Clair-obscur

Clair-Obscur Effects on the moon's surface

(Also spelled clair-obscure and clare-obscure)
(glossary entry)

Description

French for "light" (clair) and “shadow” (obscur): the term is used here to mean any effect on the Moon's surface created by the interplay of light and shadow.
  See also: Pareidolia and Trompe-l'oeil
 
 

Additional Information

• The term clair-obscur was reportedly introduced by seventeenth century French painter and art-critic Roger de Piles in a discussion of effects that could be created with color and drawing. In connection with paintings, the equivalent Italian expression chiaroscuro is now more often used to express the same idea.
• Most clair-obscur effects on the Moon are short-lived, but they are not one-time or rare events. Because lunar lighting patterns repeat in a cycle of approximately 29.5 days, each effect can be observed from somewhere on Earth once every month. The 0.5 day part of the lunar cycle gives many observers the impression that many of these effects are rarer than they actually are. Because of the 0.5 day part, a particular effect can typically be observed from a given location at most once every other month (because in the alternate months, for that observer, the event occurs during daylight hours or when the Moon is below the horizon).
• As indicated in the following list, many clair-obscur effects on the Moon have been described, and some have been given informal names. Some of these names are relatively well known; but most are pet names, often recognized by only a handful of observers. Most apply to a specific surface feature, or group of surface features; but unlike other informal and unofficial feature names, they are applied to those features only when they are seen with a particular lighting. Since the IAU does not assign names to lighting patterns, none of these are "official" lunar names, nor do they even have the possibility of becoming official.
• The names should be used with caution because, as with all other informal and unofficial names, the same name may have been used by another observer to mean something completely different; and, conversely, the same effect may be differently named by different observers.

List of Clair-Obscur Effects and Informal Optical Feature Names

Four quadrants list of well known and rather unknown clair-obscur effects, related to the lunar morning and evening terminator.

Quadrant 1

(the northeastern part of the moon's near side)

• The Zeno steps near or at crater Zeno (an evening-terminator effect which was photographed by the dedicated lunar observer Stefan Lammel).
• The Lunar V north-northeast of Ukert M (this is a rather large clair-obscur effect, observable through small telescopes, during local sunrise). This effect is observable at about the same moment of the Lunar X 's appearance.
• O'Neill's Bridge between Promontorium Lavinium and Promontorium Olivium. See LPOD; jan 22th, 2009: Fallen Bridge.
• Barrow's shaft of light (on the floor and western wall of Barrow). A morning terminator effect. See LPOD; may 13th, 2008.
• Alexander's beaded rim. The "crackled" illumination of the western "half" of the degraded crater Alexander during sunrise (a curious "Jeweled Handle" appearance, a bit like the larger "handle" at the Montes Jura/ Sinus Iridum). Observed by Danny Caes. See photograph by Stefan Lammel.
• The Hayn-'Belkovich shadow, when it's two-days-before-Full-Moon. Hayn and Belkovich are two craters near the moon's northeastern limb, and curiously enough, there's a black shadow in (or near?) these craters when it's about two days before Full Moon, which is too soon to be part of the eastern limb's evening terminator! (observed by Danny Caes during the night of march the 3th, 1996)(there was Full Moon at march the 5th). The same phenomenon observed on january the 3th, 2015 (Full Moon on the 5th).
• Posidonius' illuminated rim during local sunset (evening terminator at about 30 degrees east over Posidonius). Observed by Danny Caes on 19-9-'08 at 23.00 U.T., Orion-SkyQuest Dobsonian.
• The "L"-shaped mountain at 43° north/ 11° east (in the northern part of the Montes Caucasus) is an interesting telescopic target during Full Moon, when the western slope of its north-south oriented part, and the southern slope of its east-west oriented part, look very much like an illuminated "L" or "sunlit corner". It is unofficially called Mons Elbruz by Danny Caes (note: the Terrestrial Mount Elbruz is also located in the Caucasus range).
• The Caucasus X. An X-shaped appearance in that part of Montes Caucasus west-northwest of Eudoxus, observable during the local morning illumination. Discovered by Luka Giacomelli, see his photography of Eudoxus region (The Caucasus X is marked with red rounded rectangle).
• Cauchy's interrupted western shadow, when the morning-terminator runs at about two degrees west of Cauchy. The northern and southern parts of Cauchy's western shadow are untouched, the central part of it seems absent. A curious effect! Paolo Lazzarotti's LPOD of Cauchy and surroundings shows Cauchy's "interrupted" shadow very well! (see also at Wilkins's Lenham in Quadrant 3).
• Aratus CA's illuminated eastern inner slope. Although Aratus CA is basically a rimless depression, it can be seen as a bright dot when the sun is very low to the west. It would appear that the Sun's setting rays are able to shine down Vallis Christel and strike the east wall of the pit between Vallis Krishna and Rima Sung-Mei. Research: Jim Mosher.

Quadrant 2

(the northwestern part of the moon's near side)

• Gruithuisen's so-called lunar city, north-northwest of crater Schröter (disappears during high-sun).
• Cassini's Moon Maiden at Promontorium Heraclides (a morning-terminator effect). See Bibliography.
• The Golden Handle effect or Jewelled Handle at Montes Jura and Sinus Iridum (a classic morning-terminator effect which is observable through small binoculars!). This effect appears exactly 24 hours after the morning terminator's location at Copernicus, which, for us, is a very good indicator! See also LPOD Half A Circle and LPOD 24 Hours.
• Buzz Saw (the opposite of the Jewelled Handle effect), see: https://www.cloudynights.com/topic/551293-next-lunar-buzz-saw-early-monday-morning-sept-26/
• Two bright stars at Mons Pico and Pico Beta, when the morning terminator runs east of these two peaks. A well-known phenomenon, observable through small telescopes and binoculars.
• Babbage A's illuminated rim when the morning-terminator is at 55° West. The floor of Babbage itself is still in darkness, while Babbage A's rim is already in sunlight. Observed by Danny Caes.
• Pythagoras's illuminated central peak, when the floor of Pythagoras is still in darkness during the local sunrise (looks quite "pronounced" and "steep"). See LPOD Textures.
• Crater Plato's hook.
• Ashen Light on Plato's floor (or is it some sort of TLP?). See Phil Morgan's exquisite drawing in LPOD Milk of Plato.
• Copeland's star tip mountain (Mons Herodotus)(a morning-terminator effect). See LPOD dec 14, 2008.
• Crater Carlini's double shadow during local sunrise. See Bibliography.
• Lambert Gamma (incorrectly labeled Mons Undest -Mons Undset- on LTO-40B4). Before local sunrise it (Lambert Gamma) looks like a very bright "star" on the shadowed part of Mare Imbrium).
• The two stars at Mons Ampère and Mons Huygens (a morning-terminator effect which was discovered (or re-discovered?) by Danny Caes. See also LPOD A New Look
• Crater Dechen's flat dish appearance (a morning-terminator effect).
• Crater Lavoisier A's dish-shaped appearance when the morning terminator runs at about 75 degrees West.
• Crater Wallace's three shadow-spires (a morning-terminator effect). See also AS17-153-23591, which is the last one of a small series showing orbital views of Wallace during local sunrise. Research Danny Caes.
• Wallace's illuminated ring effect during local sunrise. See LPOD may 6th, 2009.
• Distinct short illuminated streak at 17°45' North/ 6°10' West (a morning-terminator effect).
• Crater T. Mayer A's bright illuminated ring appearance (a morning-terminator effect).
• Crater Aristillus B's rim which is "cut in half" when the shadow of one of Montes Spitzbergen's peaks is projected upon it (an evening-terminator effect). See Bibliography.
• Maupertuis' Moonmaiden at the eastern part of Maupertuis, during local sunset (evening-terminator). Should be observed through a prism-less telescope, on Earth's northern hemisphere. In other words; moon's north at the bottom, south up.
• The solemn looking face at 16° North/ 6° West (southeast of Mons Wolf). This is a rather difficult-to-recognize clair/obscur effect, because of the face itself (which looks northeast-ward in a rather "solemn" manner). A morning terminator effect. The location of the face is also visible on Apollo 17's Nikon photograph AS17-160-23946, below centre. Discovered and observed by Danny Caes.
• A peculiar Z-shape tendency on the western inner slopes of Aristarchus, which could be a Trompe-l'Oeil effect related to the angle of illumination (??). See LPOD Catching some ZZZ
• Quincunx at 14.0° North / 22.6° West, a pattern of six 'lights' on morning terminator in vicinity of Gay-Lussac and Montes Carpatus, north of Copernicus. Five points were similiar to pattern of five pips on the face of a die (a "quincunx"). The sixth sunlit point was situated southwest of the five pips. The six lights together formed a stubby cross. Observed by Mike Rowles through 90mm Mak-Cas on 5/12/2011, 0256 UT, from Maryland. Pattern began to break up after about one hour. Photographed by Dave Mitsky on same night from Pennsylvania.

Quadrant 3

(the southwestern part of the moon's near side)

• Rings in Clavius (a well known morning-terminator effect). Sometimes called the eyes of Clavius.
• Hesiodus Sunrise Ray, see APOD Hesiodus Sunrise Ray.
• Pitatus Sunset Ray, see article The Pitatus Lunar Ray.
• Shadows on the floor of Ptolemaeus and at Ammonius, during local sunrise (see LPOD may the 15th, 2008).
• The Pearl Necklace effect at craters Fra Mauro and Bonpland (a morning-terminator effect). This effect has been studied by Phil Morgan. See his online drawing.
• The illuminated cross near crater Fourier (a morning-terminator effect which was discovered (or re-discovered?) by Danny Caes.
• Crater Crüger's thin crackled illuminated rim (a morning-terminator effect).
• Crater Maginus's three illuminated dots, gradually transforming into the shape of a triangle (a morning-terminator effect).
• The Equilateral triangle/ Three illuminated dots or 3 Stars between Lacus Timoris and Epimenides (when the morning-terminator runs at 24 degrees west). Discovered by Danny Caes.
• "Long straight dark line crossing a curious curved oval-shaped ring" near Lassell. See Bibliography.
• Robert E. Curtiss's "cross" in the Fra Mauro Zeta complex (an evening-terminator effect).
• Bright and distinct appearance of a "2" (a number two) at the eastern parts of Deluc and Deluc D. This peculiar effect is visible when the morning terminator runs at 1° 30' West (say: 1° to 2° West, over Oppolzer). The appearance looks very much like a "nonchalantly written number 2" (very thin looking, especially when the effect has just started). Discovered by Danny Caes (may the 12th, 2008).
• Equilateral triangle of 3 illuminated dots at the northeastern part of Schickard's nocturnal floor, when the morning-terminator runs at 54° or 55° West (observed by Danny Caes during the night of march the 30th, 2007, through the 23-cm Cooke/Steinheil refractor of Ghent-Belgium).
• Broken Engagement Ring at 13° South/ 57° West (east of Sirsalis), when the morning-terminator runs at 55° West. This ring looks like an asterism or "hook", composed of several illuminated peaks. Observed by Danny Caes.
• Five separate sunlit summit peaks on one of the Doerfel mountains, observable during waning crescent moon. See pages 130-131 in Harold Hill's A Portfolio of Lunar Drawings.
• Face in Wurzelbauer (observed by Terry Hannan): "On the evening of 9-08-08, I was again struck by the appearance of a face in Wurzelbauer; containing 2 eyes, a nose, one ear, and a grinning mouth. Has anybody noted this?".
• Lenham's interrupted western shadow, when the morning terminator runs at about 27° West. The northern and southern parts of Lenham's western shadow are untouched, the central part of it seems absent (note that Lenham was H.P.Wilkins's name for Kies A). Paolo Lazzarotti's photograph of Kies A and surroundings shows that effect very well! See also Cauchy's western shadow, in Quadrant 1. And also LPOD Horns and Wings: drawing of the interrupted western shadow at Lenham by Lambert Spix.

Quadrant 4

(the southeastern part of the moon's near side)

• The Lunar X at craters Blancanus, La Caille, and Purbach (this morning-terminator effect is a favourite of today's many lunar observers!). This effect is observable at about the same moment of the Lunar V 's appearance.
• Larrieu's Dam at Polybius K, discovered in 1955 by A.C.Larrieu, see article by Nigel Longshaw.
• Nessie's silhouetted head and neck on the northeastern part of the floor of Ptolemaeus, an effect which is observable during local sunrise circumstances (First Quarter Moon). Discovered by Joe Adlhoch, Colorado. See LPOD Nessie.
• The illuminated cross at crater Hase (an evening-terminator effect). See Bibliography.
• The three stars or Orion's belt at crater Hipparchus (a morning-terminator effect which was discovered (or re-discovered?) by Danny Caes.
• Crater Curtius's illuminated peak at the northwestern part of its rim (a morning-terminator effect). See Bibliography.
• Hand and pointing finger of E.T. on the western part of Cyrillus's rim (a morning-terminator effect).
• La Perouse A's bright appearance when the evening-terminator is running over it (during local sunset). La Perouse A is a bright craterlet northwest of La Perouse.
• Hipparchus C's shadowed (yet dimly illuminated) eastern inner slope, a couple of days after First Quarter Moon. Although it is not known if this optical effect (dimly reflected light on the shadowed part of a bowl-shaped crater's inner slope) is indeed observable through powerful telescopes, it sure could be a very interesting experiment! It is also interesting to know that Apollo 16 (april 1972) made a series of orbital Fairchild Mapping/Metric photographs of Hipparchus C and its dimly illuminated eastern shadowed inner slope. The most convincing examples of this series of photographs are AS16-M-0165 to 0169. Extreme close-ups of these photographs are online at the APOLLO IMAGE ARCHIVE of the Arizona State University (ASU). Research Hipparchus C and its dimly illuminated shadowed inner slope: - DannyCaes Jan 10, 2009
• Oetzi The Iceman - the silhouetted face in Albategnius, when the sunrise terminator is running over Ptolemaeus. Discovered by Aleksander Božič, see LPOD Ramparts and comment section.
• The Lion's Head. In august 2014 Patricio Leon (Nunki) discovered the remarkable appearance of a lion's head on the chaotic interior floor of Faraday C. This curious appearance is always noticeable when the western interior slopes (of Faraday C) are in darkness, such as in the WAC-mosaic of the LROC's ACT REACT Quick Map.

Other Informal Optical Feature Names

• Saber's Beads -- informal name for detached points of light seen in lunar crescents.
• Aries Hoofprint -- Andrew Martin SFO: I was just reading the Astronomy Magazine email I get and they were mentioning a feature known as Aries Hoofprint. I was wondering if anyone else ever heard of this feature name?
  From the email:
  Aries the Ram certainly never stepped on the Moon, but it's fun to imagine that it left a large hoofprint on the lunar surface. By the evening of October 6 (2008), the Sun has risen over the fascinating Hyginus, Triesnecker, and Ariadaeus rille complexes. Yet, one thing that catches
  the eye in this area just north of the lunar equator is the delightful "Aries' hoofprint" feature. The combination of brightly lit mountains and two deep channels of dark lava creates this striking play of light and shade.

New terminator-page?

A special Lunar Terminator page should be constructed for Flemish moon observer Ivo Demeulenaere. He's a very active investigator of the way grazing sunlight is playing in and around the moon's craters and mountains.
 
 

LPOD Articles

Sunrise phenomena: (drawings and photographs).
Horns and Wings (the interrupted sunrise shadow at Kies A, aka Wilkins's Lenham).
24 Hours (the sunrise terminator at Copernicus (right photograph) and 24 hours later at Montes Jura, with the Jewelled Handle effect (left photograph)).
Skimming the Surface (Clavius during local sunrise).
The Pearl Necklace (Fra Mauro during local sunrise, drawing by Phil Morgan).
Tapering towards the Terminator (Bonpland Sigma, Fra Mauro A, B, and C just after local sunrise, drawing by Phil Morgan).
Two Views (sunrise at the Triesnecker region, drawing by Phil Morgan).
Half A Circle (sunrise at Sinus Iridum and Montes Jura, drawing by Richard Handy).
A Fence (the Eratosthenes-Stadius region during local sunrise).
A Bucket of Darkness (close-up of Eratosthenes, made during the local lunar morning).
A Sharper Image (the Sinus Roris region during local sunrise).
Radial Fractures? (the Aristarchus Plateau during local sunrise, note the bright appearance of Mons Herodotus, aka Copeland's Star-Tip Mountain).
New Domes? (the Hansteen-Billy-Zupus-Mersenius region during local sunrise).
Faultless Image (the Plato-to-Northpole region during local sunrise).
Levels (the Pingré-Hausen basin and Inghirami during local sunrise).
Travelogue (Waxing Crescent Moon, with sunrise terminator at 34° east).
Cuarto Creciente (moon slightly before First Quarter, sunrise terminator over Ptolemaeus).
Two for One (sunrise terminator over Eratosthenes).
Western Tales (Eddington and surroundings during local sunrise).
Whose Rilles? (the Agatharchides-Hippalus region during local sunrise).
Classical Imbrium (dawn at Mare Imbrium, with sunrise terminator over Timocharis).
Ramparts (the Albategnius-Hipparchus-Ptolemaeus region, with sunrise terminator over Ptolemaeus).
Pseudo or Real Scarp? (the Hippalus region after local sunrise).
LPOD News (Gassendi and environs during local sunrise).
Don't Burp (southern half of the Waxing Crescent Moon, upside-down view).
An Ancient Ring? (sunrise at the Lansberg region, with the terminator at Lansberg itself).
Beyond the Polar Explorers (the Southpole region during the post-sunrise days).
14 Rilles, not counting Gassendi (the Gassendi-Mersenius region during the post-sunrise hours).
Wow! (Eddington, Seleucus, and Briggs during local sunrise, with sunrise terminator over Eddington).
The last of Eddington (the region of Eddington during local sunrise, drawing by Frank McCabe).
Corner of Mystery (Mons Rumker after local sunrise).
Mystery Lines (sunrise at Montes Jura, a couple of hours after the appearance of Patrick Moore's Jewelled Handle effect).
Delayed Sunrise (Sunrise Terminator west of Montes Caucasus).
The Beetle (sunrise at Birt, Birt A, and Rupes Recta).
Swell Wichmann (sunrise at Wichmann and environs).
Finding the Past (sunrise at Philolaus and environs).
One TLP explained! (sunrise terminator over Ptolemaeus).
Mount Sirsalis (the Sirsalis region during local sunrise).
Three things to notice (the Bianchini-Bouguer-Foucault-Harpalus-South region, with South at the local sunrise terminator).
Petavian View (sunrise terminator just west of Petavius).
A Museum Piece (Montes Apenninus after local sunrise, note the "illuminated ring effect" at Wallace!).
Classic View (Montes Alpes after local sunrise, and morning terminator at Plato).
A Classic View (Plato and Vallis Alpes during local sunrise, with shadow spires on Plato's floor).
Convergence (Waxing Crescent Moon).
Procellarum Twins (Cardanus and Krafft during local sunrise).
Lavafall (morning terminator at the mountains surrounding Flamsteed G)(west of Flamsteed itself).
Space Station in the Moon (source: APOD)(part of the moon's northern half during First Quarter, with the International Space Station in front of Mare Serenitatis!).

Sunset phenomena: (drawings and photographs).
Don't give me any of your lip (Howard Eskildsen's evening terminator photograph of the Neander Fault region).
Crossing the Lines (Phil Morgan's drawing of the Sabine-Ritter-Manners-Dionysius region during local sunset).
Milk of Plato (Phil Morgan's exquisite drawing of a peculiar phenomenon on the floor of Plato, which is perhaps related to the optical effect known as Ashen-Light, or maybe it's some kind of TLP?).
Fractured, Tilted, and Flooded (Fracastorius during local sunset).
The Left Side of Hell (Deslandres during local sunset).
Why Here? (the Cauchy region during local sunset).
Rille Cut (the Gutenberg region during local sunset).
Third Discovery (Fabricius just before local sunset).
An Old Fault? (the Lacus Somniorum region during local sunset).
A Rim? (Janssen during local sunset).
Pyrenees Perusal (the Cook-Colombo-Magelhaens region during local sunset).
The Oklahoma Orbiter (Montes Taurus and Apollo 17's Taurus-Littrow site during the pre-sunset hours).
A Little Bit of Mystery (the Cleomedes-Macrobius-Newcomb region during the pre-sunset hours).
Zoroaster (the Mare Humboldtianum region during local sunset).
Oddities (the Atlas-Hercules-Endymion region before local sunset).
Fallen Bridge (the so-called "O'Neill's Bridge" between Promontorium Lavinium and Promontorium Olivium, during the pre-sunset hours).
Jumbilation (the Rimae Gutenberg-Vallis Capella region just before local sunset).
Lovely Rheita (the Vallis Rheita region during local sunset).
Two Views (sunset at the Triesnecker region).
Land Lost in Time (the Geminus-Messala region during local sunset).
Modes of Devastation (Montes Haemus during the pre-sunset hours).
A Tortured Terrain (the Delambre-Hypatia-Torricelli-Zollner region during the pre-sunset hours).
Sharing the Spotlight (Petavius and environs shortly before local sunset).
Somewhere over the Rainbow (the Albategnius-Ritchey-Andel-Abulfeda region during the pre-sunset hours).
Leftovers (sunset at the Campanus-Mercator-Kies-Golubiz Cluster-Konig-Bullialdus region).
Volcanic Hinterland (sunset at the Gruithuisen group).
Terminator tales (sunset at Plato and environs).
Half a Golden Quarter (a wonderful and detailed drawing of the Last Quarter Moon's northern half, by Josef Klepesta and Ladislav Lukes).

Miscellaneous:
The Tyranny of an Orbit (an artificial "east-west" sunrise/sunset terminator at the Montes Caucasus region and environs).

APOD articles

Hesiodus Sunrise Ray

Bibliography

• The Jewelled Handle effect at Montes Jura/ Sinus Iridum:
  • In almost each of Patrick Moore's books about the moon and lunar observing.
• The illuminated cross at Hase:
  • Patrick Moore's New Guide To The Moon, Page 298.
• Cassini's Moon Maiden and Gruithuisen's lunar city:
  • Almost every book about lunar observing has descriptions (and drawings) of these well-known effects!
• Crater Carlini's double shadow:
  • Op Ontdekking In Het Maanland, by A.J.M. Wanders (1949), pages 241, 242.
• Crater Curtius's illuminated rim-peak:
  • A Portfolio of Lunar Drawings, by Harold Hill, page 187.
• Crater Aristillus B and the shadow of one of Montes Spitzbergen's peaks:
  • A Portfolio of Lunar Drawings, by Harold Hill, page 61.
• The long straight dark line near Lassell:
  • A Portfolio of Lunar Drawings, by Harold Hill, pages 102, 103.

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