Nautical Mph/Knots

Date: 08/12/2003 at 19:28:07
From: Gretchen
Subject: Nautical mph/knots

I want to know how fast the wind is blowing when the weather report 
states 5 knots.

6,076 feet per hour (nautical)
5,280 feet in a mile (land)

Date: 08/12/2003 at 21:08:37
From: Doctor Warren
Subject: Re: Nautical mph/knots

Hi Gretchen,

The 'nautical mile' or 'knot' is a unit used in air and sea travel all
over the world.  It has a fairly ingenious definition:

   A NAUTICAL MILE is 1/60th of 1/360th of the distance around the
   Earth at its equator.

The equator is a circle, and all circles are composed of 360 degrees.
That's where the 1/360 factor comes from. There's also a standard unit 
called the 'arcminute' (sometimes just called a minute) that is 1/60th 
of one degree. That's where the 1/60 factor comes from.  One nautical 
mile is therefore one minute of arc along the Earth's equator.

It happens that the nautical mile is very close to the statute (land)
mile. The statute mile is 5,280 feet, while the nautical mile is 6,076 
feet. Therefore, 1 nautical mile is approximately equivalent to 1.1508 
statute miles, as can be seen here:

                   6,076 feet    1 st. mile
   1 naut. mile * ------------ * ---------- = 1.1508 stat. mile
                  1 naut. mile   5,280 feet     

where 'naut.' is an abbreviation for 'nautical' and 'st.' is an
abbreviation for 'statute.'

In practice, when someone uses nautical miles, they just call them
knots, and when someone uses statute miles, they just call them miles.
To make matters a little more confusing, people also refer to speeds
like '40 nautical miles per hour' as simply '40 knots.'  It is more
customary, however, to express speeds in statute miles specifically as
'miles per hour.'

If the weatherman says the wind is blowing at 5 knots, he really means
"5 nautical miles per hour," and you just have to multiply by 1.1508
to obtain the wind speed in statute miles per hour:

   5 knots * 1.1508 miles per knot = 5.754 miles per hour

Let me know if you have any more questions!

- Doctor Warren, The Math Forum
  http://mathforum.org/dr.math/