A Watch Built for Sixteen Sunrises a Day

Astronauts aboard the International Space Station see up to 16 sunrises and 16 sunsets every 24 hours, which makes a traditional 12-hour watch dial useless for tracking time in orbit. The solution the aerospace world adopted is the same one aviation has used for decades: the 24-hour clock set to Coordinated Universal Time (UTC), or Zulu time. A feature published this week by AOPA examines the specialized watches built for spaceflight—and the lesson behind them applies directly to pilots fighting fatigue on long or overnight flights.

Why a Normal Watch Doesn’t Work in Orbit

The ISS travels at roughly 17,500 miles per hour, circling the planet once every 90 minutes. At that pace, the sun rises and sets every hour and a half—light, dark, light, dark—far faster than anyone can track.

A standard 12-hour watch dial has always relied on a simple assumption: it loosely maps to the rhythm of a single day, with morning on one side and evening on the other. In orbit, that assumption collapses. There is no morning and no evening, just a rapid cycle that strips away every environmental cue your brain uses to sense the passage of time.

What Time Zone Does the ISS Use?

The Station runs on Coordinated Universal Time (UTC)—the same Zulu reference pilots already use to copy clearances and file flight plans.

The logic is identical to aviation’s. A pilot doesn’t say “a quarter past two in the afternoon”; they say 1415 Zulu. That gives one unambiguous reference shared by a pilot in Anchorage and a controller in London, regardless of where the sun sits in the sky. Aviation learned long ago that local sunlight is a poor way to coordinate anything crossing time zones.

Space takes that principle to its extreme. When you cross every time zone on Earth every 90 minutes, local time becomes meaningless. So mission control, the crew, and the experiment schedules are all anchored to a single clock that doesn’t care whether the window is bright or dark.

What Makes a Spaceflight Watch Different

A watch built for orbit has to survive conditions an everyday timepiece was never designed for: wild temperature swings as the spacecraft moves between sunlight and shadow, and launch vibration that would shake an ordinary movement apart. It also has to stay readable when daylight—the most basic cue your brain uses to estimate time—is firing 16 times a day and misleading you every time.

That’s why watches flown on crewed missions tend to be deliberately, almost stubbornly simple:

• Mechanical movements that don’t depend on a battery you can’t easily replace
• High-contrast dials legible in harsh light or near darkness
• A 24-hour reference so a crew member can read mission elapsed time and Zulu time at a glance—without guessing whether it’s “supposed” to be day or night outside

Why This Matters for Pilots

The underlying problem isn’t unique to astronauts—it’s circadian disruption. Your body runs on an internal clock that expects light and dark on a roughly 24-hour schedule. When that schedule breaks, your judgment, reaction time, and alertness all degrade, whether you feel it or not.

Astronauts get the most dramatic version: 16 sunrises a day will scramble anyone’s internal sense of time. But a pilot crossing six time zones, or flying the back side of the clock on an overnight leg, faces a milder form of the same thing. Your watch says one thing, your body clock says another, and the sun out the window says a third.

The professional answer is the same in the cockpit and in orbit: stop trusting the sun. Commit to one reference clock, discipline yourself to it, and manage your rest and duty time against that clock rather than against how bright it is outside. Zulu time isn’t just a bureaucratic convention—it’s a defense against your own scrambled instincts.

The Takeaway for Everyday Flying

If you fly any real distance, get comfortable thinking in Zulu—not only for filing, but as your honest anchor for how long you’ve actually been awake and how long you’ve actually been flying. The crew on the ISS lives their entire mission that way out of necessity. The rest of us can borrow the same discipline without ever leaving the atmosphere.

This story was reported by AOPA, whose full feature on the watches built for orbit is worth reading in detail.

Key Takeaways

• Astronauts on the ISS experience up to 16 sunrises and sunsets per 24 hours, making a 12-hour watch dial useless in orbit.
• The Station runs on Coordinated Universal Time (Zulu)—the same 24-hour reference aviation uses for clearances and flight plans.
• Spaceflight watches are built simple and rugged: mechanical movements, high-contrast dials, and a 24-hour display that survive temperature swings and launch vibration.
• The real issue is circadian disruption, which degrades judgment and alertness for astronauts and time-zone-crossing pilots alike.
• The professional fix in both worlds is identical: commit to one reference clock and manage rest and duty time against it, not against the sun.