Depth 1 · 37 s
One system for everyone
Every country kept its own copy of the metre, and copies drift. So in 1875 seventeen countries agreed to share one set of standards, and in 1960 the system got a name, the SI - built on base units like the metre, kilogram, second and kelvin. In 1999 a spacecraft was lost at Mars because one team used pounds and the other SI.
About this video
Is a metre in Japan the same as a metre in Brazil or Germany? For much of the 1800s each country kept its own copies of the standards, and copies can slowly drift apart. In 1875, at the Metre Convention in Paris, 17 countries agreed to share one set of standards. In 1960 the shared system got its name, the International System of Units, or SI. It is built on a few base units, such as the metre for length, the kilogram for mass, the second for time and the kelvin for temperature. The kelvin starts at absolute zero, the coldest anything can be, and its steps are the same size as degrees Celsius.
This animation ends with what happens when people don't share units. In 1999 NASA's Mars Climate Orbiter reached Mars, but one team's software gave its numbers in pounds while the other team expected SI units. The spacecraft flew about 60 km above the surface instead of about 150 km and was lost. One system means everyone's numbers mean the same thing.
This video shows depth 1 of The SI system.
Learn The SI systemTranscript
- 0:00A metre in Japan, Brazil, Germany. Is it the same metre?
- 0:04Each country kept its own copy, and copies can drift apart.
- 0:091875, Paris: 17 countries agree to share one set of standards.
- 0:13In 1960 it got a name, the SI, built on base units like these.
- 0:18The kelvin starts at the coldest anything can be, with Celsius-sized steps.
- 0:231999, Mars: one team used pounds, the other SI units.
- 0:27The spacecraft flew far too low and was never heard from again.
- 0:31One system, so everyone's numbers mean the same.