There are many instruments used to measure length including rulers, tape measures, micrometers, gauges and vernier calipers. There are also instruments that make use of scientific principles to deduce length where it may not be directly measured. Measuring lengths accurately has been a driving force behind much of human civilization.
Measuring the length of waves of light, sound, X-rays and radio waves is accomplished by analyzing the composition of the waves using scientific techniques that can be incorporated into the design of instruments. Measuring distances in outer space requires instruments that make use of radar, parallax, cepheids, supernovae, red-shift and Hubble's Law. In 225 B.C., Eratosthenes was the first person to accurately measure the circumference of the Earth. He did this by comparing the lengths of shadows in two different cities on the summer solstice and applying geometric principles.