What do you call the line where the sky appears to meet the sea?

The horizon, and how far away it is depends entirely on how high your eyes are. Standing on a beach it is about five kilometres off. Stand on a ten-metre dune and it is eleven. From the top of a tall building it is fifty or sixty. The rough rule for a person at sea level is a little over three kilometres for every metre of height, square-rooted.

This is also the oldest and simplest proof that the Earth is round, and sailors knew it long before anyone argued about it. A ship does not shrink to a dot as it leaves; it sinks, hull first, until only the mast is showing. Watch it in reverse and the mast appears before the hull.

The line is not perfectly reliable. Air near the surface is often a different temperature from the air above it, which bends light and lifts or lowers the apparent horizon - the effect that makes distant objects appear to float, and that occasionally lets observers see things which are, geometrically, well over the edge.