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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