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I came across this image taken on the "Balcony" at over 27,000 ft (8,200 m) elevation, beneath the summit of Mount Everest. So many stars can be seen on it even though the Sun has risen. Do Everest climbers really see stars after sunrise or is the image a long-exposure one or something?

Glorfindel
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  • The more I look the more I'm certain that's a composite image. The shadows don't point away from the sun, and I don't think the fisheye lens is enough to explain that (looking at the most central shadows especially. And the host page is an advertising company after all. It's linked from a page about an image capture/augmented reality project, which certainly means some processing and implies some combination of images – Chris H Nov 10 '21 at 13:23
  • Related video. The last scene matches some of this image, but with a tighter crop and a different camera angle, though the preview if you hover over the progress bar looks like a closer match (the preview appears to be of a 3d version that I can't enable here) – Chris H Nov 10 '21 at 13:31
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    @ChrisH Oh it's just snow. blush –  Nov 10 '21 at 14:06
  • It's not implausible that you'd see planets near sunrise (even at sea level, possibly even a few stars at such high altitude) and the shadows are long enough that the sun is low in the sky. Without the distortion we could work out the sun elevation. Wikipedia has examples that might be of interest. Note that astronomical magnitude is one of the strangest scales you'll ever meet: it's logarithmic, with negative being brighter, and an arbitrary zero point (Vega). – Chris H Nov 10 '21 at 14:33
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    @ChrisH One can reportedly see Venus in daytime, if you know where to look and have eagle eyes. But I meant seeing stars easily, with likelihood of spotting one randomly without searching the sky. Just like after sunset or before sunrise. –  Nov 10 '21 at 15:19
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    I've certainly seen Venus with the sun low in the sky, but haven't looked in broad daylight – Chris H Nov 10 '21 at 17:00
  • Despite the fact you have identified the dots on that image being snow, I think the general question is okay and may be on topic on Astronomy so will migrate it. – Rory Alsop Nov 11 '21 at 16:00
  • As I mentioned here, when Venus is near its maximum brightness it's certainly possible to see it with the naked eye at any hour of the day, if you know where to look (assuming the sky is sufficiently clear). – PM 2Ring Nov 13 '21 at 00:51
  • German Wikipedia has a page on daylight star observations, which states that with a 4“ instrument mag 1-2 stars can be found easily (and regardless of observer height): https://de.wikipedia.org/wiki/Tagbeobachtung (use your favourite translator). I‘ve looked for weather ballon footage or high altitude ballon observations (as done in the „early“ days, when cosmic rays were discovered), but didn‘t find anything. Maybe your googlefu is better. – Grimaldi Nov 13 '21 at 12:57
  • For ISS I believe you can see stars, even if it is in daylight (as long as you‘re not looking into the direction of the sun). This leads to the existence of a height where it is possible (for the atmosphere changing continuously) – Grimaldi Nov 13 '21 at 13:01

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The reason that you can't see stars in the daytime is that the atmosphere scatters sunlight and effectively blots them out. But even where I live, a dark sky area, the brightest stars are visible during twilight. Not just the planets, Sirius for example becomes visible before the sky is dark.

With the sun on or near the horizon, the effect of scattering is reduced.

At high altitudes the atmosphere is much thinner. The scattering is therefore similarly reduced and yes, you can sometimes see the brightest stars in daytime. This would not necessarily be the case every day and at all hours of the day.

Although I haven't been to Everest myself, friends who have report that some stars can be seen on clear days. I haven't interrogated them as to whether it was near sunrise or sunset.

(But your picture does look like a composite)

Chenmunka
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  • My picture actually shows snow as ChrisH found out. Why do you consider Wiltshire a dark sky area? The highest point of the UK is the Ben Nevis at just over 4,400 ft. The daytime sky becomes considerably darker above ~25,000 ft I guess. When in an airliner at cruise altitude the sky above me is much darker up there, but I think I've never seen stars out of a plane during day. Once I have seen a star at twilight out of a plane, when the Sun might still have been above the horizon, but it's a long time so I don't remember exactly. I've seen possible stars on a picture of a view out of Concorde. –  Nov 10 '21 at 16:14
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    From https://nightblight.cpre.org.uk/maps/ there are some pretty dark region of Wiltshire (e.g. much of Salisbury Plain). But when looking low in the sky that's a bit misleading as I found the other riding round the edge of the Cotswolds Gloucestershire when the Northern Lights came much further S than normal: I was in the 2nd darkest band, maybe even the darkest, but the light pollution from Gloucester, Stroud etc. was scattered by a few low clouds. Car lights didn't help either. Even straight up, I couldn't see as many stars as in the Scottish Highlands, but far more than in a city – Chris H Nov 10 '21 at 16:46
  • So you're talking about light pollution, but during day we can't see stars because of the brightness of the closest hydrogen-burning star, the Sun, and therefore the lit blue atmosphere. –  Nov 10 '21 at 17:43
  • @Giovanni that's the only way I can interpret "dark sky area", but this still helps when the sun is low – Chris H Nov 11 '21 at 09:30