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Last night I saw a bright light moving across the sky. It was probably as bright (or even brighter) than some of the brightest visible stars like Vega and moving quite slowly (so no shooting star) at roughly the speed satellites appear to move. I was quite surprised to see a satellite that bright but I was pretty certain that it couldn't be the ISS because of the position and direction in the sky.

However, from the moment that I spotted it it continuously dimmed until it wasn't anymore visible to the naked eye - maybe 5-10 seconds and 30° of movement later.

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    $\begingroup$ Aliens, probably $\endgroup$
    – Valorum
    Aug 11 at 17:42
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    $\begingroup$ @Valorum, we do not acknowledge the existence of aliens until after the uprising. Quit skipping the meetings. $\endgroup$ Aug 12 at 4:11

2 Answers 2

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From your description, this was most likely a Satellite Flare. This is the Sun reflecting off a highly reflective part of the satellite. The most famous type was the flares from the Iridium Satellites, but they have all been replaced with satellites that no longer flare. Flares still happen though, just not at the predictable level the Iridium satellites did.

I do not believe this was just a normal satellite pass, with it entering the Earth's shadow for two reasons:

It was brighter than all other stars: Under otherwise normal circumstances, a satellite reflects light in pretty much all directions, and are generally fairly dim. Only during a flare do you get a nearly mirror-like reflection of the sun.

It dimmed continuously over 30 degrees: A flare will brighten and dim as more/less of the Sun's image is reflected towards you. When a satellite enters the Earth's shadow, it generally stays the same brightness, and quickly fades to nothing.

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  • $\begingroup$ Thanks! Could a satellite flare also be visible during the day? Before yesterday's sunset I also saw a bright spot of light about 20-30° above the horizon. Although the background sky was still pretty bright with no stars visble at all, I spotted a bright point source of light appearing about as bright as a 0 to -1 mag star would appear at night. It seemed to move towards the horizon (quite slowly because of the proximity to the horizon) and it was dimming. However, I couldn't see if and when it wasn't anymore visible because it disappeared behind trees. $\endgroup$
    – 299792458
    Aug 12 at 11:50
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    $\begingroup$ Is there something like a "plane flare"? I highly doubt satellite flares can be that bright! $\endgroup$
    – 299792458
    Aug 12 at 11:52
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    $\begingroup$ Yes, I have been fooled by a "plane flare" more than once. But there's a very short period when they can happen after sunset since they only fly a few miles off the ground. There are flares bright enough to be seen in the daytime, I had seen some Iridium flares during the day, and the ISS can often be seen even when it's not a flare. $\endgroup$ Aug 12 at 12:14
  • $\begingroup$ Oh, thanks a lot! Didn't know that. Guess I'm quite lucky to see several flares in the span of 2-3 days ... $\endgroup$
    – 299792458
    Aug 12 at 22:53
  • $\begingroup$ They are actually quite common in my experience, though I'm unaware of any attempts to track them on a large scale. I notice they tend to come in groups where I'll see quite a few in a week, then none for a while. $\endgroup$ Aug 13 at 2:49
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It very likely IS a satellite which is moving from the sunlit part into the shadow of the earth, dimming more and more as it crosses from the day side through twilight (partial earth shadow) to night.

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