# Why nautical miles?

Why does NASA use nautical miles when talking about spaceship take-off? We don’t when we talk about distance between planets, etc. Airlines use statute miles when they give distance between cities and when the captain tells passengers how fast they’re flying.
I don’t recall which measure private pilots use to measure airspeed.
Peace,
mangeorge

Airlines don’t give distances in nautical miles to the passengers, because most people wouldn’t want to bother with converting to statute miles (even if they know the conversion). Airline pilots use nautical miles. Private pilots use nautical miles for navigation, though they use statute miles for weather minimums.

Since space flight is a realm of ‘flight’, it makes sense to use nautical miles.

EDIT: My dad’s 1970 Cessna 172K’s airspeed indicator read in miles per hour. Newer aircraft all seem to use knots.

A nautical mile is one minute of arc of latitude along any meridian. In other words, any polar circumference of the world at sea level is 21,600 nautical miles. It is exactly 1852 metres or juast over 1.15 statute miles.

Since space flight tends to deal with earth orbit, and the Navy was important in early aerospace research, the measurement is more useful than any other kilometer-range unit.

I’d forgotten that bit. It’s a better answer than ‘That’s the way pilots and sailors do it.’

Up to a point. The length of a minute of arc chenges from the equator to the poles and the nautical mile has been defined by convention as 1852 m which is close to the average.

It doesn’t change for latitude, only for longitude, no?

There are small changes in the length of a degree of latitude with a degree of latitude at the equator being slightly shorter than a degree of latitude at the poles (59.7 nm and 60.3 nm respectively.) The difference is due to the non-spherical shape of the Earth.

Latitude calculator.

Ah, I see.

Those fluctuations are important to note for precision measurement and cartography, but to cut a quick fix in navigation they don’t matter much - they are on the order of the thickness of the pencil lead on most charts. We happily placed our dividers on the latitude markings to get a distance reading, and this never caused a problem in the years that I was active-duty Navy.

Also, nautical miles work much better than statute miles for relative and absolute motion calculations without computers - you can use the old familiar three minute, six minute and sixty minute rules.