Why do boats use nautical miles and why are they different than normal miles? Couldn't you just convert it to normal miles or km?

[LittleSister]

Or if more than 1 mile it may be described as a mile and a bit!

Only by the lay person.

The cognoscenti would distinguish between a 'fair mile', a 'goodly mile' and a 'long mile'. The differences can matter, especially if it's uphill, a hot day, and closing time is imminent!

 

[Malabar]

Definition of a nautical mile is a pillar of celestial navigation. I sense a degree of confusion when having to take into account the measurement of nautical miles takern from a mercator chart. Then, beacuse of the chart's projection then indeed the NM does vary in measured distance according to your latitude.

 

[AntarcticPilot]

Definition of a nautical mile is a pillar of celestial navigation. I sense a degree of confusion when having to take into account the measurement of nautical miles takern from a mercator chart. Then, beacuse of the chart's projection then indeed the NM does vary in measured distance according to your latitude.

It also varies slightly on the earth's surface because the earth is not a sphere, but is more closely described by an ellipsoid. However, changes in the length of a minute of arc in latitude are well below the precision of navigation!

The change is that the nautical mile is 19m longer at the pole than at the equator.

 

[B27]

French Nautical mile, 19th century, 10,000km /(90x60)= 1851....m
UK admiralty mile 6080 feet
US Navy mile 6080.2 feet
International nautical mile 1852 meters. Exactly. Everywhere. Even when we've changed the definition of a metre

The polar and equatorial radii of the earth are only 1 part in 300 different, the circumferences are only 1 part in 588 different so it doesn't make much difference at sea level.

 

[AntarcticPilot]

French Nautical mile, 19th century, 10,000km /(90x60)= 1851....m
UK admiralty mile 6080 feet
US Navy mile 6080.2 feet
International nautical mile 1852 meters. Exactly. Everywhere. Even when we've changed the definition of a metre

The polar and equatorial radii of the earth are only 1 part in 300 different, the circumferences are only 1 part in 588 different so it doesn't make much difference at sea level.

19m, as stated above.