The Evolving Universe

Smithsonian Institution

How Fast is Light?

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Light whizzes from Earth to the Moon and back in about 2.5 seconds - that's only 768,000 km (480,000 miles).

The distances between stars and galaxies are much greater, so astronomers measure them in light-years. One light-year equals the distance light travels in one year - almost 9.5 trillion km (6 trillion miles).

In technical work scientists also use the parsec, which is 3.26 light-years.

Distances within the Solar System are measured in smaller increments. A light-minute is 18.0 million km (about 11 million miles) ; a light-hour is 1.08 billion km (almost 700 million miles).