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    Distance & light

    Light Travel Time Calculator

    How long light or a radio signal takes to cross any distance — from the Moon to other stars.

    t = d / c

    Inputs

    Results

    One-way light time
    499.005 s
    8 min 19.0 s
    Round-trip (signal + reply)
    998.01 s
    16 min 38.0 s
    Worked solution
    1. t = d / c = 1.496 × 10^11 m / 299,792,458 m/s = 499 s

    How it works

    Radio commands travel at the speed of light, so distant spacecraft cannot be joystick-controlled. A command to a Mars rover takes between about 3 and 22 minutes one way depending on planetary positions.

    Astronomical distances are often expressed as light-travel times: the Sun is about 8.3 light-minutes away and the nearest star about 4.25 light-years.

    Variables

    • t — one-way travel time
    • d — distance
    • c — speed of light, 299,792,458 m/s (exact)

    Assumptions and limits

    • Vacuum propagation in a straight line
    • Ignores relativistic Shapiro delay (microseconds) and processing delays

    For education only. These models are simplified and are not suitable for spacecraft navigation or safety-critical engineering. See our scientific accuracy policy.