Length

Length is how far it is from one end of something to the other. It is one of the most basic things people measure. The length of a pencil, the height of a tree, the distance between two cities: all of these are length.

People measure length using units. In the United States, the most common units are inches, feet, yards, and miles. Twelve inches make one foot. Three feet make one yard. A little over 5,000 feet make one mile. Most other countries use the metric system, with millimeters, centimeters, meters, and kilometers. One meter is a little longer than one yard.

For a long time, people measured length using parts of their bodies. A "foot" really did start as the length of a person's foot. A "cubit," used in ancient Egypt, was the distance from a person's elbow to the tip of the middle finger. The problem was obvious. Different people had different bodies, so the same wall could be 20 cubits to one builder and 22 cubits to another.

Today, scientists define units very carefully so that everyone gets the same answer. The meter is now defined using the speed of light. Light travels one meter in about one 300-millionth of a second. That is a strange way to measure a tabletop, but it gives the world one length that never changes.