The power of human eyesight is often debated with a single question: How far can the human eye see a single candle flame? A quick Google search turns up guesses ranging from three to 30 miles, but no one has tried to find out for certain—partly because of the complications of such an experiment—until now. Astronomers Kevin Krisciunas and Don Carona at Texas A&M came up with a way to ignore the issue of the Earth's curve—which causes the horizon to drop slightly at a distance of about three miles away—and obstructions, like trees and buildings, reports the MIT Technology Review. Essentially, they compared the brightness of a candle to that of the star Vega, one of the brightest stars in the night sky, which has a magnitude 0. Using an astronomical SBIG camera to measure photons, they found the candle flame burned with the same brightness as Vega at 392 meters, or a quarter of a mile away.
The distance surprised the astronomers because the flame appeared to the human eye to be the same brightness at 338 meters, but was actually almost 2.5 magnitudes brighter. As Vega is 251.2 times brighter than the faintest stars visible to the naked eye, which are classified as magnitude 6, Krisciunas and Carona used a simple calculation to arrive at their conclusion: that a candle flame would be just as visible at approximately 1.6 miles. At 10 miles away, the flame would still be detectable but only with a telescope or binoculars as "this is far beyond the capabilities of the most sensitive human eyes," the astronomers write in the arXiv study. Is the debate over? Probably not: "What we need now is the definitive experiment that actually measures...the maximum distance at which a human or group of humans can see a candle," notes the Technology Review. (Find out what spending too much time indoors is doing to your eyesight.)
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