Sky Lights

How NASA got lost in space over meters, feet, newtons, and pounds

Wednesday, March 01, 2000
RELATED TAGS: MATH




Last September, Americans got a nasty reminder of how the rest of the world measures weights and distances. NASA's $165 million Mars Climate Orbiter stumbled disastrously off course because of a small mistake: One team of flight controllers had programmed the spacecraft with English units (pounds and feet), while the other had entered numbers using a metric scheme (newtons and meters).

Back in 1964, the U.S. National Bureau of Standards officially endorsed the metric system, with its convenient multiples of ten. Not much happened. In the late 1970s, President Jimmy Carter pushed for national metrication. Not much happened again. Scientists and technologists, not to mention auto repair shops--deal in centimeters, as they have for years, while everyday life still chugs along inch by inch.

The resistance to going metric is not just laziness. America's common English units often fall into convenient sizes that do not have simple equivalents. There's no easy metric substitute for a cup or a gallon--who wants a 3.785-liter milk container? A shot of whisky rolls off the tongue, but three cubic centimeters doesn't. Our customs, from 45-caliber pistols (0.45-inch-diameter barrel bore) to recipes calling for tablespoons, are awkward at best to metrify.

Scientists sometimes blame the media, including Discover, for perpetuating antiquated and irrational units of measure, but the press needn't apologize. The meter is not especially logical or meaningful either. Originally it was supposed to represent 1/ 10,000,000 of the distance between the equator and either pole. Then it was redefined by a particular bar of platinum in a vault in France and later specified as 1,650,763.73 wavelengths of the orange light emitted by krypton-86 atoms under certain conditions. These days it's designated as the distance light travels in a vacuum in 1/ 299,792,458 of a second--not exactly a household concept.

Fahrenheit, with its 180 divisions between water's freezing and boiling points, is nearly twice as fine as the Celsius system. One degree Fahrenheit is about the smallest temperature differential a person can discern. And the 180-degree theme is familiarly repeated in surveying, geometry, longitude, and astronomical positioning.

Problems arise not from the choice of units but from our stubborn inconsistency. The aviation community insists that altitudes be expressed in feet, but that speed be in knots. Astronomers use 24 hours of "right ascension" to mark east-west positions but degrees of "declination" for north-south. Metric measurement is dumped overboard for jewelry (carats), sound measurement (decibels), and wherever people prefer the simplicity of the status quo.

The issue of scientifically correct terms goes beyond the English versus metric battle. Astronomers, for example, have replaced the easily understood "light-year" with mind-numbing "parsec." A parsec is an arcane unit, 3.26 light-years long, defined as the distance at which an object would appear to shift back and forth by an angle of one arc-second as Earth orbits the sun. The light-year is a logical unit that depends on the amazing constancy of light's speed. The Orion nebula is some 1,500 light-years away, meaning its image has traveled 1,500 years to reach us: simple and easy. Today's specialists who prefer to think of Orion as 500 parsecs away are all too reminiscent of ancient priests, drawn to obscure terminology that kept the common folk baffled.







The National Institute of Standards and Testing offers a slick history of the struggle for standardized weights and measures: museum.nist.gov/exhibits/ex1/index.html
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSC-JanFeb15
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »