PROVIDENCE, R.I. — A new study finds the way that baseball outfielders catch fly balls is simple: by keeping their eyes on the ball.

A paper by researchers at Brown University delves into how fielders snag balls batted high in the air — a long-running question among scientists who study perception and a puzzle to sports fans who wonder how baseball greats such as Willie Mays made seemingly impossible catches.

The researchers asked varsity baseball and softball players from Brown to catch virtual balls in a virtual reality lab. Players wore special goggles that allowed them to watch simulated fly balls and ran around the 40-by-40-foot lab trying to catch them.

Some of the balls they caught followed trajectories that were physically impossible, allowing researchers to sort out what figures into a player's calculation when going after a fly ball.

One theory was that players predict where a fly ball will land based on its trajectory, said professor William Warren, who co-wrote the paper, published last month in the online Journal of Vision.

Instead, the researchers discovered that players watch the ball and position themselves so that it appears the ball is neither speeding up nor slowing down, he said. If the ball appears to be speeding up, the player should move back, and if it's slowing down, the player should move forward, said Warren, chair of Brown's Department of Cognitive and Linguistic Sciences.

"It's actually very simple," he said.

The results confirm the findings of a 2008 study that found soccer players use a similar technique when attempting to head the ball, according to Peter McLeod, a retired professor of psychology at Oxford University, who co-authored that paper. McLeod calls it "the mystery of catching" and says that, put another way, players running to catch a fly ball are making sure the angle of their gaze increases the closer they get to it.

"It's at its highest just before they catch the ball," he said.

Warren said the Brown research, which was funded by the National Institutes of Health, eventually could be used in robotics or to help people with vision problems. But it also tells us something about what makes someone as good as Mays, which could be that top-notch outfielders are especially sensitive to this variable, Warren said.

Still, even science can't explain some of baseball's most spectacular catches, such as Mays' famous grab in Game 1 of the 1954 World Series between the New York Giants and the Cleveland Indians. In that game, a sprinting Mays caught a fly ball hit by the Indians' Vic Wertz deep into center field — over his shoulder and without looking.

"He took his eye off the ball and managed to catch it anyway," Warren said. "Perhaps luck is what made it the most famous catch in history."