If the photosphere was blocked, before the sun rises on an airless moon in the outer solar system, would either the inner or outer corona be visible to the naked eye?
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$\begingroup$ I think you are describing a total solar eclipse; or a coronagraph. britannica.com/science/corona-Sun $\endgroup$– Daddy KropotkinCommented Jan 5, 2019 at 2:59
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$\begingroup$ If there was the equivalent to a total solar eclipse much further out in the solar system, then yes I am. $\endgroup$– Bob516Commented Jan 5, 2019 at 4:03
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$\begingroup$ There could be solar eclipses anywhere in the solar system if there's an object in space of the right size, meaning it has to appear exactly the same size as the sun in the sky. $\endgroup$– PicachieuCommented Jan 5, 2019 at 4:04
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$\begingroup$ Related information astronomy.stackexchange.com/questions/28139/… $\endgroup$– ProfRobCommented Jan 5, 2019 at 7:48
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$\begingroup$ I understand that the OP asks if without atmosphere one can see the rising of the corona before the rising of the disk. Discriminate between the two is not possible here on heart as for refraction and diffusion propagate the light from the photosphere before the latter emerges from the horizon. I would say yes because this is similar to what is happening during a total eclipse. Contrast should be even better. Perhaps I should make this an answer. $\endgroup$– AlchimistaCommented Jan 5, 2019 at 9:18
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2 Answers
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If the sun is ever perfectly eclipsed as it is on Earth, then yes, the corona would be visible. The only reason we can't usually see the corona is because it is way dimmer than the rest of the sun. If the entire sun other than the corona is ever covered fully by another object, then the corona will be visible.
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$\begingroup$ How far out in the solar system would the corona still be visible in such a circumstance? $\endgroup$– Bob516Commented Jan 5, 2019 at 4:01
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1$\begingroup$ @Bob516 about as far as you would be able to see the sun. Maybe a little less because the corona's much dimmer. $\endgroup$ Commented Jan 5, 2019 at 4:03
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1$\begingroup$ I do not understand how the corona could be seen about as far as you would be able to see the sun. Alpha Centauri A is in the same class as the Sun and is the fourth brightest star in the night sky, solstation.com/stars/alp-cent3.htm#ac-a, but we cannot see the corona of that star. $\endgroup$– Bob516Commented Jan 5, 2019 at 4:15
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1$\begingroup$ Alpha Centauri A is also 1.5 times more luminous than the Sun. It might be visible with a telescope, but probably not with the naked eye (which is what you ask for) $\endgroup$ Commented Jan 5, 2019 at 13:21
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When the sun is about to rise for a given location is disk or photosphere is totally covered by the planet. The situation is similar to that of a total eclipse but we can't see the corona rising first because of two and at least overlapping reasons :
the corona image undergo multiple diffusion and reflection and is not bright enough to contrast with the skydome which is already illuminated by the same;
when the sun further approach a "geometrical sunrise" refraction makes it already visible so its light is overwhelming the corona anyway
https://en.m.wikipedia.org/wiki/Atmospheric_refraction
The corona is somehow hidden in the twilight.
If we assume the star not too far or its corona bright enough then on a planet without atmosphere the latter can be indeed seen rising. The conditions for its observation would be even better that those on earth during a total eclipse as for the skydome in this terrestrial case still receive light from the penumbra and illuminated nearby regions.
edit after I had a look at this pic that I have taken just yesterday and I realised it is related to the topic here. It makes me wonder about the nature of the observation without altering the answer above when a planet without atmosphere is concerned.
The bright spot was deep red to the naked eyes. And it was there minutes before the photosphere made its appearance! Its red tint is well explained as usual by diffusion but its brightness confuses me a bit.
Might it be indeed a corona picture superimposed to the twilight of the morning? Or is just an ill-defined image of the disk originating as in the scheme above but with an even lower (as compared to horizon) Sun? Probably this is the explanation but I wonder to observe always something new. It is not the first time that I see a sunrise :) but I never observed a sun-like bright spot lasting so long before the actual photosphere rising.
