What Time Is It in Space: A Full Guide to Understanding Time in Space

Have you ever wondered what time it is in space or how astronauts keep track of time while orbiting Earth?

This fascinating guide will explain the complex relationship between time and space travel in a way that is easy to understand.

From time dilation to atomic clocks, you’ll learn how time works differently in the weightless environment of space.

What Time Is It in Space?

time in space
Photo By Starlust

Time in space is relative and flexible compared to how we experience it on Earth. Due to the effects of time dilation, astronauts orbiting the planet age slightly slower than people on the ground. Atomic clocks help keep accurate time on spacecraft by measuring the vibrations of atoms. Understanding time is crucial for navigation and communication during space missions. While the passage of time feels normal to astronauts, it doesn’t precisely match how time passes on Earth.

Understanding Spacetime and Relativity

theory of relativity
Photo By Nicolle Rager Fuller

To comprehend time in space, you need to understand Einstein’s theory of relativity. This theory revolutionized physics by showing that space and time are interwoven into a single continuum known as spacetime. The closer you travel to the speed of light, the slower time passes relative to a stationary observer.

This time dilation effect allows astronauts on fast-moving spacecraft to age just a tiny bit slower than people on Earth. The curvature of spacetime due to gravity also influences the passage of time. Gravity slows down time, meaning clocks tick slightly faster when they are farther away from a massive object like Earth. These counterintuitive properties of spacetime are crucial to factor in when navigating the cosmos.

Time Dilation Effects in Space

time dilation in space

Due to the principles of relativity, time passes slower for objects moving at high speeds relative to a stationary observer. This phenomenon is called time dilation and it has been verified by atomic clock experiments. For example, clocks on GPS satellites orbiting Earth at 14,000 km/h tick slower by about 7 microseconds per day than clocks on the ground.

This difference may seem tiny, but without accounting for time dilation effects, GPS would become inaccurate very quickly. For astronauts, the time dilation effect is more extreme. Astronaut Scott Kelly aged roughly 0.007 seconds less during his year on the International Space Station as compared to his twin brother on Earth. Though the time discrepancy is minimal, it illustrates the mind-bending consequences of traveling at high velocities through spacetime.

How Time Passes Differently on Spacecraft

time in spacecrafts
Photo By Carl De Torres

Due to relativistic effects, time passes slower for astronauts on spacecraft compared to people on Earth. For example, astronauts on the International Space Station orbiting at 17,500 mph experience time dilation effects. After 6 months in space, an astronaut would have aged 0.007 seconds less than their twin on Earth.

This effect becomes more pronounced the faster the velocity. Astronauts traveling to Mars at speeds up to 150,000 mph would experience more significant time dilation. A 1-year round trip to Mars would cause astronauts to age less by about 0.02 seconds compared to people who remained on Earth. While not hugely impactful over short missions, the cumulative effects of time dilation would need to be considered for hypothetical multi-generational interstellar space travel. The closer to the speed of light the spacecraft travels, the slower time would pass for its crew relative to observers on Earth.

Key Concepts: Ert, Ground UTC, UTC, Gmt

time zone map
Photo By Eclipse Wise

There are some key concepts to understand when it comes to timekeeping in space:

ERT (Ephemeris Time) is the time system used onboard spacecraft to track their position. It is a uniform time scale based on the motion of the Earth.

Ground UTC is the Coordinated Universal Time used on Earth by mission control teams. It is the standard time reference for all operations.

UTC (Coordinated Universal Time) is the primary time standard used worldwide. It is based on atomic clocks and occasionally has leap seconds added to account for changes in Earth’s rotation.

GMT (Greenwich Mean Time) is the mean solar time at the Royal Observatory in Greenwich, London. It is the same as UTC without leap seconds.

Timekeeping on the International Space Station

time keeping in the ISS
Photo By ESA / NASA

When it comes to timekeeping on the International Space Station (ISS), it gets even more complex. The ISS orbits Earth every 90 minutes, seeing a sunrise or sunset every 45 minutes. So what time is kept on the ISS?

The ISS uses Coordinated Universal Time (UTC) as its standard, the same as mission control on the ground. Clocks on the ISS are synchronized with ground control and set to UTC. This allows everything to be scheduled properly between the astronauts working on the ISS and teams on the ground.

But the ISS also has its own Station Time zone, which is Greenwich Mean Time (GMT). This means clocks on the ISS show GMT, even though everything is scheduled in UTC. The reason for using GMT is that this makes it easy to plan work shifts and tasks without worrying about time zone conversions.

Calculating Time Delays Over Vast Distances

space probe communication
Photo By Astronomy

When communicating across vast distances of space, there can be significant delays between sending and receiving messages. This time lag occurs because information can only travel so fast – at the speed of light.

For example, it takes light 1.3 seconds to travel from Earth to the Moon. So if we send a signal to the Moon, it will take at least 2.6 seconds total for the message to reach the Moon and for us to get a reply back on Earth. That’s because the signal has to make a round trip.

For even farther destinations like Mars, communication lags can stretch into minutes or hours. When Mars is at its closest point to Earth, it takes about 3 minutes for a radio message to reach Mars from Earth. Due to the planets’ orbits, the transmission delay can be up to 22 minutes when Mars is farthest away.

So when communicating across the solar system and beyond, mission controllers here on Earth need to account for these time delays. The long communication lags make real-time control of distant space probes impossible. Instead, the spacecraft needs to operate autonomously while controllers await those faraway signals.

The Challenge of Synchronizing Clocks Between Earth and Spacecraft

synchronizing clocks in space
Photo By Jean-Daniel Deschênes et al

With such immense distances involved, another challenge is synchronizing clocks between Earth and distant spacecraft. Even when accounting for speed-of-light delays, errors can still accumulate over time.

One factor is the relativistic effects from the curvature of spacetime around large masses like the Sun and planets. Due to differences in gravitational time dilation, clocks tick at different rates in space compared to on Earth.

For example, the atomic clocks on the GPS satellites in orbit experience faster time relative to clocks on the ground. Without adjustment, GPS satellite clocks would gain about 7 microseconds per day compared to Earth.

To keep spacecraft clocks in sync, NASA uses its Deep Space Network to regularly send time synchronization signals. Correcting for relativistic effects is also required for accurate navigation.

So while we often think of time as constant, it’s anything but in the vastness of space. Keeping precise time across such immense interplanetary distances requires advanced physics and extremely precise atomic clocks.

The Future of Timekeeping as We Venture Deeper into Space

what is deep space
Photo By Elite Dangerous

As we continue to explore deeper into space, even more advanced timekeeping will be needed. Atomic clocks are already precise to within a second over billions of years, but improvements will likely still be made.

Optical lattice clocks and nuclear clocks show promise for the future. Their precision could enable new tests of fundamental physics, like detecting subtle spacetime distortions from gravitational waves.

Entirely new timekeeping methods may also be developed to deal with the great distances and relativistic effects encountered. For example, pulsars could potentially be used as “celestial clocks” for deep-space navigation.

More autonomous timekeeping will also be critical as we send probes farther out, like the Voyagers, which are now over 14 billion miles from Earth. At those distances, any signals take over a day to reach the spacecraft, so relying on Earth for time synchronization becomes impractical.

As we venture to Mars and beyond, spacecraft will need to keep and coordinate time themselves across large fleets. The exploration of space and time will continue to be intimately linked.


What Time Zone Is in Space?

There is no official time zone in space. Astronauts on the International Space Station use Coordinated Universal Time (UTC) to keep track of time.

How Much Time Is 1 Hour in Space?

1 hour in space is the same as 1 hour on Earth – 60 minutes. Time passes at the same rate in space as it does on Earth.

What Time Is 1 Day in Space?

A day in space is approximately 24 hours long, the same as on Earth. However, the ISS orbits the Earth every 90 minutes, so astronauts see 16 sunrises and sunsets in a 24-hour period.

Is One Hour in Space 7 Years?

No, this is a myth. One hour in space is equal to one hour on Earth. There is no time dilation effect that causes time to pass slower in space.


Time works the same way in space as it does on Earth. While astronauts on the ISS experience more sunrises and sunsets due to their high orbital speed, the passage of time itself remains unchanged. So one hour in space is equal to one hour on Earth. What time is it in space? It’s the same time as wherever you are right now!

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