The video series “Measured World”, dedicated to the 150th anniversary of the Metre Convention, continues with a new and exciting episode. This time Jako Siim Eensalu, a researcher in the field of length at Metrosert, takes us on a journey through the history of the metre, from the French Revolution to the speed of light.
The metre, one of the base units of the SI system, was born during the French Revolution in 1790. At the time, the goal was to create a system of units based on natural phenomena rather than the whims of rulers. The metre was initially defined as one ten-millionth of the distance from the North Pole to the equator along the Paris meridian.
Finding this exact measure, however, proved to be quite a challenge. The Earth is not a perfect sphere but an oblate spheroid, which caused inaccuracies in meridian measurements. Problems also arose from the different measurement systems used by the French and the British.
The proof of the constancy of the speed of light in the early 20th century, combined with the ultra-precise timekeeping of atomic clocks, eventually linked the definition of the metre with quantum physics. This brought together everyday activities, such as measuring with a ruler or a tape measure, with cutting-edge fundamental physics, according to which a free photon travels one metre in just over three nanoseconds. In that tiny span of time, a modern computer processor can perform ten calculations.
Although today’s definition of the metre has changed, the legacy of the French Revolution still shines through. In 1983, the metre was redefined in terms of the speed of light in a vacuum, marking a significant step forward. The length of the metre no longer depends on the imperfect dimensions of the Earth but is based on a universal and immutable cosmic phenomenon that applies everywhere in the known universe.
Thanks to the centuries-old pursuit of accuracy, we now have a unified system of measurement that transcends national borders and ensures equality in how everything is measured for everyone.
The video series Measured World, dedicated to the 150th anniversary of the Metre Convention, introduces SI units, various measurement fields, and future perspectives on precise measurements. The series features metrologists and scientists who explain how accurate measurements contribute to our daily lives and the development of the world. Watch other episodes.
The series received second prize in the 2025 Science Popularization Awards in the category “Popularization of Science and Technology through Audiovisual and Electronic Media.”