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Units of Length

Defining Length: Can You Please Hold the Other End of this Rule?

Length can be defined as the distance between two points.  Saying that the length of room is eight feet is equivalent to saying that the distance between two points at either end of the room is eight feet.  If someone holds a rule to measure the length of a room, the endpoint of the measurement is the mark on the rule that delineates the inches that coincides with the edge of the room.  And the beginning of the measurement is the beginning point of the rule.

Typically the length of a room is measured by using a rule and holding it at either end of the room. The beginning and end of the rule serve as points of measurement, and the length is determined by reading the inches or meters off the rule.

Uniting on a Unit

What is essential to measuring length is having a unit of measurement.  The American system uses the units of inch, foot, yard, and mile.  This is also called the imperial or U.S. system of measurement.  The units are derived from English (British) units.  This system is still in use in parts of the United Kingdom (UK), although the metric system is used in the UK for much of the measurement in scientific and industrial contexts.  Likewise, the metric system has made inroads in the United States, but the imperial units still dominate.

The European system uses the centimeter, meter, and kilometer as the fundamental units of length.  This is called the metric system, and it is one most commonly used in countries outside the United States.  The metric system originated in France in the 17th and 18th centuries.  It is now called the International System of Units and is abbreviated SI.  It is based on multiplying or dividing its basic units by ten and its powers.

The Importance of Establishing a Standard of Measurement

While the units of length vary from one society to another, they are derived in a similar way.  A standard of length serves to define a unit of length, and this unit is used to measure length

In order to establish a unit of measurement, whether it is an inch, a foot, a meter, or any other measurement unit, the unit has to be defined in terms of some known standard.  Unless the unit is defined in terms of a standard, it will not have a reference and cannot be used as a unit of measurement.  The standards for the units of measurement we use today have changed many times over the centuries.

U.S. Standard and the Evolution of English Standards

In ancient times, units of measurement were defined in terms of parts of the body.  This was convenient in that it provided a standard readily available to all.  The obvious drawback was that these units varied from one person to another.

It is generally accepted that the standard for a foot was the length of the average man’s foot, although the 12-inch foot as defined today is somewhat longer the length of the modern human foot.  The inch was commonly defined in terms of the width of a man’s thumb.  The term ‘yard’ was derived from the Old English word ‘gyrd’, meaning “rod” or “measure.”  The yard came to be known as the distance from the nose to the end of the middle finger of the out-stretched hand.  In the 12th century, King Henry I decreed the yard to be the distance from his nose to the tip of his out-stretched thumb.  This closely approximates the distance we call a yard today, which is three feet.

The definition of a foot as consisting of 12 inches goes back to Roman times.  When the Normans conquered England in 1066, they brought with them the Roman tradition of measurement of using a foot with 12 inches.  At some point there was an attempt to standardize the length of a foot.  Some attribute this to Henry I, who reigned from 1100 to 1135.  Later in the 1100s, a “foot of St. Paul’s,” which was similar to our modern foot, was inscribed at St. Paul’s church in London at the base of a column so all could see.  During this time the idea arose of defining the length of a foot as being 1/3 yard, making the yard the primary unit of measurement.  This idea appears to have come from Henry I, who is said to have ordered the construction of three-foot standards, called yards. 

Units of Length: One Furlong and a Doorway

The following are some common units of length measurement that you may encounter in a variety of contexts.

Egyptian cubit – The length of the arm from the elbow to the extended finger (about 3000 BC)

Ancient Greek foot – The width of 16 fingers

Ancient Roman inch – The Greek foot divided into 12 sections (the unicae)

Yard – Decreed by King Henry I in the 12th century who ordered a yard to be constructed to be the length of three feet

The distance between the end of your thumb and the first joint is about one inch

The width of a standard doorway is one yard

1 meter = 3.28 feet

1 inch = 25.4 millimeters

1 foot = 0.3048 meters

1 mile = 5,280 feet

1 mile = 1760 yards

1 mile = 1.609344 kilometers

1 kilometer = 0.62137 miles

1 yard = 914.4 millimeters

1 yard = 0.9144 meters

1 nautical mile = 1.150779 miles

1 furlong = 660 feet

1 furlong = 7920 inches

1 centimeter = 0.3937 inches

1 inch = 2.54 centimeters

1 centimeter = 10 millimeters

1 meter = 100 centimeters

1 kilometer = 1000 meters


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