|International Space Station|
The ISS only appears in the sky for a short while after sunset, or before sunrise. It is in orbit about 225 miles above the surface of the Earth; for example, shortly after a sunset the ISS catches sunlight and shines as it travels across the sky. When you get a very good alignment and it passes nearly overhead, it can reach the same magnitude as a bright Venus, and therefore be easy to spot. On such a pass, it takes about 5 minutes to travel from horizon to horizon, covering over 1000 miles in that time.
ISS visibility depends upon your location on Earth, since the best times to see the ISS in one part of the globe won't be the same as another. NASA has an an excellent web resource for this; for ISS sightings in San Francisco, we have morning passes right now, and will soon have good visibility in the evenings starting on February 5th. The iPhone app ISS Visibility is quite helpful for locating the ISS, with maps and easy-to-follow directions.
Best of luck seeing this wonderful sight. Image courtesy of NASA.