APOD: An Airplane Glory (2016 Dec 22)

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APOD: An Airplane Glory (2016 Dec 22)

Post by APOD Robot » Thu Dec 22, 2016 8:12 am

Image An Airplane Glory

Explanation: Looking out the window of an airplane, you might be lucky enough to see "the glory" in the direction directly opposite the Sun. Before airplanes, the phenomenon, known to some as the heiligenschein or the Specter of the Brocken, was sometimes seen from mountaintops. There, when conditions were right, one could look away from the Sun and see what appeared to be the shadow of a giant surrounded by a bright halo. The giant turns out to be the observer, as in the modern version a silhouette of an airplane frequently occupies the glory's center. This bright glory was photographed two weeks ago over Michigan from an airplane on approach to O'Hare International Airport. The cause of the glory is still being researched and is relatively complex. Surely, small droplets of water in some way reflect, refract, and diffract sunlight backwards towards the Sun. The phenomenon has similar counterparts in other branches of science including astronomy, where looking out from the Earth in the direction opposite the Sun yields a bright spot called the gegenschein.

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Guest2 » Thu Dec 22, 2016 11:45 am

Main link not working...?

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Guest » Thu Dec 22, 2016 11:46 am

Years ago I saw a similar feature looking out of an airplane window. For me though, the conditions were such that there was some kind of interface producing a reflection of the glory. What I saw was almost a complete number 8 in the clouds below. No camera - this was before cell phones. A great memory though. We were in the climb right after take off, and I had a friend across the aisle. I insisted that he unbuckle and come over for a look. What rogues we were.

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by orin stepanek » Thu Dec 22, 2016 12:13 pm

You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by BillLee » Thu Dec 22, 2016 12:32 pm

The cause of the glory is still being researched and is reflect, refract, and diffract sunlight backwards towards the Sun.
Isn't this the same physics/geometry as a rainbow?

R/

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by zoomer » Thu Dec 22, 2016 1:36 pm

The photographer was in just the right seat here. But the 'right seat' would have a photo of an off-center shadow.

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Blackstrap » Thu Dec 22, 2016 2:11 pm

Isn't this the same thing as a sun dog ?

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Chris Peterson » Thu Dec 22, 2016 2:17 pm

orin stepanek wrote:You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
There can only be one, unless you are flying on a planet with two suns!
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Chris Peterson » Thu Dec 22, 2016 2:20 pm

BillLee wrote:
The cause of the glory is still being researched and is reflect, refract, and diffract sunlight backwards towards the Sun.
Isn't this the same physics/geometry as a rainbow?
No, it's a completely different optical phenomenon.
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by ta152h0 » Thu Dec 22, 2016 3:08 pm

I carry my camera with me all the time ( and two batteries ) got lucky once and became schooled. Next time I will become famous.
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by neufer » Thu Dec 22, 2016 3:20 pm

BillLee wrote:

The cause of the glory is still being researched and is reflect, refract, and diffract sunlight backwards towards the Sun.
Isn't this the same physics/geometry as a rainbow?
The 3D ray optics for rainbows & glories are pretty straightforward:

1) Glories are simply the incoming cylindrical set of solar rays onto water droplets which are reflect right back as an outgoing cylindrical set of solar rays (rather than being dispersed conically).

2) Rainbows are those unique outgoing conical caustics where an incoming finite width cylindrical set of solar rays all exit as an outgoing zero width conical set of solar rays.
However, things get much more complicated when one uses
sophisticated wave optics that involve diffraction & interference.
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by tdurow3 » Thu Dec 22, 2016 3:33 pm

I took a picture like this five years ago when I was flying from Sydney, Australia to Los Angeles, California. The picture is greater than the allowed 900 pixels limit for this forum so here is the reddit link.

https://www.reddit.com/r/Astronomy/comm ... the_glory/

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Chris Peterson » Thu Dec 22, 2016 3:41 pm

tdurow3 wrote:I took a picture like this five years ago when I was flying from Sydney, Australia to Los Angeles, California.
I always take a window seat when I fly, and I've probably observed this phenomenon on more than half of all the flights I've taken. It's extremely common.
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by zoomer » Thu Dec 22, 2016 4:32 pm

Chris Peterson wrote:
orin stepanek wrote:You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
There can only be one, unless you are flying on a planet with two suns!
My imagination, admittedly, runs a bit wild at times. So give me a break if my mind's eye can see two layers of cloud deck. The closer is thin and diffuse. The second is further down and reflecting a smaller image which is possibly blocked from view, too dim to shine back through the upper diffuse layer.

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Tszabeau » Thu Dec 22, 2016 5:58 pm

"I'm directly under the Earth's sun right... NOW! Yeah"... Chief Wiggums.

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by neufer » Thu Dec 22, 2016 6:14 pm

Tszabeau wrote:
"I'm directly under the Earth's sun right... NOW! Yeah"... Chief Wiggums.
`...there are three hundred and sixty-four days when you might get un-birthday presents — ‘

`Certainly,’ said Alice.

`There’s glory for you!’

`I don’t know what you mean by “glory,”‘ Alice said.

Humpty Dumpty smiled contemptuously. `Of course you don’t — till I tell you.
I meant “there’s a nice knock-down argument for you!”‘

`But “glory” doesn’t mean “a nice knock-down argument,”‘ Alice objected.

`When I use a word,’ Humpty Dumpty said in rather a scornful tone,
`it means just what I choose it to mean — neither more nor less.’
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Guest » Fri Dec 23, 2016 3:33 am

zoomer wrote:
Chris Peterson wrote:
orin stepanek wrote:You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
There can only be one, unless you are flying on a planet with two suns!
My imagination, admittedly, runs a bit wild at times. So give me a break if my mind's eye can see two layers of cloud deck. The closer is thin and diffuse. The second is further down and reflecting a smaller image which is possibly blocked from view, too dim to shine back through the upper diffuse layer.
I have seen that. But the glories are slightly different sizes...

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by neufer » Fri Dec 23, 2016 3:43 pm

Chris Peterson wrote:
witchcat wrote:
It is not possible that the gegenschein is not a focus of light frequencies from the sun as they deferentially refract passing around the earth due to gravity?
The gravitational effect of the Earth on light is incredibly small.
The focus of light frequencies from the sun as they deferentially refract passing around the earth due to gravity is ~15,000 AU away! A better 'alternative gegenschein theory' might involve a possible concentration of particles at the L2 Earth–Sun Lagrangian point.
https://en.wikipedia.org/wiki/FOCAL_(spacecraft) wrote:
<<Fast Outgoing Cyclopean Astronomical Lens (FOCAL) is a proposed space telescope that would use the Sun as a gravity lens. In order to use the Sun as a gravity lens, it would be necessary to send the telescope to a minimum distance of 550 AU away from the Sun, enabling very high signal amplifications: for example, at the 203 GHz wavelength, amplification of [~7 x105].

Even without using the Sun as the lens, FOCAL could perform various, otherwise impossible measurements: a telescope could be used to measure stellar distances by parallax, which would, using the baseline of 550AU, measure the precise position of every star in the Milky Way, enabling various further scientific discoveries. It could also study the interstellar medium, the heliosphere, observe gravitational waves, check for the possible variation of the gravitational constant, observe the cosmic infrared background, characterise interplanetary dust within the Solar system, more precisely measure the mass of the Solar system and similar.

FOCAL does not require any non-existing technology; however, it has various limitations. A space mission of this duration and distance has never been attempted. A gravity lens will bend objects behind it, so that images from the telescope would be difficult to interpret. FOCAL would be able to observe only objects that are right behind the Sun from its point of view, which means that for every observed object a new telescope would have to be made.>>
Last edited by neufer on Fri Dec 23, 2016 7:24 pm, edited 1 time in total.
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Chris Peterson » Fri Dec 23, 2016 3:58 pm

neufer wrote:
Chris Peterson wrote:
witchcat wrote:
It is not possible that the gegenschein is not a focus of light frequencies from the sun as they deferentially refract passing around the earth due to gravity?
The gravitational effect of the Earth on light is incredibly small.
The focus of light frequencies from the sun as they deferentially refract passing around the earth due to gravity is ~15,000 AU away!
I was thinking of doing that calculation, but was feeling too lazy yesterday. Glad you did it (but I'm surprised it's actually so close... I was thinking it would come out in light years). Were you considering light rays that are tangent to the surface? Those will be refracted much more strongly by Earth's atmosphere than by gravity, of course. The presence of an atmosphere adds a fair bit of complication to a ray tracing problem for sunlight around the Earth.
A better 'alternative gegenschein theory' might involve a possible concentration of particles at the L2 Earth–Sun Lagrangian point.
I'm not sure I'd call that an alternative, though. The diameter of the gegenschein is much too large for L2 dust alone to explain it, and it clearly is part of the zodiacal light band structure. The nature of the scatter is well understood, as is the size distribution for the particles involved. The only real question around the effect of L2 is the degree to which captured dust in that region enhances the dust concentration and therefore slightly alters the brightness distribution of the gegenschein.
Chris

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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by neufer » Fri Dec 23, 2016 5:12 pm

Chris Peterson wrote:
neufer wrote:
Chris Peterson wrote:
The gravitational effect of the Earth on light is incredibly small.
The focus of light frequencies from the sun as they deferentially refract passing around the earth due to gravity is ~15,000 AU away!
I was thinking of doing that calculation, but was feeling too lazy yesterday. Glad you did it (but I'm surprised it's actually so close... I was thinking it would come out in light years).
  • If the Earth were only as dense as the Sun the minimal focus
    would scale inversely with the Earth's size or ~ 109 x 550 AU.
Because the Earth is denser than the Sun the minimal focus is proportionately closer.
----------------------------------------------------------------------
  • Newtonian Gravitational focus = b2/rs
    Einsteinian Gravitational focus = 0.5 x b2/rs
Where b is the impact parameter (which is greater than or equal to the body's radius)
And rs is the Schwarzschild radius (which roughly scales with the body's radius cubed)
----------------------------------------------------------------------
Chris Peterson wrote:
Were you considering light rays that are tangent to the surface? Those will be refracted much more strongly by Earth's atmosphere than by gravity, of course. The presence of an atmosphere adds a fair bit of complication to a ray tracing problem for sunlight around the Earth.
I was considering light rays tangent to the Kármán line
where atmospheric refraction effects quickly start to drop below the gravitational effects for light rays.
--------------------------------------------------------------------------------------
The average amount of atmospheric refraction at the Earth's surface is 2x34 arcminutes.

The average air density at 100 kilometers altitude
(i.e., the Kármán line) is about 1/2,200,000 the density on the surface.

Hence a simple estimate of atmospheric refraction at 100 kilometers altitude
(i.e., the Kármán line) is 1.85mas for a focus at ~5,000 AU.
--------------------------------------------------------------------------------------
Chris Peterson wrote:
neufer wrote:
A better 'alternative gegenschein theory' might involve a possible concentration of particles at the L2 Earth–Sun Lagrangian point.
I'm not sure I'd call that an alternative, though. The diameter of the gegenschein is much too large for L2 dust alone to explain it, and it clearly is part of the zodiacal light band structure. The nature of the scatter is well understood, as is the size distribution for the particles involved. The only real question around the effect of L2 is the degree to which captured dust in that region enhances the dust concentration and therefore slightly alters the brightness distribution of the gegenschein.
All five Earth–Sun Lagrangian points are just the centers of broad relatively stable potential wells.

The L2 potential well is particularly broad (not to mention unstable) as viewed from Earth.
https://en.wikipedia.org/wiki/Lagrangian_point#L2 wrote:


A contour plot of the effective potential due to gravity and the centrifugal force of a two-body system in a rotating frame of reference. The arrows indicate the gradients of the potential around the five Lagrange points—downhill toward them (red) or away from them (blue). Counterintuitively, the L4 and L5 points are the high points of the potential. At the points themselves these forces are balanced.
(OTOH, any focusing of sunlight by the Earth's atmosphere would, indeed, be quite localized as demonstrated by the Moon's ability to generally avoid that particular dark red spot.)
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by orin stepanek » Sun Dec 25, 2016 12:27 pm

Chris Peterson wrote:
orin stepanek wrote:You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
There can only be one, unless you are flying on a planet with two suns!
I'm not so sure: I do believe that the windows are probably have double panes! That would allow a second reflection! Any was I don't know for sure; so I will let it go! :wink:
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Re: APOD: An Airplane Glory (2016 Dec 22)

Post by Chris Peterson » Sun Dec 25, 2016 3:48 pm

orin stepanek wrote:
Chris Peterson wrote:
orin stepanek wrote:You could almost see a second glory but it is cut off by the window post in the middle right side of the photo! :wink:
There can only be one, unless you are flying on a planet with two suns!
I'm not so sure: I do believe that the windows are probably have double panes! That would allow a second reflection! Any was I don't know for sure; so I will let it go! :wink:
Heck, you could even take a picture out the window and then view it on your camera at the same time you see it out the window. Two glories for sure!
Chris

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