You've probably seen a very thin sliver of our moon after sunset or before sunrise. Imagine it thinner. And even thinner. Eventually all that's left is sunlight shining off the mountain peaks while leaving the valleys in darkness. This arched 'string-of-pearls' is known as Saber's beads after a comment I made about the visual similarity to what is seen during a solar eclipse.
Just before (and after) the moon passes in front of the sun, light shines at us through the valleys but is blocked by the mountains- the mirrored effect of Saber's beads. After reporting this resemblence, my esteemed stargazing colleagues honored me with its namesake.
While promoted as an outreach term, further scientific study may also provide new insights regarding local albedo values within the lunar libration zones.
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EXTREME CRESCENT SPOTTING TIPS
Set up at a site with as much altitude as possible overlooking an unobstructed horizon.
Optimal sky transparency allows the crescent to be detected and tracked down to, or up from, the horizon.
Using a telescope or binoculars (mounted binos are recommended), fine tune the focus on Venus, Jupiter, or one of the brighter stars beforehand.
For dusk attempts, have Sol's setting azimuth on hand- making note after sunset of a random landmark at that position for reference- as well as Luna's altaz position at sunset thru moonset. Accordingly, for dawn attempts, have Luna's altaz info for moonrise thru sunrise.
As dawn slivers have the advantage of possible detection with dark-adapted eyes, wearing sunglasses during the day prior to sunset attempts is recommended for maximum 'dusk' adaptation.
Once the crescent is acquired in binoculars, walk the bino down to the horizon/random landmark in consecutive FOVs for the approximate naked-eye altaz.
A favorable elongation is important. In the 24 hours before or after New Moon, Luna's angular separation from Sol can vary by several degrees. With a favorable ecliptic, net elongations (as altitude) of 6° or more at sunset or moonrise offer the best window for detection. 8-10° is necessary to catch Saber's beads in optimal/deep twilight.
Illuminated fractions of same-age crescents within 24 hours of New Moon can vary by 200% and a full magnitude of brightness. Slivers near perigee help present a thicker and brighter lunar profile for personal record crescent spotting.
Last but not least, don't always count-out a shallow ecliptic. Occasionally our moon's extreme northern or southern declination will compensate for a less than favorable ecliptic angle.
BRACKETING THE SLIVERS
Another rare and challenging notch for ones lunar bedpost is to catch the consecutive waning and waxing crescents within 24 hours on each side of New Moon.
For instance, July 2008's Buck Moon offered such an opportunity as I was able to spot both the -16.5 hour illumination before sunrise on the 2nd and the +23.5 hour sliver just after sunset on the 3rd.
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EXTREME CRESCENTS AND COLONGITUDE
A big piece of the puzzle concerning the appearance of a thin crescent's degree of segmentation may be predicted by its colongitude at New Moon which, not suprisingly, can vary by more than 10°, trumping the effects and cycling independently of the apogee-perigee lunar profiles. VMA simulations show New Moon colongs less than 270° to favor segmented waxing crescents while those over 270° favor segmented waning crescents. (Those over 270° would be considered 'early' colongitudinal moons).
Equally important to those hunting Saber's beads, colongitude may also be used inversely to determine the best profiles and visibilities for personal record crescent spotting.
The table below shows the lunation, colongitude at New Moon, duration in hours of the most segmented waxing limb (-280° colongitude), and the duration in hours of the most segmented waning/preceding limb (+260° colongitude). Data derived from thin crescent images and my own experiences with extreme crescents also concur with a 260-280° colong window. Of note; the youngest naked-eye crescent sighting to date (May 1990/aged 15.5 hrs), my May '06 sighting (20.5 hrs), and the Lodriguss example above (27.5 hrs) all have a colongitude of 276-278°.
Exact formations involved may vary at each lunation due to libration and other factors. The illuminations caused by sunlit mountainous regions and/or albedo effects also bear further study.
lun/colong/h-280/h+260
1068 274.7 10 28
1069 273.0 13 25
1070 271.1 17 21
1071 269.3 20 18
1072 267.6 24 14
1073 266.0 27 11
1074 264.9 29 09
1075 264.7 29 09
1076 266.0 27 11
1077 268.5 22 16
1078 271.3 16 22
1079 273.5 12 26
1080 274.7 10 28
1081 274.9 09 29
1082 274.4 10 28
1083 273.3 12 26
1084 271.5 16 22
1085 269.3 21 17
1086 267.0 25 13
1087 265.3 28 10
1088 264.7 29 09
1089 265.3 28 09
1090 266.9 25 13
1091 268.9 21 17
1092 271.1 17 21
1093 273.1 13 25
1094 274.7 10 28
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For the record, here's the ELCD from my first observation
of Saber's beads:
New Moon 27 May 2006 0526ut
(@+24hours)
Angular Size/Distance 31.2'/383356km
Colongitude 277.6°
Declination +27°04'
Solar Elongation 13.5°
Mag/% Illumination -1.02/.012
41.5n 90.5w
-24h waxing
moonset window: 01h10m
sunset/azimuth: 0126ut/300*
altaz at sunset: 9.3*/298*
moonset/azimuth: 0236ut/308*
age at sunset: 20h00m
First detected at +20.2 hours, this instance of Saber's beads peaked in deep twilight with the infant crescent hanging 2.0° above the horizon at a solar elongation of 12.2°.
By this time I was fully immersed in the imagery of a total solar eclipse- as if in the next few moments I would either be basking in our sun's corona or forced to turn away from its glare.
This experience left me with the mystery of why the phenomenon had escaped my attention in then over 10 years of diligent young/old crescent spotting, and more so, gone unreported after four centuries of magnified scrutiny.
As a drummer and not a scientist, I've done my best to make some headway into the window parameters for viewing this rare and beautiful lunar aspect.
Note the extreme northern declination and sunset altitude combined with a favorable colongitude and ecliptic which allowed one of the longest moonset windows and highest altitudes at sunset possible for a crescent that age. It is this rarity of catching the fully/near-fully segmented limb in a dark sky that defines the complete phenomenon.
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| Lunar |
