Sunbeam
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A sunbeam, in meteorological optics, is a beam of sunlight that appears to radiate from the position of the Sun. Shining through openings in clouds or between other objects such as mountains and buildings, these beams of particle-scattered sunlight are essentially parallel shafts separated by darker shadowed volumes. Their apparent convergence in the sky is a visual illusion from linear perspective. The same illusion causes the apparent convergence of parallel lines on a long straight road or hallway at a distant vanishing point.<ref>Template:Cite book</ref> The scattering particles that make sunlight visible may be air molecules or particulates.<ref name="Lynch&1995">Template:Cite book</ref>
Crepuscular rays
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Crepuscular rays or god rays are sunbeams that originate when the sun is just below the horizon, during twilight hours.<ref>Template:Cite book</ref> Crepuscular rays are noticeable when the contrast between light and dark is most obvious. Crepuscular comes from the Latin word "crepusculum", meaning twilight.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Crepuscular rays usually appear orange because the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high midday sun. Particles in the air scatter short wavelength light (blue and green) through Rayleigh scattering much more strongly than longer wavelength yellow and red light.
Loosely, the term "crepuscular rays" is sometimes extended to the general phenomenon of rays of sunlight that appear to converge at a point in the sky, irrespective of time of day.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Anticrepuscular rays
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In some cases, sunbeams may extend across the sky and appear to converge at the antisolar point, the point on the celestial sphere opposite of the Sun's direction. In this case, they are called antisolar rays (anytime not during astronomical night) or anticrepuscular rays (during the twilight period).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This apparent dual convergence (at both the solar and the antisolar points) is a perspective effect analogous to the apparent dual convergence of the parallel lines of a long straight road or hallway at directly opposite points (to an observer above the ground).<ref name="cresp">Template:Cite book</ref>
Alternative names
- Backstays of the sun,<ref>Template:Cite book</ref> a nautical term, from the fact that backstays that brace the mast of a sailing ship converge in a similar way
- Buddha rays<ref name="Lynch&1995" />
- God rays, used by some members of the computer graphics industry<ref>E.g. this term is mentioned in: Template:Cite conference</ref>
- Jacob's Ladder<ref name="Lynch&1995" />
- Light shafts, sometimes used in the computer graphics industry, such as the game engine Unreal Engine<ref name=":0">{{#invoke:citation/CS1|citation
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- Ropes of Maui, originally taura a MauiTemplate:Sndfrom the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer<ref name="Lynch&1995" />
- Sun drawing water, from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)<ref name="Lynch&1995" />
See also
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