Knot to Mile per Second

One nautical mile equals one arc-minute of latitude anywhere on Earth. This means that at any position, a navigator can directly read distances from a chart's latitude scale without conversion. At 60 knots, for example, you cover 1 degree of latitude per hour. No equivalent mathematical elegance exists for km/h or mph, making knots uniquely convenient for celestial and GPS-assisted navigation.

The Soviet Navy's Alfa-class submarines could sustain about 44 knots submerged. On the surface, experimental high-speed craft have gone faster: the Spirit of Australia set a water speed record of 317.6 knots (588 km/h) in 1978. Modern destroyer escorts cruise at 28–34 knots. The USS Gerald R. Ford aircraft carrier sustains over 30 knots despite displacing 100,000 tonnes.

Sailors used a "chip log" — a wooden panel attached to a rope with knots tied every 47 feet 3 inches (14.4 m). The log was thrown overboard and the rope allowed to run freely for 28 seconds (timed with a sand glass). The number of knots that passed through a sailor's hands equalled the ship's speed in nautical miles per hour. The 47-foot 3-inch spacing and 28-second interval were calculated to give a 1-to-1 ratio with the nautical mile.

"Knots per hour" is a common mistake — since a knot already means nautical miles per hour, saying "knots per hour" is like saying "miles per hour per hour," which is acceleration, not speed. The correct phrase is simply "knots" or "20 knots" not "20 knots per hour." This is a persistent error even in media reporting, as the phrase rhymes well and sounds natural.

The HSC Francisco, a high-speed catamaran ferry operating between Buenos Aires and Montevideo, reaches 58 knots (107 km/h) — the world's fastest commercial passenger vessel. Most transatlantic container ships cruise at 20–25 knots for fuel efficiency. During the Blue Riband era of ocean liner competition, ships like the SS United States set crossing records at 35+ knots in 1952, a record that still stands.

The speed of light in a vacuum is exactly 299,792,458 m/s, which converts to approximately 186,282.397 mi/s. Before the SI system was globalised, US and British scientific texts routinely used this figure. The value 186,000 mi/s appears as a rounded approximation in older textbooks. Today, physicists universally use c = 3 × 10⁸ m/s or the exact SI value.

The solar wind — a stream of charged particles ejected from the Sun's corona — travels at roughly 250–500 mi/s (400–800 km/s) depending on whether it is the slow or fast component. During coronal mass ejections, bursts can exceed 1,200 mi/s (2,000 km/s). At 500 mi/s the solar wind crosses the 93-million-mile Earth–Sun gap in about 2 days, which is why space weather forecasters can give roughly 48 hours' notice before a geomagnetic storm hits.

Yes — several. NASA's Parker Solar Probe reached about 430,000 mph (119 mi/s or 192 km/s) at perihelion, making it the fastest human-made object ever. High-powered railgun projectiles tested by the US Navy reached around 2.5 km/s (1.5 mi/s). Intercontinental ballistic missiles (ICBMs) reach approximately 4 mi/s during their boost phase.

Exactly one second — that's the definition. At 1 mi/s you would cross the Atlantic Ocean (about 3,400 miles) in under an hour, reach the Moon (238,855 miles) in about 2.8 days, and reach the Sun (93 million miles) in roughly 12 days. The speed is conceptually useful for calibrating astronomical distances in imperial terms.

Rarely. The main residual use is in some US military ballistics documents and historical astrophysics references. The International Astronomical Union and NASA primarily use km/s for planetary and solar system speeds, and AU/year or c (fraction of light speed) for interstellar scales. Mi/s survives mostly in popular science writing aimed at American audiences unfamiliar with metric.