Speed of light
Light travels at 299,792 kilometres per second, so we always see distant things as they were when the light left them.
Light is fast but not infinitely fast. In empty space it travels at 299,792.458 kilometres per second, a value now fixed by definition, and nothing carrying information can go faster. Because light takes time to cross distances, everything we see is slightly in the past: the light travel time is simply the distance divided by the speed of light.
Over everyday distances the delay is far too small to notice, but in space it adds up. Sunlight takes about 8 minutes 19 seconds to reach Earth, so if the Sun vanished we would not know for over eight minutes. Radio signals to spacecraft at the outer planets take hours. A light-year is a distance, not a time: how far light travels in a year.
The first evidence that light has a finite speed came in 1676. The Danish astronomer Ole Rømer noticed that eclipses of Jupiter's moon Io, as it passed into Jupiter's shadow, came earlier than predicted when Earth was nearer Jupiter and later when it was farther away. He realised the difference was the time light took to cross the extra distance, about 22 minutes for the width of Earth's orbit by his estimate. The true figure is under 17 minutes, but the principle was right.
A common misconception is that a light-year measures time. It is a measure of distance.
See it in action
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