Science Puzzle
Why Can You Only See the Space Station at Dawn and Dusk?
The International Space Station is often one of the brightest things in the night sky: a steady white dot that glides from one side of the sky to the other in a few minutes. It goes round the Earth every hour and a half, yet you'll only spot it in the couple of hours after sunset or before sunrise, never in the middle of the night.
Why can you only see the space station at dawn and dusk?
The Answer
The station has no light of its own worth seeing; it shines by reflecting sunlight. You can only see it when the sky around you is dark but the station, about 400 kilometres up, is still in sunshine, and that only happens around dawn and dusk.
After sunset, the ground where you stand is in Earth's shadow, but high above, the Sun hasn't set yet. Anything up there is still lit, rather like the top of a tall tower catching the last light. For an hour or two after sunset, the space station passes over in full sunlight against a dark sky, and its big solar panels make it very bright.
Later in the night, the station is in Earth's shadow too, so it goes dark and you can't see it at all. You may even see it fade away halfway across the sky as it flies into the shadow. In the daytime it's sunlit, but the sky is far too bright to pick it out.
It moves so quickly because it's travelling at about 28,000 kilometres an hour, going round the Earth about 16 times a day. Websites and apps tell you when it will pass over your home.
The principle: Seeing by reflected light. A satellite shines only by reflecting sunlight, so it's visible when it's still in sunshine high up while the ground below is already dark.