Tuesday

Rocket paces off the Field. "Ten of my steps," he announces. "The Field is ten steps long."

Raven paces it too and gets twelve. "What do you notice? Same Field, different number."

"Your steps are shorter," Rocket says. "So whose number is right?"

Nova hovers between them. "A hundred years from now, who would know how long a Rocket step was?"

Rocket laughs. "Nobody. We need a unit that does not change."

Raven pulls out the tape measure and stretches it along the grass. "Metres. Everyone on Earth agrees on a metre."

"And seconds for time," Rocket adds. "Then our numbers mean the same thing to anybody who reads them."

NIST, the national measurement institute of the United States, describes the International System of Units, or SI. People often call it the metric system.

NIST says the SI is the international standard for measurement. It is built on seven base units.

Three of them matter most at the Field. Length is measured in metres, time in seconds and mass in kilograms.

Other units are built from the base units. Speed in metres per second comes from the metre and the second.

The Treaty of the Meter was signed in Paris on May 20, 1875 by seventeen countries, including the United States.

The Next Generation Science Standards put it plainly. Positions and motions must be described in a chosen reference frame and chosen units.

To share information with other people, those choices must be shared too. A Rocket step is not a shared unit.

NIST explains how the units stayed the same. For more than a century, the kilogram was a polished platinum and iridium cylinder cast in 1879.

Until 1960, the metre was passed on using platinum and iridium bars. Today the second is counted by atomic clocks.

An atomic clock counts 9,192,631,770 tiny vibrations of cesium atoms. That stretch of time is one second.

Good thinking about units. Tomorrow is Field Lab: you will time walks, jogs and rolls over the same distance.