Knots, mph, and why Mach 1 isn't a fixed speed

A car goes 60. A ship goes 20. A plane cruises at Mach 0.85. Three different speed units in three different contexts, and only one of them (mph or km/h, depending on where you are) has anything to do with the roads most people drive on. The other two exist because cars, ships, and planes solve completely different navigation problems.

Why ships and planes use knots

A knot is one nautical mile per hour, and the nautical mile itself is the important part: it's defined as one minute of latitude, 1/60th of one degree, measured along a meridian. Since the Earth is divided into 360 degrees of latitude, each with 60 minutes, that makes the planet's circumference exactly 21,600 nautical miles by definition, a clean, round number specifically because the unit was built around the planet's own geometry rather than an arbitrary stick length. That matters for navigation in a very direct way: a ship or plane that travels one nautical mile has moved exactly one minute of latitude, which makes plotting a course from lat/long coordinates dramatically simpler than it would be in statute miles. It has nothing to do with the mile used on land, and it isn't meant to.

1 knot ≈ 1.15 mph ≈ 1.852 km/h
1 nautical mile = 1 minute of latitude ≈ 1.852 km

mph vs km/h is really just geography

Almost the entire world posts speed limits in km/h. The holdouts, the US, the UK (for road signage specifically, despite being otherwise metric), and a small handful of others, use mph for the same reason feet and pounds stuck around: those systems were already built into infrastructure, driver training, and vehicle speedometers before metrication efforts had any real momentum, and nobody forced the switch. It's not a difference in accuracy or usefulness, just a difference in which system got locked in first.

Mach 1 isn't actually one fixed speed

This is the one that surprises people: Mach 1, the speed of sound, is not a constant number of mph or km/h. Sound travels through air as a pressure wave, and how fast that wave moves depends on the air's temperature, not its density or altitude directly. Colder air means a slower speed of sound. At sea level on a standard day, Mach 1 works out to roughly 767 mph. At a typical airliner's cruising altitude of around 35,000 feet, where the air is much colder, Mach 1 drops to around 660 mph. A plane cruising at a genuinely constant airspeed would actually see its Mach number change as it climbs or descends, purely from the temperature shift, with no change in how fast it's physically moving.

That's why pilots track both numbers separately: true airspeed for the physical speed through the air, and Mach number for how close they are to the local speed of sound, which matters enormously for aerodynamics near the transonic range regardless of what the mph gauge says.