 - .4-.1= 11 47 16.2 ± .3 13.0 1 IN 11 47 36.96 - .4= 11 47 36.6 ± .2 33.4 OUT 11 47 37.48 - .4= 11 47 37.1 ± .2 33.9 IN 11 47 38.04 - .4= 11 47 37.7 ± .2 34.5 OUT 11 47 39.26 - .4= 11 47 38.9 ± .2 35.7 IN 11 47 40.50 - .4= 11 47 40.1 ± .2 36.9 OUT 11 47 42.73 - .4= 11 47 42.3 ± .2 39.1 2 IN 11 47 43.15 - .4-.1= 11 47 42.7 ± .2 39.5 3 OUT 11 47 43.50 - .4= 11 47 43.1 ± .2 39.9 IN 11 47 45.57 - .4= 11 47 5.2 ± .2 42.0
NOTES
1. This call out was in mid-sentence, catching the
observer by surprise, so I have assumed an additional 1 sec reaction
time, and increased the uncertainty to .3.
2. This callout was
just the beginning of the word 'off' or 'on', interrupted by the
callout of the next event. I have assumed 'off' because this best
fits the sequence.
3. This 'On' callout interrupted the preceding
presumed 'Off' callout. Because of the overlap, requiring additional
mental processing, I have arbitrarily added .1 sec to the reaction
time.
Timing reduction by Larry Dunn,
November 24, 1991