The human eye can see up to 48 kilometers away
Earth's surface curves out of sight at a distance of 3.1 miles (5 kilometers). But our visual acuity extends far beyond the horizon.
If Earth were flat, or if you were standing atop a mountain surveying a larger-than-usual patch of the planet, you could perceive bright lights hundreds of miles distant.
On a dark night, you could even see a candle flame flickering up to 30 miles (48 km) away.
Source: livingscience.com
