How long is 24 hours in space?

The Endless Sunrise: How Long is a “Day” in Space?

We often take for granted the simple rhythm of our planet. A sunrise, a sunset, and the comforting predictability of a 24-hour day dictating our routines. But imagine breaking free from Earth’s gravitational embrace, venturing into the inky blackness of space. Suddenly, that familiar clock face looks a little different, and the definition of “day” becomes surprisingly complex.

For astronauts aboard the International Space Station (ISS), that complexity is their daily reality. Instead of experiencing a single sunrise and sunset, they witness a jaw-dropping sixteen. This isn’t because time is warped in the science fiction sense, but rather due to the ISS’s incredible speed and orbit.

The ISS races around the Earth at approximately 17,500 miles per hour. At that velocity, it completes a full orbit in roughly 90 minutes. This means that for every 90 minutes, astronauts experience a complete cycle of daylight and darkness as the Earth rotates beneath them. So, in what we consider a standard 24-hour period, they witness the sun crest the horizon, flooding their modules with light, a staggering sixteen times.

So, if an astronaut sees 16 sunrises in a 24-hour period, is each sunrise a “day”? Not quite. While they experience the visual cues of day and night multiple times, the actual duration of time within a single rotation relative to their orbit is only 90 minutes. The concept of a “day” in space shifts from a measurement based on Earth’s rotation to a measurement tied to the orbital period of the spacecraft.

This constant cycle of light and darkness presents significant challenges for astronauts. The human body thrives on consistent routines and regulated exposure to light. To mitigate the disruptive effects of these accelerated cycles, the ISS operates on Greenwich Mean Time (GMT), allowing astronauts to maintain a semblance of a 24-hour schedule. They adhere to a carefully planned work-rest cycle, complete with designated sleep periods, meal times, and exercise regimens. Sophisticated lighting systems within the station mimic the natural progression of a terrestrial day, further helping to regulate their circadian rhythms and minimize sleep disturbances.

The relentless cycle of the space station’s orbit serves as a constant reminder of the unique temporal landscape of space travel. It highlights the challenges and adaptations required to thrive in an environment where the familiar rules of time are bent, redefined, and managed with meticulous precision. While 24 hours remains the unit of measurement for their day, the astronauts aboard the ISS experience a reality where the sun never truly sets, and the concept of “day” becomes an ever-evolving, 90-minute sprint around our planet.