Knots Vs Miles?

  In most anything people deal with daily, speed is given usually in miles per hour.  We have speed limits in mph, we talk about running in mph, bikes, motorcycles, etc everything is in mph. 

The only two forms of transportation that do not use miles per hour are marine vessels such as ships and airplanes. They record speed in knots per hour.

There is an interesting reason why sailors use knots.  Its because they didn't have any other way of figuring distance.  One knot is a nautical mile which is 1.852 kilometers.  The knot comes from tying knots in a really long rope every 14.4 meters.  They would tie one end of the rope to the ships stern while they'd tie a wooden board to the other end.  The board would be dropped into the water.  A sailor has a 30 second timer and would watch it, while the other sailor counted knots that slipped through is figure.  They took regular readings to tell how fast they were going.  Over time, they declared that one nautical mile is equal to one minute of latitude or 1.15 land miles per hour. 

Now we know why they use knots for maritime travel but why is wind speed in knots rather than miles per hour?  Wind is made of air and is classified as a fluid just like water so both are measured in knots.  In addition, wind actually pulses rather than flowing steadily so knots give a more accurate reading. 

From what I can tell, many airplanes list speed in both knots and miles per hour depending on when it was built.  It also appears that airspeed is listed  in knots or nautical miles per hour while ground speed is in miles per hour.  Air traffic controllers expect speeds to be given in knots when a pilot calls in for information and the automatic weather information radio broad casts give wind speed in knots.

Now if you'd like to include some activities on this topic, here are two suggestions.

1. NASA has an activity which requires students to convert from knots to miles per hour and back again.  This is a nice way to introduce the converstion.

2. This site uses information on the USS Mauretania from the turn of the 20th century when it cossed the Atlantic in under 5 days.  The activity has students calculate how much income was generated through ticket sales, calculate the average speed in knots and miles per hour, statute miles, and kilometers.  Lots of nice conversions.

Why did I choose this topic today?  I wondered why there is a difference in speed choices for marine and land vehicles.  I thought it was interesting.  Have a great day.  Let me now what you think, I'd love to hear.