A measurement of speed relative to the speed of sound; Mach 1 means traveling at the speed of sound.
Named after Ernst Mach, an Austrian physicist who studied supersonic motion in the late 1800s. The term became standard in aeronautics to describe high-speed flight.
Mach numbers make more sense than miles per hour for jets because the speed of sound changes with temperature—at sea level it's about 761 mph, but at high altitude it's slower. When pilots say they're flying Mach 2, anyone instantly knows they're going twice the speed of sound, no matter the conditions.
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