It is said that the aircraft is flying at Mach 1 if its speed is equal to the speed of sound in air (which is 332 m/s or 1195 km/hr or 717 miles/hour.) An aircraft flying at Mach 2 is flying at twice the speed of sound in air, etc. Mach numbers are named after Ernst Mach (1838-1916), an Austrian philosopher and physicist. The term Mach number came into use in 1929.

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