Time for coffee and a scone. As I step into Al’s coffee shop he’s taking his Jupiter poster down from behind the cash register.

“Hey, Al, I liked that poster. You decide you prefer plain wall?”

“Nah, Sy, I got a new one here. Help me get it up over the hook.”

A voice from behind us. “Ya got it two degrees outta plumb, clockwise.” Vinnie, of course. Al taps the frame to true it up.

“Hey, Sy, in the middle, that’s the same seven units we just finished talking about — amps for electric current, kelvins for temperature, meters for length, kilograms for mass, seconds for time, moles for counting atoms and such, and that candela one you don’t like. What’s all the other bubbles about? For that matter, what’s the poster about, Al?”

“What it’s about, Vinnie, is on May 20 the whole world goes to a new set of measurement standards, thanks to some international bureau.”

“*Le Bureau International des Poids et Mesures.*” It’s Newt Barnes in from the Physics building. “The bubbles in that central ring are the BIPM’s selections for fundamental standards. Each one’s fixed by precisely defined values of one or more universal physical constants. For instance, a ruler calibrated on Earth will match up perfectly with one calibrated on Mars because both calibrations depend on the wavelength of radiation from a cesium-based laser and that’s the same everywhere.”

“How about the other bubbles and the rings around them?”

“They’re all derived quantities, simple combinations of the fundamental standards.”

“Hey, I see one I recognize. That °C has gotta be degrees centigrade ’cause it’s right next to kelvins. Centigrade’s the same as kelvins plus , uh, 273?”

“There you go, Al. What’s ‘rad’ and ‘sr’, Newt?”

“Symbols for *radian *and *steradian*, Vinnie. They both measure angles like degrees do, but they fit the BIPM model because they’re ratios of lengths and length is one of the fundamentals. Divide a circle’s circumference by its radius and what do you get?”

“Twice pi.”

“Right, call it 2π radians and that’s a full circle. Half a circle is π radians, a right angle is π/2 radians and so on. Works for any size circle, right? Anyone remember the formula for the area of a sphere?”

“4π*r*^{2}, right?”

“Exactly. If you divide any sphere’s area by the square of its radius you get 4π steradians. Any hemisphere is 2π steradians and so on. Steradians are handy for figuring things like light and gravity that decrease as the square of the distance.”

Something occurs to me. “I’m looking at those bigger bubbles that enclose the derived quantities. Seems to me that each one covers a major area of physical science. The green one with *newtons *for force, *pascals *for pressure, *joules *for energy and *watts *for power — that’d be Newtonian physics. The red circle with *volts *plus *coulombs *for charge, *ohms *for resistance, *farads *for capacitance, *siemens *for electrical conductance — all that’s electronics. Add in *henries *for inductance, *webers *for magnetic flux and *teslas *for flux density and you’ve got Maxwellian electromagnetism.”

“You’re on to something, Sy. Chemistry’s there with *moles *and *katals*, also known as moles per second, for catalytic activity. How does your idea fit the cluster attached to seconds?”

“They’re all per-second rates, Newt. The *hertz *is waves per second for periodic things like sound or light-as-a-wave. The other three are about radioactivity — *bequerels *is fissions per second; *grays *and *sieverts *are measures of radiation exposure per kilogram.”

“Vinnie says you don’t like *candelas*, so you probably don’t like *lumens *or *luxes *either. What’s your gripe with them?”

“All three are supposed to quantify *visible* light from a source, as opposed to the total emission at all wavelengths. But the definition of ‘visible’ zeros in on one wavelength in the green because that’s where most people are most sensitive. Candelas aren’t valid for a person who’s color-blind in the green, nor for something like a red laser that has no green lightwaves. I call bogosity, and lumens and luxes are both candela-based.”

“These 29 standards are as good on Mars as they are here on Earth?”

“That’s the plan.”

~~ Rich Olcott