In the TV Show Black Sails a pirate ship furls its sails and uses the curvature of the earth to hide the hull of the vessel whilst someone in the crows nest to watch over another ship without being spotted themselves.
Now I remember googling this at the time, and had to work really hard to find info on it, but I did find it was a technique that was used, but I'm trying to find that info again and can't. Can anyone tell me a bit more about it as a technique?
Caveat lector: I've never seen Black Sails.
However, yes, in the days of sailing ships, ships would often be sighted by their sails peeking over the horizon, and a ship whose sails could be seen but whose hull could not would be called "hull down" from the perspective of the observing ship. The visible horizon to someone on a mast at, say, 100 feet high is a bit more than 12 miles away (assuming that there's no refraction and the earth is a perfect sphere, neither of which is true). Obviously objects that are tall, such as a ship's rigging, will be seen past the horizon.
A sail, being usually white or white-ish, will stand out against the horizon and be seen much more easily than a mast would -- also because the mast is quite a bit smaller than the unfurled sail. So taking in topsails just before dawn and having lookouts at the mastheads was a tactic that was practiced by ships in the Royal Navy, especially cruisers who were interested in prizes or scouting an enemy ship or enemy fleet. Merchantmen would generally travel under reduced sail at night, so dawn was a good time to take in one's topsails and wait for merchants to shake out theirs, to see what might be nearby.