Phases of the ISS


2013-11-14 – I am on a mailing list to receive notices of when the International Space Station will be visible from where I live. The ISS is generally visible for an hour or so before dawn and an hour or so after sunset—when it is dark down here on earth but illuminated by the sun some 260 miles above.

Most of the time I’m busy when I get a notice or it’s cloudy. Evenings are not a great time. The best time is between 5:30 and 6:00 in the morning, which is when I walk to the train. I’ve been luckier to see it in the past, but I haven’t seen it much lately. Wednesday morning, the ISS passed over right in the middle of that time slot and the sky was clear. It moved from northwest to northeast.

From September to March, I watch the sky in the morning if the sky is clear. When daylight savings time is off, I also catch a little astronomy action on the way home. Now in the morning, I see Jupiter and Mars east(ish) of Orion and in the evening Venus appears brightly in the southwest.

The ISS is a fairly bright object in the sky. I’d say it was somewhere between the brightness of Venus, which is very bright, and Jupiter, which is somewhat bright. It appears to move at the speed of an airliner. I did some calculations to confirm that my comparison is fairly correct. The ISS really moves at a way higher speed, but it is much further away, so it seems about the same.

I’ve seen the ISS many times before, but this time I noticed something that I haven’t noticed before. As it moved from west to east, its brightness diminished significantly. Considering the position of the sun (soon to rise in the east), I guessed that what I was seeing was the “phases” of the ISS. That is, when I first caught sight of it, my line of sight revealed the ISS fully illuminated (like an almost full moon). But as it moved east I saw (or really didn’t see) more of the dark side.

You can’t really see the phases of the moon while you wait. You can see slight changes from day to day, but nothing within a few minutes. The moon circles the earth only once a month. The ISS circles the earth every hour and a half. And it’s much closer. So I was able to see its phases change in only a couple of minutes.

It’s pretty impressive.

And there are people in it.

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