Amateur astronomers scattered along a shadow's predicted path point stopwatches at the sky, time an asteroid blinking out a distant star to a fraction of a second, and stitch hundreds of those timings together into the only accurate outline of that asteroid's shape anyone has ever drawn.

Amateur astronomers scattered along a shadow's predicted path point stopwatches at the sky, time an asteroid blinking out a distant star to a fraction of a second, and stitch hundreds of those timings together into the only accurate outline of that asteroid's shape anyone has ever drawn.

Citizen astronomers with stopwatches and folding chairs are mapping asteroids with precision that rivals billion-dollar space missions—by simply timing how long a star disappears.