In New York City's Times Square celebration, the descending ball symbolizes the new year arriving at midnight.
The ball reaches the bottom of its pole at the instant the calendar changes from December 31 to January 1. The visual countdown turns an abstract time change into a shared public event, with crowds and broadcasters following the final minute together.
The tradition began in 1907 after fireworks were prohibited for the newspaper-sponsored celebration. The first ball was a practical illuminated device, but later versions became increasingly elaborate. Today the sphere is covered with Waterford crystal panels and lit with programmable LEDs.
The ball is sometimes described as a crystal ball, but that wording can be misleading: it is a large geodesic-style sphere assembled from illuminated panels. The event is also commonly called the ball drop, although the ball is lowered rather than released into free fall.