A decimeter is a measurement of length based on the metric scale of measurements, and equals one tenth of a meter. The exact length of a meter has changed over time, and was initially based on the distance between the Earth’s equator and the North Pole. Since the 1960s, however, the length of a meter has been based on light, with initial measurements using the wavelength of a particular element and newer measurements based on the speed of light. A decimeter therefore can vary in length depending on what standard is used, but in relation to other measurements in the metric system it remains the same.
The metric system, in which measurements such as the meter and decimeter can be found, is based on the idea of creating a system of measurements that can easily utilize decimals. Other systems of measurement, such as the American standard of feet and inches, do not always break down using a system of tens or hundreds. The inch is one-twelfth of a foot, for example, and other measurements are often based on a half-inch, quarter-inch, and further halving of the distance. Metric measurements were developed to create a standard that could easily be divided and multiplied further using a base-ten approach, using prefixes like "deci-" and "centi-" to indicate one tenth and one hundredth.
Initial measurements used to establish the length of one meter, on which the decimeter is based, were based on the distance from the Earth’s equator to the North Pole. A team of scientists was charged in the 1790s with determining this distance for use in establishing the length of one meter. Once they had this measurement, then the total length was used to establish the length of a meter as one ten-millionth of this distance. The shape of the Earth is not a perfect geometric shape, however, and so this distance was not calculated perfectly, and the actual circumference of the Earth is just over 40 million meters (over 131 million feet).
Since these initial measurements, the length of a meter and related measurements like the decimeter have been revised several times. The first major change came in 1960 when the length of a meter was defined based on the wavelength of light from an atom of krypton-86. Due to certain variables that can cause imperfections in measuring light wavelengths, however, this was again changed in 1983 to determine the length of a meter and decimeter based on the speed of light. The official length of a meter since 1983 is the distance that light travels in 1/299,792,458 of a second in a vacuum; a decimeter is then one tenth of this length.