Re: Why is ISS still visible in Earth's shadow?

From: Robin R. Wier (
Date: Mon May 02 2011 - 08:07:35 UTC

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    ----- Original Message ----- 
    From: "Robin R. Wier" <>
    To: <>; <>
    Sent: Monday, May 02, 2011 12:05 AM
    Subject: Re: Why is ISS still visible in Earth's shadow?
    > ----- Original Message ----- 
    > From: <>
    > To: <>
    > Sent: Sunday, May 01, 2011 10:48 PM
    > Subject: Re: Why is ISS still visible in Earth's shadow?
    >> I've been thinking about this for some time. Just how bright is a city as
    >> seen from LEO? It's not an easy question since we're dealing with a light
    >> source that has a very large angular size. Then again, cities at night 
    >> are
    >> popular photography targets for the ISS astronauts while photos of stars
    >> and constellations are almost non-existent. So maybe there are clues
    >> there...
    >> Here's a link to an image of Italy and parts of the Balkans at night 
    >> taken
    >> from the ISS on October 28, 2010:
    >> It was popular in the mainstream media, and you may have seen it before.
    >> Some things to notice:
    >> 1) There are stars visible above the horizon towards the northwest 
    >> (center
    >> of the image just above the horizon). You can clearly see Vega with the
    >> little equilateral triangle and attached parallelogram that make the
    >> recognizable pattern of the constellation Lyra. This provides a nice
    >> angular scale. Closer to the horizon further west, you can see Altair 
    >> with
    >> the distinctive companion stars on either side of it. The images of the
    >> stars are slightly blurred since this was a time exposure of a few
    >> seconds, probably handheld. Stars down to magnitude 3 or a little fainter
    >> are clearly visible.
    >> 2) There are shadows and some illumination on the space station with the
    >> source of light behind the astronaut to the right. Given the positions of
    >> Vega and Altair and the known date, the source of illumination is clearly
    >> the Moon which was about two-thirds full. There is no similar 
    >> illumination
    >> from the cities below. Then again, the developed area in frame, Naples,
    >> Rome, and the Po valley, are well to the north, with the latter close to
    >> the horizon.
    >> 3) The cities in view are bright compared to the stars. Naples was
    >> probably about as bright per square arcminute as Vega, but much larger in
    >> angular size. I would estimate a total apparent magnitude for the greater
    >> Naples area of perhaps -6. Since it's well out towards the horizon at a
    >> range of around 600 miles, it would probably be about two magnitudes
    >> brighter directly beneath the space station, so around -8.
    >> Naples is a small city in terms of the actual area that is developed. A
    >> bigger city with more continuous lighting from horizon to horizon, like
    >> the greater New York area, could be a couple of magnitudes brighter. You
    >> would have a light source with a magnitude of perhaps -10 but 30 or more
    >> degrees wide. So the net illumination here could begin to approach the
    >> total brightness of the Moon, though since the source is diffuse there
    >> would be no sharp shadows like in the linked photo. Are there any
    >> astronaut reports of a "glow" on the station while passing over big
    >> cities? Would they even notice? Given that city lights would always be
    >> illuminating the side of the space station facing an observer on the
    >> ground, it does seem to me that a large city could make the ISS faintly
    >> visible (magnitude 9? 10?) for an observer with a telescope and good
    >> tracking. One would need a dark sky site near a very big city. Hard to
    >> find, but such places do exist.
    >> Please correct me if I've gotten my math wrong, or if you have a better
    >> way to estimate the total brightness of a city.
    >> Just to be clear, I'm not disputing the fact that the primary 
    >> illumination
    >> after entering the shadow is from "twilight" around the Earth's limb. 
    >> It's
    >> the same source of illumination for the Moon in the middle of a total
    >> lunar eclipse. I'm just wondering if there's enough illumination from
    >> cities when the space station is in deep eclipse to make it faintly
    >> visible from the ground.
    >> -FER
    >> _______________________________________________
    >> Seesat-l mailing list
    > Thanks Frank,
    > Yes, all observations that resulted in my response were from using an 8"
    > LX200 with good tracking at high magnification from well within the 
    > Phoenix,
    > AZ, USA light dome. The affect was much more pronounced with the shuttle, 
    > if
    > it was "turned over" with the "white" sound down.
    > I would also note that sometimes the shuttle would pass over, top down, 
    > with
    > the bay open, and some sort of lights were on, lighting up the bay area.
    > I see the same affect with large airliners at altitude while tracking them
    > manually. The airliners are moving slow enough (relatively) to see the
    > pinpoint reflections of light flashing by on their wings.
    > Robin
    > _______________________________________________
    > Seesat-l mailing list
    Maybe pinpoints was a bad word choice, more like soft flares, varying in 
    intensity, yellow, to orange, all dull colors, not bright. 
    Seesat-l mailing list

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