The “Best Possible Unit Bar None”

Han_Solo_With_Nano-Second_GunBy The Metric Maven

Bulldog Edition

My friend Kat does her best to make certain I have a contemporary cultural education, but often it does not adhere. Other than the first film, that is actually Episode IV—I had never seen any of the other Star Wars films. She sought to remedy my ignorance and showed me the entire cannon canon. During Episode IV came this infamous statement from Hans Solo:

“You’ve never heard of the Millennium Falcon?… It’s the ship that made the Kessel Run in less than twelve parsecs.”

I noticed this confusion of time and distance when I first saw the movie. I had read Isaac Asimov’s The Universe: From Flat Earth to Quasar, and knew that  a parsec is a single word formed from the two words parallax and second. I also knew what stellar parallax is, and the history of the search to find it. I cringed in the original Star Wars showing. The statement reminded me of 1950s “science fiction” movies filled with hilarious scientific eye-rollers. I love watching those old movies for their campy nature and appearance. Star Wars had Peter Cushing, and so I saw it as more in line of the old Hammer Films. Unlike the 1958 Hammer film Dracula, Star Wars made an impact on culture which is still hard for me to fathom.


Definition of Parsec (Wikimedia Commons)

A parsec is defined using a right triangle that has a side, which is one astronomical unit, and whose opposite angle is one arc-second. The astronomical unit (AU) is a unit that astronomers have created which is roughly the distance from the Sun to the Earth. I say roughly because the distance between the two bodies varies as the Earth orbits the Sun. The parsec is accepted for use with SI and is defined as exactly 149 597 870 700 meters or 149.6 Gigameters. Why not just make it 150 Gigameters? The angle opposite the triangle side of AU length is 4.848 136 8 microradians or one arcsecond. For those who prefer degrees this is 277.778 microdegrees.

When the second leg of the triangle (not the hypotenuse) is computed it has a length of 30.857 Petameters. In summary the Astronomical Unit is 150 Gigameters, and the Parsec is 31 Petameters. Astronomers however don’t use Gigameters and Petameters for distance, they use light years. We have all been told this is the distance that light travels in one year. This distance is 9.4607 Petameters. When we compute the length of a parsec in terms of a light year it is 3.2 light years.

In an earlier essay, I took astronomers to task for not using the large metric prefixes. They encompass the entire observable universe—why not use them? The large metric prefixes even allow for astronomical classifications in terms of metric prefixes, which might be useful for visualization.

I wrote a contributor to an astronomy periodical, and also to their editorial board, inquiring about why there has been no effort to change to metric units. The editorial board did not respond, but the contributor wrote back and said that he was trying to convey:

“….astronomy and sometimes other sciences to the general nontechnical public in a way that makes sense to them. As things now stand in the US, that’s not metric. If we say that rain falls at a speed of 9 meters per second, that’s not as meaningful as 22 miles per hour.

As for planetary distances, I’m not sure if a few hundred million miles can be “felt” by the average person but gigameters would be worse.

As for the light year, it’s the best possible unit bar none. The notion that light, the fastest thing there is, requires 4.3 years to get here from alpha Centauri makes that distance meaningful. Nothing else can do that. Remember, astronomy articles have [a] job to do….and it’s not to force feed an alien seeming system of units, but to help people grasp vast distances. Light years accomplishes that.”

The chicken and egg-little argument is always the first drawn upon. We must become metric as a country to use metric, and no metric will be used popularly until we do, which in turn makes certain the public never sees metric usage, which in turn makes them ignorant and unwilling to change to an “alien seeming system of units.” Doing otherwise, that is using metric, could bring the sky falling down on those who chose to violate this precept. The astronomy periodical would fail, and science “communicators” would all be unemployed.

I see the light year as exploiting a common unconscious substitution, that of time and distance—just like Hans Solo. Often when I ask how far a nearby town might be I hear: “oh….about 45 minutes.” The question was how far, but the answer was time. This time value almost certainly assumes a speed of about 60 miles per hour which is one mile per minute. So the distance is actually 45 car minutes or 2700 car seconds, not just 45 minutes. Traffic jams can radically alter the time, but not the distance.

In my view the light-year is not a unit, it is a culturally accepted mathematical product without a singular definition. A light-year is the speed of light (in a vacuum) multiplied by the length of a year. No units are specified. I could argue that a light year is 1.95 trillion leagues, or 1.117 quintillion barleycorns. What a light year does is allow us to substitute a time metaphor for a defined distance. I heard a person in a recent podcast discuss the idea of intentionally sending out radio signals for aliens to receive. It was pointed out that I Love Lucy broadcasts are about 70 light years out at this point. This could be estimated quickly by assuming that I Love Lucy went on the air about seventy years ago. No distance was computed, it does not tell us how far those VHF electromagnetic waves have traveled, it only is an expression of time as a metaphor for distance.

The light year is the kilowatt hour of astronomy. One could ask their meter reader how much electricity was used, and believe that 20 kilowatt hours is an answer in energy, when it is a time metaphor for energy. I direct you to my essay Joule in the Crown where I argue an actual unit of energy, the joule, should be used to meter electricity and gas rather than kilowatt hours and therms. Therms are a familiar unit?—right?

When we use word descriptions in place of units, we continue to encourage innumeracy. The astronomy contributors insistence on time rather than distance to describe distance is a public disservice in my view, but in his view he is giving the readers “what they want” without ever actually investigating or debating the issue. Hans Solo didn’t make a mistake confusing time and distance, he gave the audience “what they expected—what they wanted” a metaphor in place of a measurement.


A video of my short lecture at Nerd Nite Denver on 2014-09-25 has been posted here.

8 thoughts on “The “Best Possible Unit Bar None”

  1. Is it possible that Hans Solo was talking about the Millennium Falcon being able to bend space to make the distance shorter for it’s trip?

  2. “but in his view he is giving the readers “what they want” ”

    No, he is giving the readers what he wants them to want. It is an old excuse to claim others want it a certain way because the person giving out the information wants it that way. When was the last poll taken to determine the validity of such a comment.

    Consider when you ask the Soda Industry why they have a 20 ounce bottle and not a 600 mL bottle and they will tell you people want that size. Yet can anyone ever remember being asked whether they would prefer a 20 ounce bottle over a 600 mL bottle?

  3. While I normally agree with you, let me paste my comment in from the reddit post ( ) I made:


    While I normally agree with him, in this case, he’s missing his physics.

    Fundamentally, the distinguishing between temporal distance and physical distance is entirely artificial. It’s the source of the random *c*^2 in the metric equations. Time is just another distance.

    So, a light-year is, in effect, a “dimensionless” year, properly putting (*x*,*y*,*z*) on the same playing field as *t*.

    So, while of course [natural units]( are the most pure, a light year is one step divorced from that (eg, arbitrary time constant), and then metric is two steps divorced from that (each fundamental quantity is a [different] constant divorced from natural units).

    Imperial, of course, is a clusterfuck.

    • Joshua, Great article except for “morpheme” under item #6. A morpheme is the smallest meaningful unit or form in a language. If you meant morphine, a narcotic, you should have used a 1000 ml or 1 liter in description, not “cubic centimeters.” But 1000ml or 1 liter of morphine would be enough to kill a substantial number of people. There are several factors to determine the correct dose of morphine, or any other drug, given to a patient. I can only presume that you were making a point. Also, I disagree about not using a manual typewriter or cursive writing. I have a manual typewriter which I find very useful for various small tasks and that it is often faster than turning on my computer and burning electricity. I also believe that both cursive writing and print are great to do such things as notes, thank you cards, etc. All of the modern devices many people are using constantly are fine until a solar incident or loss of power occurs. I speak positively about and use metric whenever possible and look forward to the day that the U.S. is finally forced to adopt it. And I do believe that we will be forced to go to SI.

      • Thanks! I was a linguistics minor in college. I guess I prefer language about language, ha! Other points taken as well. Keep up the good work.

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