APOD: The Gegenschein Over Chile (2014 Jan 14)

Post a reply


This question is a means of preventing automated form submissions by spambots.
Smilies
:D :) :ssmile: :( :o :shock: :? 8-) :lol2: :x :P :oops: :cry: :evil: :roll: :wink: :!: :?: :idea: :arrow: :| :mrgreen:
View more smilies

BBCode is ON
[img] is ON
[url] is ON
Smilies are ON

Topic review
   

Expand view Topic review: APOD: The Gegenschein Over Chile (2014 Jan 14)

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

by DavidLeodis » Wed Jan 15, 2014 10:17 pm

Chris Peterson wrote:
DavidLeodis wrote:I would be grateful if someone could please confirm (or not) for me that the gegenschein is the faint bluish diagonal feature and not the brightish area directly above the mountains.
The gegenschein is the brighter spot fairly central in the blue band, which is zodiacal light.
Thanks Chris for that very helpful information which I would not have known, as I had not realised that the zodiacal light is also in the image. Doing a more detailed look at the image I now see the brighter spot area that will be the gegenschein.

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

by Chris Peterson » Wed Jan 15, 2014 9:35 pm

DavidLeodis wrote:I would be grateful if someone could please confirm (or not) for me that the gegenschein is the faint bluish diagonal feature and not the brightish area directly above the mountains.
The gegenschein is the brighter spot fairly central in the blue band, which is zodiacal light.

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

by DavidLeodis » Wed Jan 15, 2014 9:32 pm

I would be grateful if someone could please confirm (or not) for me that the gegenschein is the faint bluish diagonal feature and not the brightish area directly above the mountains. As it is said to be an "extremely dark sky", and assuming that it is not the gegenschein, then what is the glow above the mountains as surely the Sun would have been well down for it to be a very dark enough to take the photograph? Thanks for any help. :)

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

by Beyond » Wed Jan 15, 2014 2:15 pm

neufer wrote:
geckzilla wrote:
Nope, no words edited except to change Sirius to Rigel. Honest mistake, sir.
[marks the day on the calendar] Never forget...
Honest APOD mistakes are rarely schein.
Gads, you pulled that one out of a l-o-n-g sock. :yes:

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

by neufer » Wed Jan 15, 2014 1:54 pm

geckzilla wrote:
Nope, no words edited except to change Sirius to Rigel. Honest mistake, sir.
[marks the day on the calendar] Never forget...
Honest APOD mistakes are rarely schein.

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

by geckzilla » Wed Jan 15, 2014 4:03 am

Nope, no words edited except to change Sirius to Rigel. Honest mistake, sir.
[marks the day on the calendar]

Never forget...

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

by Chris Peterson » Wed Jan 15, 2014 12:17 am

Nitpicker wrote: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.
I don't disagree. When I made my original comment, I thought I was reading something different- something that explicitly called the phenomenon rare. But I made that post before going to bed, when I was tired. I don't know if the wording got changed, or if I simply read it wrong. Anyway, rarely seen is different from rare, and I have no issue with the caption.

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

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

AWESOME PICTURE!!!!!

And not just the Gegenshein....

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

:---[===] *

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

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.

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

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

Wow. Amazing picture. I want to go there.

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

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.


Ann
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.

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

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.

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

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

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

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

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.
- RJN

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

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.>>

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

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.

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

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?

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

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

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

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

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

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.

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

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.

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

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?

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

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.

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

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!!

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

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:

Top