When do Fourth of July fireworks go off?

July 4th commemorates the Declaration of Independence of the United States, adopted in 1776, and has been celebrated with fireworks since the first anniversary, in Philadelphia in 1777. What booms that night are firework shells: cardboard spheres packed with gunpowder and stars that a mortar launches into the air. Although in Spanish they are called "cohetes" (rockets), they only share with space rockets the reaction principle and the gunpowder of their Chinese origins.

There is no official time. Major shows, like the one over New York's East River or the one on the National Mall in Washington, kick off between 9:00 and 10:00 p.m. local time, once the sky is dark enough, and last 15 to 30 minutes. In the northern part of the country, where July nights fall late, they're pushed back toward 10:00 p.m.; in the south they can start earlier. Private fireworks, legal in many states, go off from mid-afternoon until well past midnight, and in some cities also on the days before.

A shell rises in two or three seconds to 100-300 m and bursts. The light arrives instantly; sound travels at about 340 m/s, so it arrives with a delay of one second for every 340 m separating us from the burst: at one kilometer, about three seconds. The deepest booms come from the largest-caliber shells, whose bursting charge displaces more air; the dry, chained bangs are "salutes," charges designed to make noise rather than color. A professional show launches hundreds or thousands of shells over 20 to 30 minutes, with calibers from 75 to 300 mm.