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  • My guess is that it is in fact Venus. Were it the ISS, it would not be relatively stationary with respect to the sun. You didn't actually say that it was always in the same place when you saw it, but I infer from

    It shows up by dusk, before stars can be seen.

    that it is at least always above the horizon at about dusk. The thing is, Venus is (currently) pretty well above the horizon at just around dusk. The International Space Station would be moving, noticeably even to the naked eye. You would, for example, be able to notice the ISS's movement with respect to orion []. It would not, therefore, be visible just before dusk on at a given location from day to day. Coincidentally, as I type this, the ISS is right over the Iberian peninsula [].

    Also, I really really doubt the ISS is bright enough to be one of the first stars visible.

    Venus and Mercury are both fairly spectacular at the moment. They would certainly be easily visible just before dusk, following after the setting sun. Mercury is quite faint with respect to Venus, but it's still one of the first stars to come out as the sun sets. At the moment, Venus and Mercury are fairly close together, with Mercury a bit lower (closer to the sun as it sets). Mars, in case you're curious, should rise at around 12:30 or 1:00.

    There's this really cool site that'll give you a snapshot of the sky overhead, which is useful to help find where the planets are at any given time from any given location. You can, for example, set it up for Lisbon []. The same site has another cool feature called "Solar System Live []" that shows the relative positions of the planets in the solar system. Finally, there's this really cool program called XEphem [] that does both of those things (and more) on your local machine. I have it running in X on Tiger.

    • I don't know how fast it moves, but it definitely changes places.
      • Noticeably as you watch it, or just from day to day? Venus is a fast-moving planet: you'd notice a change in position from day to day, but not from second to second like the ISS.
        • I remember there was a night when I saw it moving really fast. Not that I could keep track of it moving, but each time I'd look at it it seemed to be in a different position. And eventually it was on top of the ocean, while before it was above Europe. That was one of the things that made me notice it.

          But usually it seems to be on the same place...

          I'm confused :-\ I don't think this makes any sense at all...

          Is the ISS *always* moving?

          I think I need to take a picture of whatever I'm seeing...
          • Yeah, always. The ISS has to remain moving at the same speed because of the orbit it's in.

            As for Venus, since it's always near the horizon (since it's always near the sun, whether in the evening or morning), the change in its apparent position due to the rotation of the earth could well be noticeable. But if it's "moving" at the same speed as the sun, it's not moving at all.