Your guess
You're on the beach, your eyes 1.70 m above the sea. How far away is the horizon?
Even higher
369kmis how far you can see from a plane at about 10,700 metres
From the very top of the Burj Khalifa, about 834 metres above the sea, the view reaches 103 kilometres. From an airliner at a typical cruising altitude of about 10,700 metres (35,000 feet) it's 369 kilometres.
The air cheats a little
The calculation ignores the air. In reality you usually see farther: when cold, dense air lies over water or ground, it bends light downward, a little way around the Earth's curve. Over hot ground the opposite happens, as often in deserts – that's when mirages appear.
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