Light Travel Time
Compute light travel time across a distance (km, AU, light-years).
Light travel time: t = d / c
The speed of light in vacuum is c = 299,792,458 m/s — an exact value by definition in the SI since 1983 (the metre is derived from c). For a distance d, light travel time is t = d / c. Examples: Moon ~1.28 s, Sun ~8.3 min, Jupiter 33–53 min (Earth-orbit dependent), Proxima Centauri ~4.24 years. Voyager 1 has a light-time of ~22 h (December 2024). Cosmologically we always look into the past — a galaxy 1 Gly away shows light emitted 1 Ga ago; the CMB shows the universe at ~13.8 Ga (380,000 years after the Big Bang).
Applications
Deep-space communication (NASA DSN — Deep Space Network), Mars Rover command uplink (3–22 min one-way latency), interferometry, GPS (relativistic timing corrections), fibre optics (~200,000 km/s in glass), and high-frequency trading where intercity light-time is a hard limit on arbitrage.
FAQ
Why is c exact? Since 1983 the metre is defined as the distance light travels in 1/299,792,458 s, so c is fixed by definition — it has no measurement uncertainty.
Does light always travel at c? Only in vacuum. In water ~225,000 km/s, in glass ~200,000 km/s, in diamond ~124,000 km/s — refractive index slows the phase velocity.
What is a light-year? The distance light travels in one Julian year in vacuum: ~9.461 × 10¹² km. It is a unit of distance, not time.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.