If someone tells you to "keep your eye on the ball," they're asking you to stay focused on what matters. But what ball are we talking about? One popular story takes us to a harbor. An early example of the phrase takes us to a game.
The ball sailors watched
In the 19th century, sailors needed accurate clocks to work out where they were at sea. In 1833, the Royal Observatory at Greenwich began using a large sphere called a "time ball" to help ships check their clocks. The ball was raised and then dropped at a set time each day. Sailors could watch it fall and check their clocks against the signal.
It's easy to see why some people connect the saying to this practice. The sailors really did have to watch a ball closely. But the existence of the time ball doesn't show that sailors used the phrase or that it came from them.
The ball players watched
A clearer clue comes from William Henry Giles Kingston's 19th-century novel Ernest Bracebridge. In one scene, a boy named Ellis is playing rounders, a game similar to baseball. He remembers a friend's advice, keeps his eye on the ball, and sends it flying with a good hit.
The words are literal in that scene, but it's easy to see how the advice could travel beyond the game. A batter who looks away may miss the ball. Away from the field, you can lose track of what matters just as easily. The sports example doesn't prove where the saying began, but it shows how naturally "watch the ball" could come to mean "stay focused."