The Universe Is All History
It took 300 years of experiment and calculation to pin down the speed at which light travels in a vacuum: 186,282 miles per second.
Light will travel slightly slower than this through air, and some wild experiments have actually slowed light to a crawl and seemingly made it go backward, but at the scales encountered in our everyday lives, light is so fast that we perceive our surroundings in real time.
Look up into the night sky and this illusion begins to falter. Because light takes time to get here from there, the farther away 'there' is the further in the past light left there and so we see all objects at some time in the past.
We see the relatively close moon as it was 1.2 seconds ago and the more distant sun as it was about 8 minutes ago. The measurements — 1.2 light-seconds and 8 light-minutes — can be thought to describe both time and distance.
The distance to more remote objects such as other stars is so great it is measured in light-years—the distance light will travel in a year, or about 6 trillion miles (10 trillion kilometers). Even the nearest star system, Proxima Centauri, lies more than four light-years away, so it appears to us on Earth as it was just over four years ago when the light began its journey.
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The fantastic skies of Orphan Stars from NASA Science
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