Originally posted by GaryN
In all the years of EVAs from the shuttle or the ISS, has anyone ever heard an astronaut talk about seeing the stars?
Yes, live during a spacewalk. I'll get to the details in a minute.
First, we'll have to confront the tragic truth:
Stars, when viewed in space, are not much brighter than when they are viewed from Earth.
Intuition tells us they should be, because above the atmosphere there's nothing to absorb or scatter the light. As is so often the case, intuition is
for the simple reason that our atmosphere really doesn't block much light.
You don't have to take my word for it, or NASA's or some guy on TV. You can verify it for yourself with no special equipment other than your own two
eyes. Actually, you can use just one eye. Oh, and you don't have to be launched into space, either (I know - Dang!). Heck, you can even do it in
When you look straight-up, you are looking through 'one thickness' of the Earth's atmosphere.
When you look up at a slant, you are looking through more
than 'one thickness' of the Earth's atmosphere.
Obviously, it follows that the closer you look to the horizon, the more atmosphere you are looking through.
With a little trig, you can calculate that roughly 30 degrees above the horizon (~1/3rd of the way from the horizon to the zenith), you are looking
through two-times the vertical thickness of the atmosphere. At ~20 degrees above the horizon, you are looking through three-times the vertical
thickness of the atmosphere. At ~15 degrees, it's four-times.
Here is the important thing: The difference between looking through two atmospheric thicknesses and only looking through one thickness (or the
difference between two & three thicknesses, or three & four) is the same as the difference between looking through one thickness (straight-up)
and looking through no air at all (i.e. being in space).
Thus, the change in brightness of an object (the moon, the sun, another star) as it rises & sets tells us how much light the atmosphere absorbs, and
therefore how much brighter the stars.
Suppose we say, "The stars in space must be 10 times brighter than here on Earth!"
That would mean that only 10% of the light reaches the Earth's surface through one thickness of the atmosphere, and each additional thickness we look
through would reduce the light by another 10 times.
This is testable using our eyes: If one thickness of atmosphere reduces the light by 10 times, then the sun, moon & stars will be only 1/10th as
bright when they are 30 degrees up as compared to when their straight overhead. When they sink to 20 degrees, they will only be 1/100th as bright.
At 15 degrees, only 1/1000th as bright. Clearly this is not the case.
Suppose we say, "Maybe the stars are twice as bright in space, compared to here on Earth."
In that case, the sun, moon & stars will be half as bright when they are 30 degrees up as compared to when they're straight overhead, 1/4th at 20
degrees, and 1/8th at 15 degrees. This is closer to reality, but clearly the absorption is significantly less than 50%.
That's just using your eyes. If you like, I can tell you how to test it further with cameras (video & still).
The real stumbling block for astronauts seeing stars is the same one for the rest of us poor schmucks here on Earth: How well dark adapted are their
When I walk my dog at night, I'm stepping out of a well-lit room and don't see many stars. The longer I stay out the better-adapted my eyes get and
the more I can see - but only to a point, because even if I'm not looking up at streetlights, I'm looking down onto a sidewalk that's lit
streetlights. If I want to get truly dark-adapted then I have to go to nearby park and not look at any lights for several minutes. Then
sky turns beautiful.
This brings me to the answer to your question. Normally, astronauts work in well-lit cabins or, if they're on EVA then they are in the sun or using
work-lights, both of which totally kill their dark-vision. Normally they don't have time to look away from their work for the many minutes it takes
to dark-adapt. An inadvertent exception happened on STS-61
; the first Hubble repair mission. On
they had to replace a bad solar panel. To jettison it, Kathryn Thorton stood out
at the end of the robot arm with her back to the lighted Shuttle bay, and held the 5m panel with the dark solar cells facing her. This was on the
Earth's night side. They waited 12 minutes for their orbit to bring them around to the day-side so the pilot could see the panel to maneuver away
from it. When Thorton let go of the panel and the Shuttle backed away, her dark-adapted eyes suddenly had the whole universe before them. Surprised,
she exclaimed loudly at the beauty of the stars.
edit on 10-7-2013 by Saint Exupery because: Spelling