Which time reference do astronauts use on space missions?
In orbit, daylight is no help in organising a mission: the International Space Station circles the Earth at almost 28,000 km/h and completes a full orbit in about 90 minutes.
A common reference therefore has to be chosen. Should it be Houston time, Moscow time, or any other time zone linked to one of the partner control centres? Coordinated Universal Time (UTC) is the one that prevails, precisely because it sits outside this national logic.
Neutral and detached from any particular meridian, UTC allows ground teams, whether based in the United States, Russia or elsewhere in Europe, to coordinate with the crew without calculating time differences and without ambiguity. Scientific tasks, slots for calls with families, exercise and rest times: everything is organised around this same reference, shared by the whole crew whatever their nationality.
What is a day in orbit like when the Sun rises 16 times?
Seeing the Sun rise and set sixteen times a day makes it impossible to organise anything around the natural alternation of day and night: how do you set a time for lunch or bed when the sky changes colour roughly every forty-five minutes? To solve this problem, astronauts have given up the very idea of following the course of the Sun and refer exclusively to UTC.
The Station’s interior lighting is also designed to make up for the lack of solar cues: it is adjusted artificially to reproduce, as far as possible, a day/night cycle familiar to the human body. Living by a clock without a sun therefore requires constant adaptation, in which organising time becomes entirely a matter of discipline and technique rather than natural observation.
How do crews organise work, sleep and meals without a natural alternation of day and night?
The day is divided up almost to the minute:
- scientific work slots set in advance;
- meals taken at regular times, regardless of the light outside;
- compulsory exercise sessions, to preserve muscles and bones in weightlessness;
- time set aside for calls with families;
- lights out at a fixed time, simulating nightfall.
Despite this discipline, many crews report sleep problems and disorientation, with the strange feeling that the days no longer pass quite the same way. The clock imposes order, but the body takes longer to accept it, a reality that medical teams on the ground monitor closely.
Which instruments measure time on board: from the onboard clock to the atomic clock?
Behind synchronisation with UTC lies a mechanism far more sophisticated than a simple wall clock. Atomic clocks measure time from the oscillations of excited atoms and offer stability unmatched by conventional mechanical or electronic mechanisms. This precision directly determines satellite positioning, especially for GPS, where an offset of a single microsecond is enough to throw a position off by around 300 metres.
All the Station’s onboard clocks are thus synchronised to this same UTC reference, shared with the control centres on the ground. Since April 2025, the Station has even hosted a set of atomic clocks from the European Space Agency, called ACES, designed to compare time measured in orbit with time measured on the ground with unprecedented precision.
This demand for rigour is now leading space agencies to consider time references dedicated to the Moon and, in the future, to other worlds.
Why does time not pass exactly as it does on Earth for an astronaut?
On board the International Space Station, every astronaut ages very slightly differently from someone who has stayed on Earth. Two forces act at the same time, in opposite directions.
On the one hand, the orbital speed, around 28,000 km/h, slightly slows down the passage of proper time on board: this is the effect predicted by Einstein’s special relativity, according to which a fast-moving clock runs slow compared with a stationary one. On the other hand, since Earth’s gravity is weaker in orbit than on the ground, general relativity produces the opposite effect: less gravity makes time pass slightly faster.
For the ISS, the speed effect very slightly outweighs the gravity effect, so astronauts come back from their mission with an internal clock marginally behind that of the Earth. Nothing noticeable in daily life, no effect on sleep or meals timed by UTC, but a reality measured with extreme precision thanks to atomic clocks.
Two notions must therefore be clearly distinguished: the conventional time reference that astronauts follow day to day (UTC, a practical convention) and the physical passage of time, which genuinely depends on the frame of reference from which it is observed.
Gravity is in fact at the heart of our Interstellar universe. Our two automatic models, the LUNAR 1,622 and the RED 3,721, pay tribute to it in their very names: 1.622 m/s² on the Moon, 3.721 m/s² on Mars. Each one contains meteorite dust from the celestial body it represents.
How is onboard time synchronised with Earth time despite this offset?
This relativistic offset is measured only in microseconds: it has no effect on the time displayed on board or on the crew’s schedule, and the Station’s clocks do not need to be corrected to account for it.
It does, however, become decisive for satellite positioning systems, which also rely on onboard atomic clocks: these are set to take relativistic effects into account, otherwise the position error would build up by around ten kilometres a day.
For astronauts, the reference nonetheless remains simple day to day: no complex calculations to make, just clocks continuously synchronised with Earth, allowing them to fit their work, meal and sleep times into one worldwide schedule.
Which time reference for future missions to the Moon or Mars?
Future missions to the Moon or Mars raise a new question: which time reference should be adopted when no terrestrial time zone naturally applies to another world?
On Mars, a day, called a sol, lasts about 24 hours and 39 minutes, a difference large enough that teams operating Martian rovers from Earth have to gradually shift their own schedules to this offset rhythm.
Mars holds a special place in our RED 3,721 collection. Each watch contains powder from the Martian meteorite NWA 14269, discovered in 2021 in North Africa, housed in a capsule at the heart of the case. Supplied by MSG Meteorites, a British company licensed by the IMCA, this rare material comes with a certificate of authenticity including a spectrometric analysis, and every piece in the collection is numbered.
For the Moon, there is currently no universal time standard of its own, and past missions relied on the time of the control centre on the ground. This could soon change: in April 2024, the White House asked NASA to define a Coordinated Lunar Time, with a deadline set for the end of 2026.
The Moon nevertheless already has its historical landmarks. Mare Tranquillitatis, the most famous lunar sea, is where the lunar module Eagle landed on 20 July 1969, paving the way for humanity’s first step on the Moon.
Our Lunar 1969 pays tribute to it: the coordinates of the six Apollo missions that landed on the Moon are engraved on its dial, and each watch contains an authentic fragment of the lunar meteorite NWA 11515, discovered in 2017 in North-West Africa. As with the RED 3,721, this fragment is supplied by MSG Meteorites and comes with a certificate of authenticity including a spectrometric analysis.
Some of our Lunar 1969 watches have even travelled to near space. Attached to a gondola carried by a weather balloon, they rose until the balloon burst at altitude, triggering a parachute. A GPS fitted to the gondola made it possible to recover the watches, and a certificate attests to each of these flights.
This way of rethinking time, far from any natural reference, echoes the universe of space watches in the INTERSTELLAR collection by Col&McArthur, a collection of space-themed watches paying tribute to these conquests in which every mission had to reinvent its own measure of time.
“INSPIRED BY HISTORY / MADE OF MEMORY”
At Col&McArthur, every timepiece in this universe belongs to this lineage: a numbered creation, conceived as a tribute to those who had to push back the limits of humanity to explore the unknown. This limited collection is intended for those for whom a watch is never a mere accessory, but the passing on of a reference point, a memory, a moment of History worn on the wrist.
As with every creation from the house, each purchase comes with secure payment, the option to pay in 3 instalments with Alma, free delivery worldwide, a 2-year warranty and customer satisfaction rated 4.8/5.
Frequently asked questions
Why don’t astronauts use Houston or Moscow time?
Because UTC, neutral and detached from any national time zone, avoids any confusion between control centres and allows both the crew and the ground teams to work on the same schedule, whatever their nationality.
How many sunrises does an astronaut see per day in low orbit?
About sixteen, since the International Space Station completes a full orbit of the Earth every 90 minutes at around 28,000 km/h.
Does time really pass more slowly in space?
Yes, to a tiny degree: orbital speed slightly slows down the astronaut’s proper time compared with someone who has stayed on Earth, an effect measured with atomic clocks but completely imperceptible in daily life.
Is there an official time for the Moon or Mars?
Not yet for the Moon, but NASA has been tasked with defining one by the end of 2026; on Mars, ground teams follow a rhythm based on the Martian “sol”, which lasts about 24 hours and 39 minutes.
How does lighting help astronauts sleep without a natural solar cycle?
The Station’s artificial lighting is programmed to imitate a day-night cycle, giving the body a cue that the sky no longer provides.
