RE: I saw ISS!

From: John Franke via Seesat-l <>
Date: Tue, 31 Mar 2020 12:27:56 -0400 (EDT)
As a satellite approaches the shadow of the Sun, the sunlight is taking a longer path through the Earth's atmosphere and more blue light is scattered than red light, the result is redder sunlight, much the same as for sunsets. When the shadow edge is far below the satellite, the light does not pass through any of the Earths atmosphere and hence the sunlight is whiter. 


> On March 31, 2020 at 11:02 AM Daniel Hampf via Seesat-l <> wrote:
> Hello Cees and everyone,
> I found this really interesting, so I pointed my tracking telescope at the ISS on a pass last Tuesday. I could indeed follow it for more than a minute into the Earth shadow, then I lost it due to poor tracking. I hope to do this again soon.
> I recorded the brightness and colour index of the ISS during the pass and uploaded it here:
> As expected, the brightness decreases by several orders of magnitude when it goes into the shadow. Interestingly, the colour index jumps from blue-ish to red-ish after it's reached the shadow. However, from a single pass it's hard to say if that is actually an effect of the shadow a rather of another viewing angle onto the structure.
> I'm still quite puzzled as to where the light is coming from. Cannot quite believe it's the city lights, but I don't have a much better idea either. Will be interesting to hear what other people think.
> Thanks for this inspiration!
> Best,
> Daniel
> -----Ursprüngliche Nachricht-----
> Von: Seesat-l [] Im Auftrag von C. Bassa via Seesat-l
> Gesendet: Dienstag, 24. März 2020 00:27
> An: Seesat-L
> Betreff: I saw ISS!
> This subject used to be hot on SeeSat-L back in 1998, but this evening
> I saw ISS where I did not expect to see it -- while ISS was in the
> shadow of the Earth!
> I only noticed that my camera had captured ISS while processing the
> results. I was very puzzled why ISS was not much much brighter. Only
> after checking the time of the observation (21:15UTC), did I realize
> ISS was already in the Earth's shadow and no longer illuminated by the
> Sun.
> At 21:15UTC ISS was only about a minute after shadow entry, so some
> stray sunlight may still have been illuminating ISS. Hence, I set up
> the camera again for the next pass, that was predicted around
> 22:53UTC. Passing close to Procyon, ISS was seen again. Shadow entry
> on that pass was at 22:48UTC, so ISS was deep into the shadow. I
> re-positioned the camera towards Spica to catch it later in the pass.
> While not obvious, ISS was detectable after correcting for the
> expected motion and averaging the moved frames.
> I've uploaded my images to this link:
> This clearly highlights the sensitivity of my setup, but also that ISS
> is big enough to reflect what must be lights from bright cities while
> passing over Europe.
> An unexpected first!
>     Cees
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Received on Tue Mar 31 2020 - 11:28:49 UTC

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