The curvature of the earth limits what observers aboard a ship can see. Raising a lookout increases that distance, which made the crow's nest important aboard sailing ships. Slow closing speeds then gave commanders time to identify a distant opponent and decide whether to engage. As ships became faster and less dependent on the wind, that warning time shrank. Balloons and aircraft extended observation beyond the ship's horizon, helping commanders locate an enemy early enough to choose a fight or avoid one.
Chapter · 35:23-37:47
Why faster warships needed a longer warning horizon
Greater speed shortened the time between sighting an enemy and meeting it, increasing the value of observation from above the ship's horizon.