APOD: The Gegenschein Over Chile (2014 Jan 14)

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APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by APOD Robot » Tue Jan 14, 2014 5:05 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 last year is one of the more spectacular pictures of the gegenschein yet taken. Here a deep exposure of an extremely dark sky over Las Campanas Observatory in Chile shows the gegenschein so clearly that even a surrounding glow is visible. Notable background objects include the Andromeda galaxy, the Pleiades star cluster, the California Nebula, the belt of Orion just below the Orion Nebula and inside Barnard's Loop, and bright stars Rigel and Betelgeuse. 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 (2014 Jan 14)

Post by Ann » Tue Jan 14, 2014 5:11 am

What a lovely picture! :D :D :D

Note the big round emission nebula between Betelgeuse and Bellatrix at about three o'clock. That's the Lambda Orionis nebula.


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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Nitpicker » Tue Jan 14, 2014 5:21 am

Beautiful. I've just learnt a new word, too: gegenschein. But I think the notable bright stars in the background are Rigel and Betelgeuse.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Chris Peterson » Tue Jan 14, 2014 5:34 am

At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Nitpicker » Tue Jan 14, 2014 5:50 am

Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.
How rare is rare? A dark site is rare to most people. And several times a year at a dark site doesn't exactly sound common. The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by geckzilla » Tue Jan 14, 2014 6:08 am

Chris is a meteor guy so you have to imagine his concept of rare involves a space rock landing in his living room.
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by neufer » Tue Jan 14, 2014 6:58 am

Nitpicker wrote:
The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.
  • The L2 Lagrangian point is not dynamically stable so it is not clear why dust would necessarily collect there.
http://history.nasa.gov/SP-349/ch6.htm wrote: <<Pioneer 10 produced new information about the Zodiacal Light, the faint band of light along the Zodiac, believed to be an effect of sunlight reflected from particles in interplanetary space. The slight enhancement of the glow exactly opposite to the Sun in the sky - the Gegenschein - could be caused by distant particles illuminated like miniature full moons opposite the Sun, or by a stream of particles extending as a comet's tail from Earth.

The imaging photopolarimeter was turned on March 10, 1972, seven days after Pioneer 10 was launched. During the first few weeks of the mission, when the Sun angle was about 26 degrees from the spacecraft's spin axis, only that part of the sky more than 60 degrees from the Sun line could be inspected by the imaging photopolarimeter. So observations concentrated on the Gegenschein. It quickly became apparent that the Gegenschein could not be associated with Earth because, although the spacecraft had not moved much farther from the Sun, it had moved ahead along the orbit of the Earth. While the direction of the Gegenschein was directly away from the Sun as seen from the spacecraft, this direction was by this time different from the direction of the Gegenschein from Earth. So the anti-solar glow was confirmed as being associated with the light reflected from particles spread around the Solar System, not from particles associated with the Earth itself. Experimenters later measured the faint glow of the Gegenschein to near the orbit of Mars, again confirming its interplanetary nature.

As Pioneer 10 moved away from the Sun, the brightness of the Gegenschein also decreased, thereby showing that it results from particles in the inner Solar System. There was, however, a decrease in the rate at which the brightness faded within the asteroid belt, which indicates that the particles responsible for the counterglow increase somewhat within the belt. But beyond the belt there is virtually no Gegenschein.

As Pioneer moved out from Earth, it became possible to start mapping the whole of the sky to look at the Zodiacal Light. Scientists found that the Zodiacal Light also decreases in brightness as the square of the distance from the Sun. The rate of decrease slowed within the asteroid belt, indicating that particles responsible for the Zodiacal Light, although concentrated in the inner Solar System, also increase somewhat within the asteroid belt itself. But beyond three and a half times Earth's distance from the Sun, the Zodiacal Light is negligible, and experimenters were able to record the integrated starlight from the Galaxy tree of the Zodiacal Light for the first time. Since the Zodiacal Light's brightness, at 2.41 times Earth's distance from the Sun, is less than one tenth that at Earth's orbit, experimenters conclude that the asteroid belt beyond this distance does not contribute significantly to the Zodiacal Light as seen from Earth. Zodiacal Light brightness fades almost completely at a distance of 3.3 times Earth's distance from the Sun. This is where particles would have to circle the Sun in orbits having a period of half that of Jupiter. Jupiter's gravity appears to sweep the Solar System clear of such particles beyond this resonance orbit. Thus, there is virtually no Zodiacal Light nor Gegenschein beyond the asteroid belt.>>
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Nitpicker » Tue Jan 14, 2014 7:28 am

neufer wrote:
Nitpicker wrote:
The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.
  • The L2 Lagrangian point is not dynamically stable so it is not clear why dust would necessarily collect there.
Thanks neufer. I just read a little further in the Wikipedia article -- http://en.wikipedia.org/wiki/Gegenschein#Explanation -- and the L2 idea was cited from a 1962 article in (an old) New Scientist magazine. Perhaps a tad dated. :ssmile:

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by ufox » Tue Jan 14, 2014 7:34 am

sorry, but I don't see Sirius there...remember - this is the southern hemisphere! Sirius should be above and right of Orion and not below and left as seen from the northern hemisphere. Nevertheless a great pricture!!

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Pedr Fawr » Tue Jan 14, 2014 8:32 am

I agree that Sirius is off the picture at upper right. The bright orange star off to the left of (the inverted) Orion is Aldebaran.

We gather that the gegenschein is like a cosmic rainbow. The apparent visual position depends on the position in space of the observer.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by astabada » Tue Jan 14, 2014 10:02 am

Hi, where is Syrius exactly in the picture?

I can see the Pleiades, the Taurus, Orion - but shouldn't Syrius be on the right of Orion - hence outside the picture?

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Fernangarc54 » Tue Jan 14, 2014 2:17 pm

I agree with Nitpicker, the companion of Betelgeuse in the picture should be Rigel and "the red eye of the bull", Aldebaran, is close to the Pleiades. Great picture.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Fernangarc54 » Tue Jan 14, 2014 2:20 pm

I pose an open question: Can we assume that Gegenschein is a visual reflection of what would have been our modest planetary ring?
Thank you.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by tdurow3 » Tue Jan 14, 2014 2:32 pm

I took a picture of the glory mentioned in today's APOD back in 2011. This was on my way back from Sydney Australia to Chicago. Image
Also can be found at http://imgur.com/wWBXr2U

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by SteveScout502 » Tue Jan 14, 2014 3:10 pm

Thanks for the info about Sirius being missing. I figured out that this is a southern view, so it's "upside down" to me. I was a bit bewildered when I thought I had identified the easy parts of Orion- belt, Betelgeouse, Rigel, and I thought I identified the Pleides, but I was looking for Sirius to be the brightest star in the sky...
I think I'm good now.

BTW, nice picture

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Chris Peterson » Tue Jan 14, 2014 3:13 pm

Nitpicker wrote:
Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.
How rare is rare? A dark site is rare to most people. And several times a year at a dark site doesn't exactly sound common. The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.
Well, rare in this case is not in the slightest. The gegenschein is always there. So it's just a question of being in a dark site with good weather, not having a Moon, and looking. The current wording is much better- it is seldom discerned because most people don't have good observing conditions.

An observation made several times a year, for most astronomical phenomena, is not rare. I only see a full Moon a few times a year. Is a full Moon rare? I only see Jupiter at opposition once a year. Is that a rare event?
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Chris Peterson » Tue Jan 14, 2014 3:15 pm

Fernangarc54 wrote:I pose an open question: Can we assume that Gegenschein is a visual reflection of what would have been our modest planetary ring?
No. The gegenschein is backscatter from astroidal dust in orbit around the Sun, not in orbit around the Earth.
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by neufer » Tue Jan 14, 2014 3:36 pm

tdurow3 wrote:
I took a picture of the glory mentioned in today's APOD back in 2011.
The glory is due to coherent backscatter from spherical droplets.

The opposition effect of gegenschein is most likely due to shadow-hiding by millimeter sized dust:
http://en.wikipedia.org/wiki/Opposition_surge wrote: <<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted when at astronomical opposition.

Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.

The phase angle is defined as the angle between the observer, the observed object and the source of light. In the case of the solar system, the light source is the Sun, and the observer is situated on Earth. As the phase angle of an object lit by the sun decreases, the object's brightness rapidly increases. This is mainly due to the increased area lit, but is also partly due to the intrinsic brightness of the part that is sunlit. This is affected by such factors as the angle at which light reflected from the object is observed. For this reason, a full moon is more than twice as bright as the moon at first or third quarter, even though the visible area illuminated appears to be exactly twice as large.

When the angle of reflection is close to the angle at which the light's rays hit the surface (that is, when the sun and the object are close to opposition from the viewpoint of the observer), this intrinsic brightness is usually close to its maximum. At a phase angle of zero degrees, these shadow areas become negligible. The celestial body in effect becomes an imperfect mirror. When phase angles approach zero, there is a sudden increase in apparent brightness, and this sudden increase is referred to as the opposition surge.

The effect is particularly pronounced on regolith surfaces of airless bodies in the solar system. The usual major cause of the effect is that a surface's small pores and pits that would otherwise be in shadow at other incidence angles become lit up when the observer is almost in the same line as the source of illumination. The effect is usually only visible for a very small range of phase angles near zero. For bodies whose reflectance properties have been quantitatively studied, details of the opposition effect — its strength and angular extent — are described by two of the Hapke parameters. In the case of planetary rings (such as Saturn's), an opposition surge is due to the covering of shadows on the ring particles. This explanation was first proposed by Hugo von Seeliger in 1887.

An alternate theory for the increase in brightness during opposition is that of coherent backscatter. In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light. Coherent backscatter phenomena have also been observed with radar. In particular, recent observations of Titan at 2.2 cm with Cassini have shown that a strong coherent backscatter effect is required to explain the high albedos at radar wavelengths.

The existence of the opposition surge was first recorded in 1956 by Tom Gehrels during his study of the reflected light from an asteroid. Gehrels' later studies showed that the same effect could be shown in the moon's brightness. He coined the term "opposition effect" for the phenomenon, but the more intuitive "opposition surge" is now more widely used. Since Gehrels' early studies, an opposition surge has been noted for most airless solar system bodies. No such surge has been reported for gas giant, nor for bodies with pronounced atmospheres.>>
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by RJN » Tue Jan 14, 2014 5:11 pm

Nitpicker wrote:Beautiful. I've just learnt a new word, too: gegenschein. But I think the notable bright stars in the background are Rigel and Betelgeuse.
Yes, I have now changed the text to indicate that Rigel is actually one of the bright stars, not Sirius. My bad.
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by astromandan » Tue Jan 14, 2014 5:31 pm

If we're sticking with German, I believe the word for "the glory" is "heiligenschein"

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Chris Peterson » Tue Jan 14, 2014 5:43 pm

astromandan wrote:If we're sticking with German, I believe the word for "the glory" is "heiligenschein"
Although the heiligenschein and the glory look very similar, they are different things, caused by different mechanisms. Heiligenschein is a scattering phenomenon, while glories are a complex refractive phenomenon. You can often distinguish them visually because heiligenschein is the same color as the source (typically white), while glories show dispersion- a radial rainbow effect.
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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by starsurfer » Tue Jan 14, 2014 6:42 pm

Ann wrote:What a lovely picture! :D :D :D

Note the big round emission nebula between Betelgeuse and Bellatrix at about three o'clock. That's the Lambda Orionis nebula.


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Ann I can't help but think we're both on the same wavelength, that was the first thing I noticed as well! I prefer to call it Sh2-264.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Anthony Barreiro » Tue Jan 14, 2014 7:31 pm

Wow. Amazing picture. I want to go there.
May all beings be happy, peaceful, and free.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Nitpicker » Tue Jan 14, 2014 10:16 pm

Chris Peterson wrote:
Nitpicker wrote:
Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.
How rare is rare? A dark site is rare to most people. And several times a year at a dark site doesn't exactly sound common. The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.
Well, rare in this case is not in the slightest. The gegenschein is always there. So it's just a question of being in a dark site with good weather, not having a Moon, and looking. The current wording is much better- it is seldom discerned because most people don't have good observing conditions.

An observation made several times a year, for most astronomical phenomena, is not rare. I only see a full Moon a few times a year. Is a full Moon rare? I only see Jupiter at opposition once a year. Is that a rare event?
:ssmile: Okay, whilst we're down here plumbing the depths of pedantry ...

In defence of the original sentence in the explanation:
In fact, a rarely discernable faint glow known as the gegenschein ... can be seen 180 degrees around from the Sun in an extremely dark sky.
I read "rarely" and considered that only a minority of people live under dark skies. If the majority can never see the gegenschein, when looking outside from their neighbourhood, then the use of "rarely" is justified, in my humble opinion. But of course, rarity is a subjective word. Perhaps the rarity of gegenschein is magnified as it is a new word to me.

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Re: APOD: The Gegenschein Over Chile (2014 Jan 14)

Post by Boomer12k » Tue Jan 14, 2014 11:31 pm

AWESOME PICTURE!!!!!

And not just the Gegenshein....

That whole Orion Complex...et al.... OUTSTANDING!!!

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