Ask A Grown-up: How Far Can Our Eyes See? - The Guardian

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Ask a grown-up: John Brookes
Photograph: Philip Partridge/GNM Imaging
Photograph: Philip Partridge/GNM Imaging
Ask a grown-upLife and style This article is more than 11 years oldAsk a grown-up: how far can our eyes see?This article is more than 11 years oldConsultant ophthalmic surgeon John Brookes answers nine-year-old Samuel's questionJohn BrookesSat 10 May 2014 07.00 BSTShare

An old joke goes: "A man went to the optician for an eye test. The optician took the man outside and pointed to the sky. 'What's that?' the optician asked. 'The sun,' the man replied. The optician said, 'Well, how far do you want to see, then?'"

A 6ft man standing and looking out to the horizon can see approximately 5km away, as the Earth's surface curves out of sight. But our ability to see extends well beyond the horizon. It also depends on the amount of dust and pollution in the air, which usually limits normal vision to less than 12 miles. However, in 1941 a vision scientist, Selig Hecht, worked out that, with a clear, unobstructed view, the human eye could see a candle light flickering about 30 miles away.

So the answer depends on how big and bright the object is. The sun is 150m kilometres away, but you can see it because it's so big and bright.

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Tag » How Far Can The Human Eye See