Light Travel Time Calculator
How long light takes to travel a distance — kilometers, astronomical units or light-years — from the Moon to the nearest star.
The delay in seeing anything far away.
How the light travel time calculator works
Light travels at exactly 299,792,458 m/s. Divide a distance by that and you have the delay: 1.28 s from the Moon, 8.3 minutes from the Sun, 4.2 years from Proxima Centauri. Enter a distance in km, AU (149.6 million km) or light-years and the calculator gives the time in the most readable unit and in seconds.
Formula: t = d ÷ c (c = 299,792,458 m/s)
Worked examples
| Inputs | Light travel time | Note |
|---|---|---|
| Moon, 384,400 km | 1.282 seconds | 1.28 seconds |
| Sun, 1 AU | 8.32 minutes | 8.3 minutes |
| Proxima Centauri, 4.24 light-years | 4.24 years | 4.24 years — by definition |
| Mars at closest, 0.37 AU | 3.08 minutes | 3.1 minutes each way |
FAQFrequently asked questions
What is a light-year?
The distance light travels in a Julian year: 9.461 trillion km. It is a distance, not a time.
Why do Mars rovers need to drive themselves?
A radio command takes 3–22 minutes to arrive depending on the planets' positions, so real-time control is impossible.
Are we seeing stars as they were?
Yes — light from a star 1,000 light-years away left it 1,000 years ago.
How long does light take to cross the Solar System?
About 5.5 hours from the Sun to Pluto (39 AU) and over a day to reach the Voyager probes.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.