Can we see ISS from Earth? by @Astro_Clay
Answer by Clayton C. Anderson:
Why certainly we can see the International Space Station (ISS) from Earth! We just have to know when, and where, to look!
The ISS orbits our earth every 90 minutes, due to its positional orbit about 250 nautical miles up. Since it completes 16 orbits in a 24-hour period (okay teachers, have those youngsters do the math!), there are ample opportunities to see it pass overhead… as long as your location lies beneath its ground track during that 24-hour period. The ISS is pretty easy to spot because it is so large (about the size of a football field). The sun's rays hit the station, and for all intents and purposes, it lights up, becoming the second brightest object in the night sky behind our Moon. Not bad company to keep in the annals of brightness!
But, the earth also rotates on its axis every minute of every hour of every day, etc. That means –in the most simplistic sense– that the ISS orbit shifts relative to that rotation. Don't worry though… NASA has figured all of that out for you, and they've created smartphone apps and websites which will allow you to "spot the station!" Search your smart device for the apps or gototo find when it is next flying over you.
Now, you have to know where to look too. That is done via something star gazers refer to as azimuth and elevation. Whoa…rocket science! Don't sweat it. The NASA website (above) has a nice explanation, as do the apps. But for the lazy ones out there like me, it will tell you things like the direction (the azimuth) from which it will rise and then set, and how high it will be (and how long) above the horizon (elevation).
Good luck on your ISS hunting expedition. This is the PERFECT time for me to say, "…keep lookin' up!