The Physics of the Universe - Difficult Topics Made Understandable
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HOW FAST DOES LIGHT TRAVEL?

The reasoning used by Rømer in 1675 to determine the speed of light - click for larger version
(Click for a larger version)
The reasoning used by Rømer in 1675 to determine the speed of light
(Original Source N/A: phyun5.ucr.edu/%7Ewudka/Physics7/Notes_www/node65.html)

Light travels at exactly 299,792,458 metres per second in a vacuum (about 300,000 kilometres per second or just over 1 billion kilometres per hour). As a comparison, sound waves travel at a paltry 343.14 metres per second (about 1,235 kilometres per hour), almost a million times slower than light waves, and the fastest military airplane, the SR-71 Blackbird, can fly at about 980 metres per second (about 3,500 kilometres per hour).

At that speed light takes:

  • 0.0000033 seconds to travel 1 kilometre;
  • 1.3 seconds to reach us from the Moon;
  • 8.32 minutes to reach us from the Sun;
  • 4.37 years to reach us from Alpha Centauri, the nearest star system to the Solar System (Alpha Centauri is therefore said to be 4.37 light years away);
  • 26,000 years to reach us from the centre of our Milky Way galaxy;
  • 2,500,000 to reach us from the Andromeda Galaxy, our next nearest galaxy (and the most distant object visible to the naked eye, although only as a barely perceptible smudge);
  • 59 million years to reach us from the Virgo Cluster, the nearest large galaxy cluster;
  • and, theoretically, about 78 billion years to reach us from the edge of the observable universe (this is actually longer ago than the 13.7 billion year age of the universe, because the continued expansion of space has significantly increased the distance the light from these early objects has had to travel).



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