Researchers at NIST find that clocks at Mars' areoid run on average 0.477 milliseconds (477 microseconds) faster per Earth day than Earth-based clocks, because Mars' weaker surface gravity outweighs its slower orbital speed. The offset varies by about 226 microseconds over a Martian year and shows additional ~40‑microsecond changes linked to synodic cycles. Residual model errors of roughly 100 nanoseconds per day would require periodic resets (about every 100 days) unless further corrections — such as orbital precession and gravitational quadrupole moments — are included.
Einstein Was Right: Clocks on Mars Run Faster — What That Means for Interplanetary Timekeeping
Time moves slightly differently on Earth and Mars due to the two planets' differing gravitational pulls. A new study confirms this prediction of Einstein's relativity. | Credit: NASA/JPL
Scientists report that clocks referenced to Mars' areoid run slightly faster than Earth-based clocks — a small but important effect for future interplanetary missions and an eventual "internet" across the solar system. A new study by NIST physicists Neil Ashby and Bijunath Patla finds Martian clocks lead Earth clocks by an average of 0.477 milliseconds (477 microseconds) per 24-hour Earth day when measured from Earth.
How Relativity Makes Mars 'Faster'
Albert Einstein's general theory of relativity predicts that both gravity and velocity affect the passage of time. Stronger gravity slows clocks; weaker gravity speeds them up. Likewise, higher orbital velocity produces time dilation that slows clocks. Mars' relatively weak surface gravity (about one-fifth of Earth's at the areoid) outweighs its slower orbital speed, producing a net speed-up of Martian time compared with Earth when observed from Earth.
Key Findings
The authors applied general-relativity formulas to Mars' areoid and included gravitational influences from the Sun, Earth and Moon. Their principal results include:
A gravity map of Mars taken by NASA's Mars Global Surveyor (MGS). For the new study, researchers had to find a region of Mars akin to Earth's sea level, then compare the gravity and velocity of the region with Earth. | Credit: NASA/JPL
- Average Offset: Martian clocks run ahead by about 477 microseconds per Earth day when referenced to Earth clocks.
- Annual Variation: That offset oscillates by roughly 226 microseconds over a Martian year, driven largely by Mars' more elliptical orbit and changing gravitational perturbations.
- Synodic Modulation: An additional ~40 microseconds variation appears across seven synodic cycles (the interval for Mars to return to the same position in Earth's sky).
- Moon Comparison: For context, earlier work found lunar clocks run about 56 microseconds faster per day than Earth clocks.
- Residual Error: The model still leaves residual inaccuracies of about 100 nanoseconds (0.1 microseconds) per day on long timescales — enough to require resetting a Martian reference clock roughly every 100 days unless corrected.
Why This Matters
These effects are tiny in everyday terms but matter for high-precision navigation, communication and experiments across planets. A standardized time reference for each celestial body will be essential for coordinating spacecraft, rover operations and any future crewed activities, and for building reliable interplanetary communication networks.
Limitations And Next Steps
The authors note their model omits several minute influences that will be necessary for ultra-precise timekeeping: orbital precession (gradual wobble), each planet's gravitational quadrupole moment (how mass is distributed inside the planet), and extremely small, time-varying shifts in planetary motions. Accounting for those factors will be required to reduce the current residual errors and to support precision applications and tests of general relativity.
Publication: Neil Ashby and Bijunath Patla, National Institute of Standards and Technology; published Dec. 1 in The Astronomical Journal.
