A Nautical Mile is 1 minute of arc of LATITUDE.

Latitude is the key, ignore Longitude. From the globe, you can see that lines of Longitude start together, fan out over the equator and converge again at Antarctica. Because the distance between them varies they are NEVER used for measurement of any kind, they simply provide the horizontal grid reference point.

Lines of Latitude (the horizontal lines running around the globe) are at a constant spacing from N090.00.00 to S090.00.00. It is this constant distance between them that is the key to navigational measurement.

A nm is 6076.11549 feet or 1852 meters

A deg of Latitude is 60nm (think of it as an 'hour')
A min of Latitude is 1 nm (60 mins in an 'hour')
A sec of Latitude is 101.3ft (60secs in a min)
A Knot is 1nm per hour.

So when you are flying visual and you want to know the distance between two points, you get out your dividers, pin each end to the points en route and then check the distance against the scale that runs Nth/Sth(vertical) ... that's the correct distance. Just for fun, measure the same dividers spacing on the east west (horizontal) scale and it will be quite different and very inaccurate.

All aerontautical maps show the convergence of Longitude towards the poles, Atlases sometimes don't. These are called different "projections" they have names like Mercator(shows a squares in an atlas) and Lambert. The Lambert Conical Projection is the one with the lines all converging top and bottom and is what HAS to be used for navigational purposes.