Donaldson R F Light Travel Time Calculator
Light is fast, but space is big: work out how long it takes light (or a radio signal, which travels at the same speed) to cross any distance.
How this is worked out
Time = distance ÷ speed. The speed used is the exact, defined speed of light in a vacuum: 299,792.458 km per second. An astronomical unit (AU) is the IAU's defined average Earth–Sun distance (149,597,870.7 km); a light-year is the distance light travels in one Julian year of 365.25 days.
This is also the delay in any signal sent at the speed of light — radio, radar, or a spacecraft's telemetry — which is why, for example, a command sent to a Mars rover takes several minutes to arrive, and a reply takes just as long to come back.
Frequently asked questions
Why do the presets use the Sun as the starting point?
They show each planet's own mean distance from the Sun, from the same fact sheet as the Planet Facts Table, so "light travel time" here means the delay between the Sun and that planet, not between Earth and that planet (which changes constantly as both planets move).
Does anything actually travel at the speed of light?
Light itself, radio waves and all electromagnetic radiation do, in a vacuum. Spacecraft travel far slower — this tool only calculates the signal delay, not a travel time for a physical craft.
Related tools: Planet Facts Table, Age on Other Planets, Julian Date Converter.
Source / Last verified: speed of light (299,792.458 km/s, exact) from NIST, "speed of light in vacuum" (physics.nist.gov); astronomical unit (149,597,870.7 km, the IAU-defined value) from NASA/JPL Solar System Dynamics, "au (astronomical unit)" (ssd.jpl.nasa.gov/glossary/au.html); planet distances from NASA/GSFC, "Planetary Fact Sheet – Metric" (nssdc.gsfc.nasa.gov/planetary/factsheet), marked "Last Updated: 18 March 2025, DRW". All read and checked on 27 September 2026.