----- Original Message ----- From: "Robin R. Wier" <rwier@concentric.net> To: <Frank@reednavigation.com>; <seesat-l@satobs.org> Sent: Monday, May 02, 2011 12:05 AM Subject: Re: Why is ISS still visible in Earth's shadow? > ----- Original Message ----- > From: <Frank@reednavigation.com> > To: <seesat-l@satobs.org> > 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: >> http://spaceflight.nasa.gov/gallery/images/station/crew-25/html/iss025e009840.html >> >> 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 >> http://mailman.satobs.org/mailman/listinfo/seesat-l > > > 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 > http://mailman.satobs.org/mailman/listinfo/seesat-l 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 http://mailman.satobs.org/mailman/listinfo/seesat-l
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