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From certain positions in outer space the Earth will appear to have a blue ring around it when the viewer is in the shadow of the Earth and sunlight is passing through the atmosphere.

Here is a simulation from Google Earth:

image of the earth in shadow with blue ring around it

Sample sentence: The astronaut looked down at the blue ring formed by the sunlight passing through the atmosphere around the Earth.

Is there a word that describes this ring?

I had assumed the ring would be blue, based on this photograph from the Tumblr site Col. Chris Hadfield:

tumblr Col. Chris Hadfield

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    $\begingroup$ Google planet astronomy "atmospheric halo". $\endgroup$ – FumbleFingers May 22 at 14:43
  • $\begingroup$ @FumbleFingers Atmospheric halo refers to a phenomenon that is seen from the surface of the Earth not from outer space. en.wikipedia.org/wiki/Halo_(optical_phenomenon) $\endgroup$ – Bob516 May 22 at 15:02
  • $\begingroup$ Bob516: Yeah, I did actually know that. But the "astronomical" phenomenon is so recently relevant (on account of exoplanet atmospheric spectroscopy that's becoming technologically feasible) that English probably hasn't yet settled on how to refer to such things. But since you know about the existing "halo" usage already, I'm now inclined to agree with @Centaurus above. $\endgroup$ – FumbleFingers May 22 at 15:07
  • $\begingroup$ In SE Biology I was down voted on a different word request, "...it’s off-topic in Biology. It’s more appropriate on English Language & Usage." $\endgroup$ – Bob516 May 22 at 15:34
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    $\begingroup$ atmospheric limb. $\endgroup$ – stevesliva May 22 at 20:33
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There are actually a number of words / expressions related to this.

Aureole

[Bright edge of object - sometimes with colour of light source vs object] This is when an object is silhouetted against a bright light source and it gives a ring of colour. This typically occurs when the light source is smaller than the sillhoutte. I think this is the closest meaning because its meaning implies the colouration caused by diffusion (or even refraction - in this case) of light around the sillhoutte. For example, a girls head in front of a candle: the fine granularity of the hair diffuses the light, bending it and causes a bright circumference of light around the extremities of her hair. This word is associated in optical physics with a number of other diffusion phenomina too, such as the aureole effect.

Corona

[Colour of the light source, shape of light source] This is a ring of light when the sillhoutte is much smaller than the light source (or even no sillhoutte at all!), and there is practically no interaction between the light and the extremities of the sillhoutte, as a result most of the colour is from the star.

Noteable mentions:

A diamond ring

[Angle of light source to object] is where the eclipse has passed either side of total and there's a flare on one side.

Bailey's beads

[Shape of object to shape of light source] are when there is a total eclipse and there are flares all around the circumference, caused by imperfections in the planet's surface and / or the irregularity of the star's corona - entirely diffraction based.

Glory

[Entirely colour of diffraction / refraction of light source around object] This is an optical phenomina which is less common, and is almost entirely based on refraction, giving colour bands in the vicinity of the object, typically in concentric colour bands. A bit like a rainbow. This only occurs in the context of where the observer and object are in an atmosphere so is least applicable to this context - although technically the whole solar system is within the atmosphere (or heliosphere) of the sun, right up to the helioshock, where the heliosphere interacts with either other stars or forms a boundary with outter space, but it's not inconceivable that this could occur in a nebula, or even give rise to a

Brocken spectre

Incidently: the blue aura, in this case, is actually caused by a physics phenomina called Rayleigh Scattering which is where the atmosphere has the effect of appearing to change the wavelength of light as it passes through. Often this can be blue, however in some situations it might be red.

There is a whole family of words for these kinds of meanings, and a many of them are poetic. Some of the words relate to colour, some to shape, some to diffraction, some to refraction. This is one of those situations where knowing a little science could help you chose the right word.

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    $\begingroup$ Most of these words refer to very specific phenomena (e.g. Bailey's beads are seen during a solar eclipse), and are completely unrelated to the OP's question. $\endgroup$ – HDE 226868 May 23 at 1:53

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