APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

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APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby APOD Robot » Wed Aug 11, 2010 4:04 am

Image Crepuscular Rays Over Lake Michigan

Explanation: What could cause such rays of dark? Dark sky rays were caught in spectacular fashion earlier last month from Pentwater, Michigan, USA, looking west over Lake Michigan. The cause is something surprisingly familiar: shadows. Clouds near the horizon can block sunlight from reflecting off air, making columns outward from the Sun appear unusually dark. Cloud shadows can be thought of as the complement of the more commonly highlighted crepuscular rays, also visible above, where sunlight pours though cloud holes. Sometimes, on the opposite side of the sky, anticrepuscular rays can also be seen.

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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby mexhunter » Wed Aug 11, 2010 4:11 am

A beautiful picture.
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby anamait@mail.pf » Wed Aug 11, 2010 8:15 am

we call the phenomenon the zodiacal lights when at sunset and the geigenshein lights at sunrise; both pics are so beautiful that i go to work with my camera, all along the lagoon in tahiti; i took a few pics from the place overloocking the lagoon last june ; i would like to send them to everyone but i don't know how to , through your comments
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby orin stepanek » Wed Aug 11, 2010 11:49 am

http://www.youtube.com/watch?v=mQfZ_WD8iA8
Really neat the way the shadows can be seen in the sky! 8-)
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby zbvhs » Wed Aug 11, 2010 2:00 pm

I remember seeing something like this on a trip in North Dakota, except that the light fan was on the northern horizon. Time was around midnight local time, late June, early July. The night was perfectly clear, no clouds in sight. Consensus in the car was that this was sunlight coming over the North Pole perhaps reflected off polar ice. The land is relatively flat in the middle of the North American continent practically all the way to the Arctic Sea so no mountains would have interfered with sunlight coming over the pole.
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby Chris Peterson » Wed Aug 11, 2010 2:54 pm

anamait@mail.pf wrote:we call the phenomenon the zodiacal lights when at sunset and the geigenshein lights at sunrise;

Crepuscular rays, zodiacal light, and gegenschein are different phenomena. Crepuscular rays are a purely local, atmospheric effect. Zodiacal light is sunlight scattered sideways from dust in ecliptic plane, in interplanetary space. It is seen after twilight at night, or before in the morning. Gegenschein is backscattered light from interplanetary dust, seen late at night at the antisolar point.
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby León » Wed Aug 11, 2010 3:02 pm

I have had to travel for ten hours in the pampa Argentina accompanied by Crepuscular Rays like the image below
Image
As off topic let me point out that the dawn will then today we will have the alignment of Venus, Saturn, Mars and the Moon to later attend the spectacle of the Perseids
Image
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby Chris Peterson » Wed Aug 11, 2010 3:41 pm

León wrote:I have had to travel for ten hours in the pampa Argentina accompanied by Crepuscular Rays like the image below

Very nice image. But I wouldn't personally call such rays "crepuscular". The term really means "occurring near twilight". Rays coming through clouds are more generally called "sun rays", with "crepuscular rays" better reserved for those that occur around dusk or dawn- particularly when the clouds actually casting the shadows are right on the horizon.
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby León » Wed Aug 11, 2010 5:08 pm

Chris Peterson wrote:
León wrote:I have had to travel for ten hours in the pampa Argentina accompanied by Crepuscular Rays like the image below

Very nice image. But I wouldn't personally call such rays "crepuscular". The term really means "occurring near twilight". Rays coming through clouds are more generally called "sun rays", with "crepuscular rays" better reserved for those that occur around dusk or dawn- particularly when the clouds actually casting the shadows are right on the horizon.


As stated before issuing the post I thought long and I agree that the name is inappropriate, intended to make the reservation, but could not find a more appropriate, it seems that the generic name is the crepuscular rays "Crepuscular rays (pronounceable / krɪpʌskjələr / ), in atmospheric optics, are rays of sunlight That Appear to radiate from a single point in the sky. These rays, Which stream-through gaps in clouds or between' Other objects, are columns of sunlit air separated by Dark Cloud-shadowed regions. The Their name from frequent eating Occurrences During evening hours (around dawn and dusk Those) When the light and dark contrasts between' are the most Obvious "http://en.wikipedia.org/wiki/Crepuscular_rays
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby StarstruckKid » Wed Aug 11, 2010 11:58 pm

Is this phenomenon sometimes caused by mountain shadows, say over the horizon east of the Rockies?
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby Chris Peterson » Thu Aug 12, 2010 1:21 am

StarstruckKid wrote:Is this phenomenon sometimes caused by mountain shadows, say over the horizon east of the Rockies?

Yes, a very similar phenomenon, generally called (you guessed it!) mountain shadows is caused just this way. Like crepuscular/anti-crepuscular rays, they may be seen both facing the Sun or facing away from it, depending on conditions.
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Re: APOD: Crepuscular Rays Over Lake Michigan (2010 Aug 11)

Postby Sophrosuna » Thu Aug 12, 2010 3:43 am

Nice pic, I'd have gotten one from Arizona though, the effect is the basis for the state flag.
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