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Anticrepuscular rays

Anticrepuscular rays are beams of sunlight that appear to converge toward the antisolar point—the location on the sky directly opposite the Sun—creating a striking optical illusion typically observed during twilight hours when facing away from the rising or setting Sun.^[1]^[2] These rays are essentially the extension of crepuscular rays, which are visible near the Sun's position, but they manifest on the opposite side of the sky due to the Earth's curvature and linear perspective, where parallel shafts of light seem to meet at a vanishing point on the horizon.^[2]^[3] In reality, the rays are parallel, as confirmed by observations from space such as those from the International Space Station, which reveal their uniformity without the distorting effects of perspective.^[2] Formed when sunlight filters through gaps in clouds or other obstacles, the rays become visible against a backdrop of atmospheric haze, dust, or water droplets that scatter the light, while shadows from the blocking objects project across the sky.^[1]^[2] This phenomenon requires specific conditions, including low Sun angles during dawn or dusk and sufficient particulate matter in the atmosphere to enhance contrast between illuminated and shadowed regions, making it more prominent in polluted or dusty environments.^[1] Unlike true converging light, the effect is purely visual and can span the entire sky in rare instances, often accompanying other atmospheric displays such as rainbows or the Belt of Venus.^[3] Anticrepuscular rays have been documented in various locations worldwide, from coastal sunsets to mountainous vistas, and serve as a natural demonstration of geometric optics principles, highlighting how human perception interprets parallel phenomena.^[1]^[2]

Overview

Definition

Anticrepuscular rays are parallel beams of sunlight that appear to converge toward the antisolar point, which is the location on the sky directly opposite the Sun. These rays create an optical illusion of convergence due to perspective, forming visible shafts of light extending across the sky.^[1] They typically appear as alternating bands of light and shadow in the western sky during dawn or in the eastern sky at dusk, when the Sun is low on the horizon. Also known as antisolar rays, anticrepuscular rays constitute a meteorological optical phenomenon akin to crepuscular rays but observed in the opposite direction from the Sun.^[4]^[5]

Comparison to Crepuscular Rays

Crepuscular rays manifest as prominent beams of sunlight that appear to diverge from the position of the low Sun at sunrise or sunset, creating a fanning effect across the sky toward the observer. In contrast, anticrepuscular rays present as subtler beams that seem to converge on the antisolar point—the location directly opposite the Sun on the celestial sphere—typically observed when facing away from the solar disk. This oppositional geometry highlights their relational nature, with anticrepuscular rays forming the visual extension of crepuscular rays beyond the observer's zenith.^[1]^[6] Both crepuscular and anticrepuscular rays originate from the same mechanism: parallel shafts of sunlight penetrating gaps in clouds, cirrus veils, or other atmospheric occluders, such as during twilight when the Sun is near the horizon. The apparent convergence or divergence in either case is an optical illusion driven by linear perspective, akin to parallel lines on a distant road seeming to meet at the horizon; the rays themselves remain parallel in reality. Anticrepuscular rays are thus not a separate phenomenon but the far-side projection of the same cloud shadows that produce crepuscular rays, visible only from the opposite vantage.^[7]^[8] Anticrepuscular rays are typically dimmer and harder to discern than crepuscular rays due to the extended atmospheric path length the light must traverse to reach the observer, which permits greater diffusion of skylight into the shadowed regions and reduces overall contrast. In crepuscular rays, the proximity to the Sun minimizes this diffusion, preserving sharper boundaries between illuminated and shadowed areas through more direct forward propagation. This intensity disparity often renders anticrepuscular rays as faint, bluish streaks against a twilight sky, requiring clear atmospheric conditions and careful observation for detection.^[9]

Formation

Optical Perspective Effect

Anticrepuscular rays create a striking visual illusion of convergence due to linear perspective, much like how parallel railroad tracks or a receding road appear to meet at a distant vanishing point. This effect arises because the human visual system projects three-dimensional space onto a two-dimensional plane, causing parallel lines viewed at an angle to seem to draw together in the distance. In the case of anticrepuscular rays, observers facing away from the sun perceive the beams as emanating from the opposite horizon and narrowing toward the antisolar point, the location on the sky directly opposite the sun.^[10]^[11] Geometrically, these rays consist of parallel shafts of sunlight passing through gaps in clouds, with the sun's immense distance—approximately 150 million kilometers from Earth—ensuring that the incoming rays are effectively parallel despite the sun's finite angular diameter of about 0.5 degrees. The apparent convergence occurs solely because the observer's line of sight aligns progressively closer to the direction of the rays as they extend toward the antisolar point, creating the illusion of tapering. This perspective distortion is identical to that seen in crepuscular rays toward the sun but reversed in direction, emphasizing the rays' inherent parallelism across the sky.^[10]^[12] When the sun is low on the horizon, the geometry of the celestial sphere can cause the rays to follow paths along great circles, potentially giving them an arched appearance as they span the dome of the sky toward the antisolar point. This subtle curvature in projection enhances the dramatic effect but does not alter the fundamental parallelism of the light shafts.^[11]

Atmospheric Influences

Anticrepuscular rays often form when sunlight streams through irregular gaps in clouds, particularly towering cumulus or cumulonimbus formations, casting elongated shadows that delineate the visible beams across the sky. These cloud breaks act as apertures, allowing direct solar illumination to penetrate while adjacent cloud masses block light, creating high-contrast patterns essential for ray definition. Deep, optically thick clouds can project shadows extending hundreds of miles, enhancing the rays' prominence during low solar elevations. The visibility of anticrepuscular rays relies on atmospheric scattering mechanisms, where backscattering by haze, dust, aerosols, and water droplets illuminates the shadowed regions against darker backgrounds. Particles smaller than the wavelength of visible light facilitate diffuse backscattering, making the rays appear fainter and more ethereal compared to brighter forward-scattered crepuscular counterparts, as forward scattering is minimal in these antipodal views. Airborne particulates such as inorganic salts and organic aerosols increase scattering efficiency, providing the necessary contrast for the rays to stand out.^[13]^[14] Ray intensity is amplified at dawn and dusk due to the sun's low angles, which lengthen the light paths through the atmosphere and deepen shadows within cloud umbras, thereby boosting contrast via extended scattering interactions. Optimal conditions involve moderate aerosol optical thickness for balanced visibility, as excessive particles can diffuse light too broadly and reduce definition. Elevated particulate levels from pollution or wildfires, such as smoke plumes, can intensify rays by augmenting scatterers, though high concentrations may eventually haze out the effect.^[13]

Observation

Optimal Conditions

Anticrepuscular rays are optimally observed during sunrise, when facing westward, or sunset, when facing eastward, with the Sun positioned low on or just below the horizon. This timing corresponds to periods around sunrise or sunset when the Sun is low near the horizon, typically within about 6 degrees of the horizon elevation (either slightly above or below), allowing scattered sunlight to project across the sky toward the antisolar point.^[15]^[16] Weather conditions play a crucial role in enhancing visibility, requiring partly cloudy skies where gaps in the clouds are aligned toward the Sun to permit beams of light to penetrate and scatter. Hazy or moderately polluted air, with an aerosol optical depth greater than 0.11, increases contrast by providing sufficient particles for scattering without excessive attenuation, while avoiding dense overcast that would obscure the effect. Clear atmospheric conditions, such as light winds and high visibility under subtropical high pressure systems, further promote the phenomenon by minimizing turbulence and diffusion.^[15]^[16]^[17] These rays occur more frequently in clear, dry regions like deserts, where low humidity and minimal cloud cover create ideal backdrops for the beams to stand out against the sky. Winter sunsets can enhance visibility in such areas due to the Sun's lower trajectory, extending the period of low-elevation light scattering. The dim appearance of the rays results from backscattering in the atmosphere, which reduces their intensity compared to forward-scattered crepuscular rays.^[16]^[17]

Viewing Techniques and Examples

To observe anticrepuscular rays, position yourself during sunrise or sunset and face away from the Sun, directing your gaze toward the opposite horizon where the rays appear to converge at the antisolar point.^[15]^[18] This technique works best when crepuscular rays are visible near the Sun, as the anticrepuscular counterparts extend across the sky from the same cloud gaps.^[15] A common pitfall is overlooking these rays due to fixation on the more prominent crepuscular beams near the Sun, though they become evident with deliberate attention to the opposite direction.^[15] For photography, employ a wide-angle lens to encompass the full arc of convergence and the surrounding landscape, often requiring patience and adjusted exposure to capture their faint, subtle appearance.^[19]^[18] Notable examples include panoramic views over expansive plains like Bolivia's Salar de Uyuni salt flats, where rays converge eastward at sunset, enhanced by the flat terrain's unobstructed vista.^[18] Over oceans, striking displays occur along coastal areas such as New Zealand's Orewa estuary, where rays arc across the water during twilight.^[20] Aircraft contrails can also produce similar effects, as seen in curved shadow rays projecting antisolar from high-altitude flights, visible over open skies.^[21] In mountainous coastal settings like Norway's Ålesund fjords, HDR images reveal rays blending with terrain for dramatic depth.^[19]

Mountain Shadow

The mountain shadow represents a specific manifestation of anticrepuscular rays where a topographic feature, such as a mountain, casts an elongated shadow into the atmosphere at sunset, projecting toward the antisolar point opposite the sun.^[22] This shadow forms as the mountain blocks incoming sunlight, creating a three-dimensional void of unlit air that extends for tens to over 100 miles, bounded on either side by parallel shafts of illuminated and shadowed atmosphere that manifest as anticrepuscular rays.^[22] Due to linear perspective, these bounding rays and the shadow's edges appear to converge, delineating a dark triangular pyramid shape that tapers with distance.^[17] The resulting structure has its apex at the antisolar point, where the rays visually meet on the horizon, enhancing the illusion of depth and enclosure.^[23] This configuration is most visible from elevated vantage points, such as mountain summits or ridges overlooking the feature, where the observer can align their view along the shadow's axis during twilight, often requiring clear atmospheric conditions to discern the faint boundaries.^[24] The phenomenon's dramatic effect arises from the contrast between the dark pyramidal shadow and the converging rays, which can include a prominent "spike" extending from the summit's silhouette if the observer is positioned slightly below the peak.^[24] Notable instances occur frequently in prominent mountain ranges, including the Alps, where the shadow of Mont Blanc (4,810 meters) has been documented forming a triangular apparition with upward-jetting spikes visible to climbers below the summit.^[24] In the Rocky Mountains, shadows cast by peaks in Colorado or New Mexico can extend approximately 515 kilometers eastward, appearing as anticrepuscular rays observable from distant locations like Texas under favorable scattering conditions.^[16] Similarly, Mauna Kea in Hawaii (4,207 meters) produces striking examples, with its shadow tip aligning at the antisolar point amid cloud-enhanced rays, amplifying the ethereal convergence during sunset.^[23] These occurrences underscore the interplay of topography and optics, often photographed for their visual impact in regions with high elevations and low-angle sunlight.^[23]

Wagon-Wheel Spokes

Wagon-wheel spokes refer to the striking visual effect where anticrepuscular rays appear to radiate inward from the rim of a rainbow, creating the illusion of spokes on a wheel centered at the antisolar point.^[25] This phenomenon integrates the converging perspective of the rays with the circular geometry of the rainbow, which encircles the antisolar point when viewed from an appropriate vantage.^[26] The rays, typically bright beams of sunlight passing through cloud gaps opposite the low Sun, project toward the rainbow's "hub," enhancing the wheeled appearance due to the parallel nature of the actual light paths as perceived from the observer's position.^[27] The effect requires precise alignment between scattered raindrops forming the rainbow and clouds producing the anticrepuscular rays, often during post-rain conditions with the Sun near the horizon.^[28] Large raindrops in the rainbow's cone direct sunlight into specific directions, mimicking the scattering that defines anticrepuscular rays but confined within the rainbow's arc.^[25] Optimal viewing occurs when the observer faces away from a setting Sun, with rain showers in the antisolar direction and partial cloud cover to cast the rays, typically in the brief window after a storm when skies partially clear.^[26] This spectacle is relatively rare, demanding the rare coincidence of a full or near-full rainbow visibility and aligned cloud structures for ray formation, most commonly documented in temperate regions during summer or autumn sunsets.^[25] Notable examples include a double rainbow with spokes observed in Hurunui, New Zealand.^[29] where the rays fanned dramatically from the arc's center, and a rotating wheel effect captured in Quakertown, Pennsylvania, on May 10, 2019, due to fast-moving clouds.^[26] Another instance from Saskatchewan on July 17, 2003, highlighted dark and bright spokes converging on the antisolar point, amplifying the optical drama post-rain.^[25]

References

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Anticrepuscular Rays - Atmospheric Optics
Dec 16, 2024 · The appearance of anticrepuscular rays is a result of sunlight passing through openings in clouds. As the sun's rays interact with water ...
[2]
Sunbeams and the Belt of Venus Are Delightful Twilight Sights
Nov 29, 2024 · Technically, rays on the opposite side of the sky as the sun are called anticrepuscular rays, and if they meet again at the horizon, we say they ...
[3]
2025 February 4 – Anticrepuscular Rays: A Rainbow Fan over Spain
Feb 4, 2025 · Anticrepuscular rays result from sunlight, blocked by some clouds, going all the way around the sky, overhead, and appearing to converge on the ...
[4]
2020 March 18 - Anticrepuscular Rays over Florida - APOD
Mar 18, 2020 · At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Featured here is a particularly ...
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Crepuscular rays are sunrays in twilight skies - EarthSky
May 18, 2024 · Crepuscular means resembling twilight or dim. This phenomenon ... In this case, they're called antisolar rays. The image on the right ...
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Crepuscular rays | International Cloud Atlas
### Summary of Crepuscular and Anticrepuscular Rays
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Crepuscular and Anticrepuscular Rays
- **Crepuscular Rays**: Sunlight beams visible through gaps in clouds or near the horizon, appearing to converge at a point due to perspective, often seen at sunrise/sunset.
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Anti-crepuscular rays - Cloud Appreciation Society
Anti-crepuscular rays are shadows from tall clouds, visible when facing away from the rising or setting sun, and appear to converge near the horizon.Missing: scientific | Show results with:scientific
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The Role of Atmospheric Conditions in Determining Intensity of ...
Crepuscular and anticrepuscular rays are visible because their brightness contrasts with that of the surrounding sky. Even if shadows are cast into the ...
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Anticrepuscular rays - Atmospheric Optics
Like crepuscular rays they are parallel shafts of sunlight from holes in the clouds and their apparently odd directions are a perspective effect. Think of a ...
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The Optics of Sunlight | Physics Van | Illinois
Aug 20, 2016 · It's also hard to explain anti-crepuscular rays if crepuscular rays aren't parallel. These are rays from the setting sun that extend from the ...
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2016 November 26 - East to West, Light and Shadow - APOD
Nov 26, 2016 · There known as anti-crepuscular rays, they indicate the point opposite the rising sun. The cloud shadows are very nearly parallel, but converge ...
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Crepuscular Rays - Atmospheric Optics
Dec 16, 2024 · The presence of airborne particles, such as dust and aerosols, enhances scattering and contributes to the overall intensity and visibility of ...
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Crepuscular Rays, India - Gateway to Astronaut Photography of Earth
Oct 30, 2011 · The light also must be scattered by airborne dust, aerosols, water droplets, or molecules of air, providing a visible contrast between shadowed ...
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Anticrepuscular rays: How to see them - EarthSky
Bottom line: If you want to see anticrepuscular rays, look carefully opposite the direction of the sun. They are most often seen at sunrise or sunset.Missing: optimal | Show results with:optimal
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The Role of Atmospheric Conditions in Determining Intensity of ...
Jul 1, 2010 · Anticrepuscular rays (ARs) are virtually identical, except they converge on the antisolar point, which, when the sun is just below the western ...Missing: scientific | Show results with:scientific
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https://www.atoptics.co.uk/atoptics/skywide.htm
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Bolivia Antisolar (anticrepuscular) rays - OPOD
Dec 16, 2024 · Choose a location with an unobstructed view of the sky, preferably a high vantage point or a wide-open space. · Opt for a wide-angle lens to ...Missing: techniques | Show results with:techniques
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Anticrepuscular Rays, Norway - Atmospheric Optics
Dec 16, 2024 · How to Capture Anticrepuscular Rays in Photographs · Use a wide-angle lens to capture a larger portion of the sky and the converging rays.
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Anti-crepuscular rays over Orewa estuary in New Zealand - Facebook
Jan 14, 2025 · Rarely seen anti-crepuscular rays over Orewa estuary in New Zealand, captured during full moon setup, with discussion on crepuscular rays at ...Anticrepuscular Rays at the Grand CanyonAnticrepuscular rays over Boca Ciega Bay in TiMore results from www.facebook.com
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Antisolar Oddities, Curved Contrail Shadows - OPOD
Dec 16, 2024 · This article explores the phenomenon of curved contrail shadows and the mysterious antisolar oddities observed in the sky, ...
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Dec 16, 2024 · The sun's disk shape causes the tapering effect of mountain shadows. Only the shadow's end provides substantial cues about the mountain's shape.Missing: anticrepuscular | Show results with:anticrepuscular
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Mauna Kea Shadow & Rays - Atmospheric Optics
Dec 16, 2024 · This atmospheric spectacle occurs when the immense clouds in the west cast shadows across the sky, forming dark anticrepuscular rays that appear to converge on ...
[24]
Mont Blanc shadow & spike - OPOD - Atmospheric Optics
Dec 16, 2024 · It is akin to perceiving the weighty cloud above the climber as casting anticrepuscular rays. Factors Influencing the Visibility of Upper ...
[25]
Rainbow spokes and wheels - Atmospheric Optics
The result is one or more dark radial spokes centered on the antisolar point and making the rainbow sometimes resemble a wagon wheel. Bright spokes are similar ...
[26]
Rainbow spokes over Pennsylvania | Today's Image - EarthSky
May 14, 2019 · The result is one or more dark radial spokes centered on the antisolar point and making the rainbow sometimes resemble a wagon wheel.
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Crespuscular Rays - What Are They? - WeatherBug
Dec 10, 2020 · The wheel's rim is the rainbow and the sunbeam rays are the spokes, creating a “wagon-wheel spokes” anti-crepuscular rays. Another cool ...
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July 2019 - Cloud Appreciation Society
Jul 17, 2019 · And this turns out to be the reason the spokes of a rainbow wheel always align perfectly around the rainbow's rim. Anti-crepuscular rays ...