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Propagation of light

Light and distances

At human scale light seems to propagate in very short times but it's different at universe scale.
The time that light needs to propagate is even used to compare distances between stars and planets.

Here are some significative examples:
The closest heavenly body to the Earth is the Moon at roughly 380 000 km. Light emitted by the Moon takes 1,3 second to reach the Earth.
The Sun is roughly at 150 000 000 km from the Earth and its light propagates for 8 minutes and 20 seconds to reach the Earth.
As for the stars that are much farther from the Earth, their lights have to propagate for several years before to reach the Earth: for instance, light from Proxima Centauri travels for 4,2 years before we can see it.

Thus, distances between stars are usually expressed in " light-year ": a light-year is the distance that light travels in one year.
A light-year is about  1013 km.
As for the solar system its distances can also be expressed in light-second, in light-minute or even in light-hour.

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