On Earth, at some times and places, we can see many stars in the sky. When I watch rocket launch videos, or look at NASA imagery from the Apollo missions, I rarely see any stars. Why is this? The stars are obviously there, why can't they be seen in these circumstances?
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$\begingroup$ Here is a related topic. The Sun is dazzling. $\endgroup$– LocalFluffCommented Dec 6, 2015 at 23:38
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2 Answers
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Aperture. The aperture diameter is too small and frame rate too great to capture dim distant sources of light. It's really that simple. Consider how the rocket's engine, its ejecta and the illuminated Earth in the background would look if the camera's aperture was set to detect background stars. Any foreground lit object would cause frame overexposure and everything would appear white, possibly even cause permanent sensor damage if it caught a ray of light from the Sun.
You can try this at home with any point and shoot camera. Point it towards a full moon (or close to being full) during night, set it on auto-exposure, and then see if it also caught any stars appearing around the Moon. It won't. Even the Moon at night is too bright and capturing stars requires larger aperture, longer exposure and/or greater sensor sensitivity. You might succeed in capturing Venus (apparent magnitude of up to −4.89) when it's particularly bright next to a half or less lit Moon, but that's not a star. Brightest star on the night sky, as seen from Earth or indeed anywhere near our Solar system, if we exclude the Sun itself, is Sirius at mag. −1.46 (23.55 times dimmer than Venus at its brightest, and over 38,000 times dimmer than full moon at apparent magnitude of −12.92).
answered Dec 6, 2015 at 22:14
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$\begingroup$ This always makes me think to the mechanics and interpretive wetware behind the human eye -- and reminds me of looking at the sky at night wearing NVGs (a mind-blowing experience the first time you try it). $\endgroup$– zxq9Commented Dec 7, 2015 at 2:20
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$\begingroup$ "possibly even cause permanent sensor damage if it caught a ray of light from the Sun" Isn't that a pretty accurate description of what happened on Apollo 12? $\endgroup$– userCommented Dec 7, 2015 at 10:55
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It's all about apparent brightness.
Even the closest and brightest stars are so far away that very little light from them arrives in our celestial neighborhood. When you go out on a clear moonless night far away from any artificial light sources, you can see lots of stars because your eyes have adapted and they are the only source of light around.
Go out at night in a big city downtown with lots of light around you, even on a clear night and look up - you probably won't be able to see a single star because all are to faint compared to all the other light around you.
Apollo mission photography was all done in "daylight" i.e. everything was lit by the Sun. The amount of light falling on the subjects and/or lunar terrain is essentially equal to what is received at Earth's surface in daylight. To achieve correct exposure of the intended subjects, starlight will be far too faint to show up in any of the images. The cameras could have been adjusted to capture and record starlight, but the intended subjects would have been vastly overexposed, and all that excess light would have probably spilled over the entire image anyway (due to stray light reflecting/refracting around the various internals of the camera).
