Two clocks. Different elapsed time.
Choose how fast a spacecraft moves relative to a space station. Compare the time measured in the station’s frame with the time on the spacecraft.
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Your selected interval
Station reference frame
Spacecraft clock
The illustration moves only when you choose a step. Each hand makes one turn per elapsed minute; the numbers show the full durations. It is not a live view of signals arriving from the spacecraft.
What would it feel like?
Your own clock and everyday processes feel normal beside you. In this model, the two frames assign different elapsed times to the chosen interval. Neither frame is a universal “stationary” clock. Einstein Online explains time dilation.
The formula and its limits
β = v/c. The Lorentz factor is γ = 1/√(1 − β²). Spacecraft proper time = reference-frame coordinate time / γ. At 80% of light speed, 10 reference seconds correspond to 6 spacecraft seconds.
The reference display stands for a synchronized network of clocks in the station’s frame. It does not mean distant clocks can both be read at one place without correcting for signal travel time. This assumes constant relative speed and flat spacetime: no gravity, acceleration, turnaround or reunion. It does not calculate a return-trip age difference or a frame travelling at light speed. Durations are seconds, hours or 24-hour days, not calendar months or years.
Source: Max Planck Institute / Einstein Online: light clocks and time dilation. Reviewed 24 October 2026.
Real clocks, real effects
Precision-clock experiments test both motion-related and gravitational effects. GPS accounts for both: satellite motion and the difference in gravity affect clocks in different directions. This speed-only calculator does not predict the total GPS correction. NIST: Putting Einstein to the Test.