APOD: The Gegenschein Over Chile (2012 Dec 02)

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APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby APOD Robot » Sun Dec 02, 2012 5:06 am

Image The Gegenschein Over Chile

Explanation: Is the night sky darkest in the direction opposite the Sun? No. In fact, a rarely discernable faint glow known as the gegenschein (German for "counter glow") can be seen 180 degrees around from the Sun in an extremely dark sky. The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets. Pictured above from 2008 October is one of the more spectacular pictures of the gegenschein yet taken. Here a deep exposure of an extremely dark sky over Paranal Observatory in Chile shows the gegenschein so clearly that even a surrounding glow is visible. In the foreground are several of the European Southern Observatory's Very Large Telescopes, while notable background objects include the Andromeda galaxy toward the lower left and the Pleiades star cluster just above the horizon. The gegenschein is distinguished from zodiacal light near the Sun by the high angle of reflection. During the day, a phenomenon similar to the gegenschein called the glory can be seen in reflecting air or clouds opposite the Sun from an airplane.

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Ann » Sun Dec 02, 2012 5:41 am

I note that the Gegenschein is blue. I believe that the color is correct, since the colors of the stars seem correct. Possibly, however, the color balance of this picture might be "slightly blue".

If the Gegenschein is indeed blue, then it must be caused by very tiny dust grains that preferentially scatter the blue light of the Sun back to the Earth and to the rest of the solar system. In any case, the Gegenschein is definitely a reflection nebula, although an extremely faint one.

The Pleiades are just peeking their sisterly heads above the horizon in this image. How fascinating: Two blue reflection nebulae can be seen in this image, one very nearby and extremely faint, one far away and rather bright as reflection nebulae go.

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Chris Peterson » Sun Dec 02, 2012 6:48 am

Ann wrote:I note that the Gegenschein is blue. I believe that the color is correct, since the colors of the stars seem correct. Possibly, however, the color balance of this picture might be "slightly blue".

I'd say "very blue".

In fact, the gegenschein is white. Spectrally, it looks like sunlight, which is not surprising since it is just that, reflected from particles too large to preferentially scatter one part of the visible spectrum over another. Accurate color images of the gegenschein (and of zodiacal light) show either white or faint yellow (due to secondary scattering in the Earth's atmosphere- the same thing that makes the Sun appear slightly yellow).
Last edited by Chris Peterson on Sun Dec 02, 2012 3:57 pm, edited 1 time in total.
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby ritwik » Sun Dec 02, 2012 9:00 am

andromeda looks menacingly closer in this picture :o

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby revloren » Sun Dec 02, 2012 10:06 am

APOD Robot wrote:During the day, a phenomenon similar to the gegenschein called the glory can be seen in reflecting air or clouds opposite the Sun from an airplane.


Or from the top of a mountain with clouds below you and your back to the sun.

AKA Sundog. 8-)

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Boomer12k » Sun Dec 02, 2012 11:55 am

WOW, that is allot of STARS. WoW. I wish I had eyes that good!

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Matt Hiller » Sun Dec 02, 2012 12:14 pm

About the Specter of the Brocken mentioned: This phenomenon always occurs is just more visible when projected onto clouds. I noticed when I ride a bike on a street with grass growing on the bank, I can see the grass reflecting the sun light slightly brighter just around the shadow of my head. Riding a bike helps as you pass the grass and can see the change in brightness.

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby owlice » Sun Dec 02, 2012 12:23 pm

Matt Hiller wrote:About the Specter of the Brocken mentioned: This phenomenon always occurs is just more visible when projected onto clouds. I noticed when I ride a bike on a street with grass growing on the bank, I can see the grass reflecting the sun light slightly brighter just around the shadow of my head. Riding a bike helps as you pass the grass and can see the change in brightness.

That's heiligenschein, which is made a different way than a glory or Brocken spectre.
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Holy Shi#e!

Postby neufer » Sun Dec 02, 2012 3:41 pm

owlice wrote:
Matt Hiller wrote:
About the Specter of the Brocken mentioned: This phenomenon always occurs is just more visible when projected onto clouds. I noticed when I ride a bike on a street with grass growing on the bank, I can see the grass reflecting the sun light slightly brighter just around the shadow of my head. Riding a bike helps as you pass the grass and can see the change in brightness.

That's heiligenschein, which is made a different way than a glory or Brocken spectre.

Click to play embedded YouTube video.
Heiliger Dankgesang officiating...
http://en.wikipedia.org/wiki/Heiligenschein wrote:
<<Heiligenschein ("holy shine") is an optical phenomenon which creates a bright spot around the shadow of the viewer's head. It is created when the surface on which the shadow falls has special optical characteristics. Dewy grass is known to exhibit these characteristics, and creates a Heiligenschein. Nearly spherical dew droplets act as lenses to focus the light on the surface beneath them. Some of this light 'backscatters' in the direction of the sunlight as it passes back through the dew droplet. This makes the antisolar point appear the brightest.
The opposition effect creates a similar halo effect, a bright spot of light around the viewer's head when the viewer is looking in the opposite direction of the sun, but is instead caused by shadows being hidden by the objects casting them. When viewing the Heiligenschein, there are no coloured rings around the shadow of the observer, as in the case of a glory.
http://www.classicalconnect.com/archive wrote:
<<Beethoven’s String Quartet no. 13 in B-flat major op. 130, though second in order of publication, was actually composed during 1825-6 after the quartet in a minor, op. 132, making it the last of the quartets composed to fulfill the commission from the Russian prince Nikolai Galitzin. Whereas the quartet in a minor was Beethoven’s reflection on his recovery from a life-threatening illness, which gave birth to the profound and solemn "Heiliger Dankgesang" (Song of Thanksgiving) that forms the quartet’s centerpiece, the Quartet in B-flat major is quite possibly then the expression of renewed vigor and the composer’s exuberant return to his art. Hardly anywhere in the piece is there a mournful or sad measure.>>
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Matt Hiller

Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Matt Hiller » Sun Dec 02, 2012 5:48 pm

Thanks for the enlightenment! I hoped that Heiligenschein was exclusively shining for me. It would have, had nobody invented the camera.

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby quigley » Sun Dec 02, 2012 6:02 pm

I recall photos of the astronauts on the moon photographing the "halos" of light around the shadows of their heads on the lunar surface and it being labeled as "the glory" in those photos. Which phenomenon was occurring in that case?

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Chris Peterson » Sun Dec 02, 2012 6:12 pm

quigley wrote:I recall photos of the astronauts on the moon photographing the "halos" of light around the shadows of their heads on the lunar surface and it being labeled as "the glory" in those photos. Which phenomenon was occurring in that case?

That's called the opposition effect. It happens because the shadows cast by dust particles are hidden when you are viewing them at the antisolar point, so the region appears particularly bright. It's why the full Moon is much brighter than you'd expect, given its brightness on either side of full (the full Moon is seen at approximately the antisolar point).
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby JohnD » Sun Dec 02, 2012 6:44 pm

Apart from the gegenshein.......
That very deep, very wide starscene allows what appear to be 'lines' of stars.
One in particular runs from the telescope building in the middle of the pic, upwards and slightly to the left. There are many others, some straightish and some curlicued.
I'm aware that the more points on a graph, the easier it is to draw straight lines through them, so a figment of our Pareidolia.
Or is it to do with photography?

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby CapMephisto » Sun Dec 02, 2012 9:22 pm

Isn't this the same picture from, I think, May 7, 2008?

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby sinanipek » Sun Dec 02, 2012 9:44 pm

Is the Solar System a part of a nebula? I mean, if we look to the Sun from, say, Sirius, can we see the nursing nebula from which Sol came from?

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby bystander » Sun Dec 02, 2012 9:46 pm

CapMephisto wrote:Isn't this the same picture from, I think, May 7, 2008?

Why, yes, yes it is.
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Chris Peterson » Sun Dec 02, 2012 10:03 pm

sinanipek wrote:Is the Solar System a part of a nebula? I mean, if we look to the Sun from, say, Sirius, can we see the nursing nebula from which Sol came from?

No. We're currently passing through a region were the interstellar medium is a bit denser, which you might think of as a sort of instrumentally detectable nebula, but as we usually use the word, we are not in a nebula. The nebula that our system formed from is long since dissipated.
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby neufer » Sun Dec 02, 2012 10:21 pm

Chris Peterson wrote:
sinanipek wrote:
Is the Solar System a part of a nebula? I mean,
if we look to the Sun from, say, Sirius, can we see the nursing nebula from which Sol came from?

No. We're currently passing through a region were the interstellar medium is a bit denser, which you might think of as a sort of instrumentally detectable nebula, but as we usually use the word, we are not in a nebula. The nebula that our system formed from is long since dissipated.
http://en.wikipedia.org/wiki/Local_Interstellar_Cloud wrote:
<<The Local Interstellar Cloud (or Local Fluff or LIC) is the interstellar cloud roughly 30 light years or 9.2 parsecs across through which the Earth's Solar System is currently moving. It is currently unknown if the Sun is embedded in the LIC, or in the region where the LIC is interacting with the neighboring G-cloud. The Solar System is thought to have entered the Local Interstellar Cloud at some time between 44,000 and 150,000 years ago and is expected to remain within it for another 10,000 to 20,000 years. The cloud has a temperature of about 6000 K, about the same temperature as the surface of the Sun. It is not very dense, with 0.3 atoms per cubic centimeter; less dense than the average for the interstellar medium in the Milky Way (0.5 atoms/cm³), though six times denser than the gas in the Local Bubble (0.05 atoms/cm³) which surrounds the local cloud. In comparison, Earth's atmosphere at the edge of space has 12 billion atoms per cubic centimeter, dropping to 52 million at 150 km.

The cloud is flowing outwards from the Scorpius-Centaurus Association, a stellar association that is a star-forming region. The cloud formed where the Local Bubble and the Loop I Bubble met. The Sun is embedded in the Local Interstellar Cloud, as are a few other nearby stars including Alpha Centauri, Altair, Vega, Fomalhaut, and Arcturus. The Local Interstellar Cloud's potential effects on Earth are prevented by the solar wind and the Sun's magnetic field. This interaction with the heliosphere is under study by the Interstellar Boundary Explorer (IBEX), a NASA satellite mapping the boundary between the Solar System and interstellar space.>>
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Robolt » Mon Dec 03, 2012 2:12 am

Is Gegenschein ever visible to the naked eye?

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby neufer » Mon Dec 03, 2012 3:06 am

Robolt wrote:
Is Gegenschein ever visible to the naked eye?

    Yes.
Everyone who observed gegenschein prior to photography did so in dark skies with their naked eyes.

http://en.wikipedia.org/wiki/Gegenschein wrote:
<<The gegenschein was first described by the French Jesuit astronomer and professor Esprit Pezenas (fr) (1692–1776) in 1730. Further observations were made by the German explorer Alexander von Humboldt during his South American journey from 1799 to 1803. It was also Humboldt who gave the phenomenon its German name Gegenschein.

The Danish astronomer Theodor Brorsen published the first thorough investigations of the gegenschein in 1854. He was also the first to observe that the zodiacal light can embrace the complete sky, because under near-perfect conditions, a feeble light bridge connecting the zodiacal light and the gegenschein can be observed. Besides, Brorsen had already proposed the correct explanation for the gegenschein (interplanetary dust reflections).

In modern times, the gegenschein is not visible in most inhabited regions of the world due to light pollution.>>
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Chris Peterson » Mon Dec 03, 2012 5:48 am

Robolt wrote:Is Gegenschein ever visible to the naked eye?

I can see it almost anytime I'm outside around midnight if there's no Moon and I've had ten or fifteen minutes to dark adapt.
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby DavidLeodis » Mon Dec 03, 2012 12:18 pm

The explanation implies that the picture was taken in October 2008, but in the information brought up through the "Pictured above" link it however states "The image was obtained by Yuri Beletsky in October 2007". I thought I would mention what is presumably a typographical error in the explanation.

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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby neufer » Mon Dec 03, 2012 2:15 pm

DavidLeodis wrote:
The explanation implies that the picture was taken in October 2008, but in the information brought up through the "Pictured above" link it however states "The image was obtained by Yuri Beletsky in October 2007". I thought I would mention what is presumably a typographical error in the explanation.

Pictured above from 2008 May APOD of the previous October.
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Beyond » Mon Dec 03, 2012 4:21 pm

Yes, APOD time travel can be quite confusing. :ssmile:
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Re: APOD: The Gegenschein Over Chile (2012 Dec 02)

Postby Anthony Barreiro » Mon Dec 03, 2012 8:27 pm

JohnD wrote:Apart from the gegenshein.......
That very deep, very wide starscene allows what appear to be 'lines' of stars.
One in particular runs from the telescope building in the middle of the pic, upwards and slightly to the left. There are many others, some straightish and some curlicued.
I'm aware that the more points on a graph, the easier it is to draw straight lines through them, so a figment of our Pareidolia.
Or is it to do with photography?


I believe you're looking at the brightest stars of the constellations Andromeda and Pegasus. It took me a while to get oriented to the picture, because everything in this southern hemisphere picture looks backwards compared to how I'm used to seeing it in the northern hemisphere. Other than Aries, Cetus, and (I think?) Eridanus, I don't recognize anything on the right hand / southern side of this picture. Ancient peoples connected the lines of bright stars in the sky to form pictures, so this pattern recognition predates photography.

By the way, pareidolia is a dangerous term to use around astronomers. Every evocative nebula name is the result of a pareidoliac reverie.

By the way, this is a beautiful picture. You can also see the Triangulum galaxy, M33.
May all beings be happy, peaceful, and free.


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