The basic unit of length is 1 m. The standard for units of length was once determined by the length of a person's stride or arm. Along with advances in science and technology came the need for a more accurate standard. In 1799, the French adopted a standard based on the size of the earth. They defined 1/10 millionth of the distance from the North Pole to the Equator as
"1 meter."

This is called the "standard meter." These days the standard meter is being redefined using the fact that light travels in a straight line at a constant speed. Accordingly, 1 meter is now defined as the distance light travels in 1/299.792458 million seconds. This definition allows us to use light to accurately measure the distance of remote objects. We can use a laser beam to measure the distance to the moon with extremely high accuracy (error of 30 cm or less). One second, the basic unit of time, is defined by the wavelength of a laser produced by cesium 133 atoms, but laser beams and light from space can also be employed for accurate time measurements.