My Favourite Units

Since the dawn of civilization, we have needed ways to define and quantify the world around us, to conduct trade, build homes, and explore further. To do this, we need to agree on the way we measure the world, we need to agree on the units. While we no longer use the size of the king’s nose or the number of doughnuts a pregnant duck can eat to quantify things, and instead rely on carefully defined standards sweated over by the world’s best metrologists (measurement scientists), people have invented some wild and wacky ways of figuring out how much of something there is. Here are some of my favourite weird units of measurement.


First on my list, the helen, because it made me laugh.

The helen is a reference to Helen of Troy, the “face that launched a thousand ships”. One millihelen is the amount of beauty needed to launch one ancient Greek warship.

American author David Goines has further standardized the helen scale, defining a picohelen to be the amount of beauty required to “barbecue a couple of steaks and toss an inner tube in the pool”, while a terahelen will “launch the equivalent of one quadrillion Greek warships and make serious inroads on the welfare of the galaxy”.


The beard-second is a unit of length inspired by the light-year, but is considerably shorter. It is defined as the average length a beard grows in one second. Some sources define this length to be equivalent to 10 nanometres (about the size of a virus), but Google defines a beard-second to be 5 nanometres. I think Google’s measurements of average beard growth may include women.

As stupid as it sounds, beard-seconds would actually be useful in applications related to nanotechnologies such as integrated circuits.


My sternum is about 15 beard years from my chin.


A smoot is a unit of length equivalent to the height of Oliver R. Smoot when he was a college freshman in 1958 (170 cm). As part of Smoot’s initiation to the Lambda Chi Alpha fraternity, Smoot and other freshmen were tasked with measuring the length of the Harvard Bridge by laying Smoot on the ground and using him as a yardstick.

They found the bridge to be 364.4 smoots long, plus or minus an ear. Smoot’s fraternity brothers also painted marking on the bridge at roughly 10 smoot intervals.

Smoots have turned out to be surprisingly useful. When the bridge was renovated in the 1980s, the local police asked that the smoot markings be maintained because they were useful markers for indicating the position of an accident on the bridge.

Google, of course, recognises the smoot as a unit of measurement.

Smoots make the list because Oliver Smoot himself took up a career in metrology, and even became the chairman of the American National Standards Institute, so the smoot is a unit with some pedigree.


Metrology can be a real drag.

Pirate Ninja

This is probably my favourite unit in the list. The pirate ninja is a unit of power invented by author Andy Weir for his best-selling novel The Martian.

In The Martian, Weir’s hero, astronaut Mark Watney, has to carefully monitor and balance his daily power usage if he is to survive being stranded on Mars. Watney decides that “kilowatt hours per sol” (sol = Martian day) is annoying and cumbersome to say so decides he’ll call them “Pirate Ninjas” instead, making one pirate ninja equal to one kilowatt hour per sol.

The reason that pirate ninjas are probably my favourite unit is that the Mars rover teams at the Jet Propulsion Laboratory ACTUALLY USE THIS UNIT! Taking inspiration from The Martian, instead of talking about the Curiosity rover’s power generation and usage in terms of watthours per sol, they now talk in units of “millipirate ninjas”.

All this faffing about doesn’t make much sense to me though, because one kilowatt hour per sol is just a unit of power, and could be easily be converted to watts. One pirate ninja is equal to 40.6 watts. Pirate ninjas are more fun though.


Give me 5 millipirate ninjas and I’ll shoot this rock with a laser.


The shake makes the list because, although it’s not an official unit, it is actually used in practice. Equivalent to 10 nanoseconds, the shake is a reference to “two shakes of a lamb’s tail”. It was invented during the Manhattan Project developing the nuclear bomb because 10 nanoseconds is a convenient time interval for nuclear physicists, being roughly equal to the time taken for one step in a nuclear chain reaction.

Shakes might also be useful for signal propagation time in integrated circuits.


Unlike the rest of the units on this list, the barn is an officially recognized unit. Like shakes, barns come from nuclear physics and the Manhattan project and are used to quantify the cross-sectional area of atomic nuclei. It’s a reference to “couldn’t hit the broad side of a barn” and was originally coined to prevent eavesdroppers realizing that scientists were talking about nuclear reactions, but has become a standard unit in particle physics. One barn is roughly equal to the cross-sectional area of a uranium nucleus.

The nuclear physicists also took the analogy further, referring to microbarns as “outhouses” and yoctobarns (10-24 barns) as “sheds”.


The new nucleus is 1 barn wide plus or minus an outhouse…

The measure of things

While metrologists around the world work hard to carefully define and standardize the official SI units of measurement, this list shows that convenience and practicality, and a little bit of humour, will usually determine what and how a measurement is made. While I doubt we will be measuring milk in Hubble-barns and marathon times in microcenturies any time soon, imagination and creativity will continue to supply us with weird and wacky ways to measure the world around us. Though so far my colleagues have resisted all my attempts to define the power of our lasers in eye-meltyness…

Measuring Up for World Metrology Day

This post is a modified version of a World Metrology Day article I wrote for Australia’s Science Channel.

Saturday 20th May is World Metrology Day.

I’m not surprised if you hadn’t heard of it. I was three years in to a PhD in metrology before I found out we had a day for it.

Metrology is the science of measurement. It is an important foundation of other experimental sciences and is also a critical component of a nation’s ability to conduct business. Metrology considered to comprise three different fields:

Scientific metrology focusses on the definition of units of measurement (e.g. the kilogram, the metre, etc), how to actually make the measurements, and how to reliably trace a measurement back to an official reference.

Industrial metrology is about the application of measurement to manufacturing and other industrial and social applications. Industrial metrology is a very important factor in a nation’s engineering capability.

Legal metrology concerns the statutory requirements of measurement for trade, taxation, and protection of the public. Whenever farming produce is weighed, petrol is pumped, or shares are traded, the measurements (in these cases; weight, volume, and time) need to meet strict government controls on accuracy and precision.

Nearly every country has a metrology institute, involved in all of these important aspects of measurement. In Australia, we have the National Measurement Institute (NMI). America has the National Institute of Standards and Technology (NIST) and the UK have the National Physical Laboratory.

World Metrology Day is an international event commemorating the signing of the Metre Convention on 20th May 1875. The Metre Convention established international cooperation to develop the metric system and the International System of Units (SI).


Signatories of the Metre Convention. Technically, America uses the metric system. Somebody should probably tell them.

Measure for measure

For the past three-and-a-bit years I have been pursuing a PhD in optical metrology — I use light to measure stuff.

In particular, I work on the transmission of atomic clock signals for use by radio telescopes and other space science experiments. Over the years, atomic clocks have become more and more precise, and today we need new technologies in order to transmit the atomic clock signals to another location for use in scientific or industrial measurements.


My metrology lab.

Measuring up

The next few years hold some exciting developments for metrologists. We will soon re-define two of the most basic measurements we use every day, the kilogram, and the second.

The SI system has seven basic units of measurement. These are: the metre, the kilogram, the second, the ampere, the Kelvin, the mole, and the candela. All other units that we use to measure stuff, such as volts or kilowatts, a derived from these basic units.

The second is currently defined using caesium atomic clocks. The caesium within the atomic clock emits microwaves at around 9 GHz, and this frequency is used as the “tick” from which the second is defined.

As atomic clocks have got better, another type of atomic clock that uses atoms of ytterbium have proved to be more stable and precise than caesium-based clocks. These clocks emit light at a frequency around 518 THz, that is, they tick at around 518 trillion times per second. In a few years’ time, ytterbium clocks might become the new way to define time.

The second, and most of the other SI base units, are defined using physical constants. For example, the metre is defined as the distance light travels in 1/299792458th of a second. However, the kilogram is the only unit that is still defined by a physical object. A platinum-iridium ingot in a vault in France is defined to be the kilogram.


NIST’s copy of the kilogram.

Metrologists are working to find a better way to define the kilogram in terms of fundamental constants so that any metrology laboratory around the world can more easily make a precise and accurate measurement of the mass of the kilogram.

The leading contender is a device called a Watt balance that uses electromagnets to convert the mass of the kilogram into units of electrical power, Watts, which can be traced back to fundamental physical constants.

You can even download plans to build your own Watt balance. The US NIST released plans for a DIY Watt balance made out of LEGO. It’s millions of times less precise than NIST’s Watt balance, but about 10 to 100 times more precise than your kitchen scales (depending on how good your building skills are).


NIST’s LEGO Watt Balance

Metrology is an important part of our modern civilization. It is as fundamental to our way of life as electricity, or the internet. The progress of science and technology depends on the progress of our ability to make accurate and precise measurements.