owlice wrote:
Hofi, I believe the close-in halo around the sun is a corona; see
here and
here for more information.
Not to be confused with the sun's actual corona.
<<
22° halos appear when the sky is covered by thin cirrus clouds containing the ice crystals that cause the phenomenon.
Small colorful coronas much nearer the sun produced by water droplets can occasionally be confused with 22° halos.>>
http://en.wikipedia.org/wiki/22%C2%B0_halo wrote:
<<A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60° angle, the angle of minimum deviation is almost 22° (21.54° for red and 22.37° for blue). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.
Pathway of light through
a hexagonal prism in the
optimal angle resulting
in minimum deviation.
Light passing through the hexagonal ice prisms is deflected twice, which produces deviation angles ranging from 22° to 50°. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo.
As no light is refracted at smaller angles than 22° the sky is darker inside the halo. This effect is similar to Alexander's band, which lies between primary and secondary rainbows.
22° halos form when the sky contains millions of variously oriented (poorly correlated) ice crystals. Some of these happen to be aligned perpendicular to the sun's light as viewed by any given observer, which produces the illuminated 22° circle, while other crystals produce the same phenomenon for other observers.>>
http://en.wikipedia.org/wiki/Alexander%27s_band wrote:
Click to play embedded YouTube video.
<<Alexander's band or Alexander's dark band is an optical phenomenon associated with rainbows which was named after Alexander of Aphrodisias who first described it. It occurs due to the deviation angles of the primary and secondary rainbows. Both bows exist due to an optical effect called the angle of minimum deviation. Light which is deviated at smaller angles than this can never reach the observer.
http://antwrp.gsfc.nasa.gov/apod/ap070912.html
The minimum deviation angle for the primary bow is 137.5°. Light can be deviated up to 180°, causing it to be reflected right back to the observer. Light which is deviated at intermediate angles brightens the inside of the rainbow.
The minimum deviation angle for the secondary bow is about 230°. The fact that this angle is greater than 180° makes the secondary bow an inside-out version of the primary. Its colors are reversed, and light which is deviated at greater angles brightens the sky outside the bow.
Between the two bows lies an area of unlit sky referred to as Alexander's band. Light which is reflected by raindrops in this region of the sky cannot reach the observer.>>
http://en.wikipedia.org/wiki/Alexander_of_Aphrodisias wrote:
<<Alexander of Aphrodisias (Greek: Ἀλέξανδρος ὁ Ἀφροδισιεύς; fl. 200 AD) was a Peripatetic philosopher and the most celebrated of the Ancient Greek commentators on the writings of Aristotle. He was a native of Aphrodisias in Caria, and lived and taught in Athens at the beginning of the 3rd century, where he held a position as head of the Peripatetic school. He wrote many commentaries on the works of Aristotle, and still extant are those on the Prior Analytics, Topics, Meteorology, Sense and Sensibilia, and Metaphysics. Several original treatises also survive, and include a work On Fate, in which he argues against the Stoic doctrine of necessity; and one On the Soul.>>
Art Neuendorffer
(Peripathetic philosopher and the least celebrated of the
Modern Geek commentators on the writings of The Asterisk*.)
[quote="owlice"]
Hofi, I believe the close-in halo around the sun is a corona; see [url=http://www.atoptics.co.uk/droplets/corona.htm]here[/url] and [url=http://www.atoptics.co.uk/halo/circular.htm]here[/url] for more information.[/quote]
Not to be confused with the sun's actual corona.
<<[b][color=#FF0000]22° halos appear when the sky is covered by thin cirrus clouds containing the ice crystals that cause the phenomenon.
Small colorful coronas much nearer the sun produced by water droplets can occasionally be confused with 22° halos.[/color][/b]>>
[quote=" http://en.wikipedia.org/wiki/22%C2%B0_halo"]
<<A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60° angle, the angle of minimum deviation is almost 22° (21.54° for red and 22.37° for blue). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.
[float=right][img]http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Path_of_rays_in_a_hexagonal_prism.png/150px-Path_of_rays_in_a_hexagonal_prism.png[/img]
Pathway of light through
a hexagonal prism in the
optimal angle resulting
in minimum deviation.[/float]Light passing through the hexagonal ice prisms is deflected twice, which produces deviation angles ranging from 22° to 50°. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. [b][color=#FF00FF]As no light is refracted at smaller angles than 22° the sky is darker inside the halo. This effect is similar to Alexander's band, which lies between primary and secondary rainbows.[/color][/b]
22° halos form when the sky contains millions of variously oriented (poorly correlated) ice crystals. Some of these happen to be aligned perpendicular to the sun's light as viewed by any given observer, which produces the illuminated 22° circle, while other crystals produce the same phenomenon for other observers.>>[/quote]
[quote=" http://en.wikipedia.org/wiki/Alexander%27s_band"]
[float=right][youtube]http://www.youtube.com/watch?v=qPTbgvzgMZU[/youtube][/float]<<Alexander's band or Alexander's dark band is an optical phenomenon associated with rainbows which was named after Alexander of Aphrodisias who first described it. It occurs due to the deviation angles of the primary and secondary rainbows. Both bows exist due to an optical effect called the angle of minimum deviation. Light which is deviated at smaller angles than this can never reach the observer.
http://antwrp.gsfc.nasa.gov/apod/ap070912.html
The minimum deviation angle for the primary bow is 137.5°. Light can be deviated up to 180°, causing it to be reflected right back to the observer. Light which is deviated at intermediate angles brightens the inside of the rainbow.
The minimum deviation angle for the secondary bow is about 230°. The fact that this angle is greater than 180° makes the secondary bow an inside-out version of the primary. Its colors are reversed, and light which is deviated at greater angles brightens the sky outside the bow.
Between the two bows lies an area of unlit sky referred to as Alexander's band. Light which is reflected by raindrops in this region of the sky cannot reach the observer.>>[/quote]
[quote=" http://en.wikipedia.org/wiki/Alexander_of_Aphrodisias"]
<<Alexander of Aphrodisias (Greek: Ἀλέξανδρος ὁ Ἀφροδισιεύς; fl. 200 AD) was a Peripatetic philosopher and the most celebrated of the Ancient Greek commentators on the writings of Aristotle. He was a native of Aphrodisias in Caria, and lived and taught in Athens at the beginning of the 3rd century, where he held a position as head of the Peripatetic school. He wrote many commentaries on the works of Aristotle, and still extant are those on the Prior Analytics, Topics, Meteorology, Sense and Sensibilia, and Metaphysics. Several original treatises also survive, and include a work On Fate, in which he argues against the Stoic doctrine of necessity; and one On the Soul.>>[/quote]
Art Neuendorffer
(Peripathetic philosopher and the least celebrated of the
Modern Geek commentators on the writings of The Asterisk*.)