Astronauts aboard the International Space Station experience sixteen sunrises and sunsets every single day because their vehicle is traveling at a blistering seventeen thousand five hundred miles per hour.

Why the horizon changes so quickly

The International Space Station flies in low Earth orbit, roughly 250 miles above the planet. At that height, it must travel at about 17,500 miles per hour to keep falling around Earth rather than back toward it. One circuit takes about 90 minutes.

As the station races along its path, it repeatedly crosses the boundary between the side of Earth lit by the Sun and the side in darkness. That boundary is called the terminator. From the crew’s moving vantage point, the Sun appears to sink below Earth’s curved edge, then emerge again on the next stretch into daylight. The atmosphere softens the transition, so it can look more like a glowing band than a sharp switch.

The crew does not live by 16-day cycles

Those rapid light changes are striking outside, but they are not a useful clock for work and sleep. Astronauts follow a planned schedule tied to a standard time, with lighting inside the station helping distinguish work periods from rest. Otherwise, a crew member could face a sunrise every time the station completed another lap.

The changing view is also a reminder that “day” depends on where you are. On the ground, Earth’s rotation carries a location through daylight and night over roughly 24 hours. In orbit, the station makes that journey around Earth in about an hour and a half—so its crew can watch the same horizon brighten and darken again and again during a single shift.