Is It Time For A New Astronomical Yardstick?

By Alison Klesman | April 23, 2018 10:18 am
An illustration of the scale of our solar system. Distances here are measured in Astronomical Units (AU). (Credit: NASA)

An illustration of the scale of our solar system. Distances here are measured in astronomical units (AU). (Credit: NASA)

Click on an article about space on Discover, and you’ll likely run into a measurement in terms of light-years, solar masses, astronomical units, or arcminutes. These units are unique to astronomy, and all can be expressed in terms of other, more fundamental units, such as meters, grams, and degrees.

In a paper published April 1 in Astronomy & Geophysics, Keith Atkin, a retired associate lecturer in physics at the University of Sheffield, UK, argues that while the professional field of astronomy has moved away from the imperial units of miles, pounds, and degrees Fahrenheit, “this transition has not been complete,” according to the abstract of his paper. The use of units such as light-years (the distance light covers in a year: 5.88 trillion miles [9.5 trillion kilometers]) and astronomical units (abbreviated AU, the average Earth-Sun distance: 93 million miles [150 million km]) persists, he says, when “simpler logical units would help both within the subject and in multidisciplinary research.”

It’s true that astronomy is a bit weird, in general. Our brightness system of magnitudes, for example, is “backwards” — the smaller the number describing its magnitude, the brighter a star appears. And sometimes terms are outdated: Planetary nebulae are not planets or even associated with planets at all, but are the expanding outermost layers of stars in their late stages of life. They were originally called planetary nebulae because they often appear round, like planets, but the term remains confusing and even misleading, particularly for the public.

Proposed Replacements

In the paper, Atkin first focuses on units of distance measurement, noting not only the seemingly random and often redundant nature of units unique to the astronomical field, but sometimes their completely strange construction as well. “My bête noire is the megaparsec — a clumsy and ugly fusion of an SI prefix and a non-SI unit,” he writes.His solution? “To encourage the use of SI units of length in all astronomical work: all distances and lengths should be based on, and simply related to, the metre. The metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”

Thus, he argues, all units of astronomical distance, from the AU to the parsec (equivalent to 3.26 light-years, and derived from the apparent motion of nearer stars against the background that occurs as Earth orbits the Sun), can really be expressed in meters, with the appropriate SI prefix attached.

For example, Atkin proposes that megameters (Mm, 106 m), gigameters (Gm, 109 m), and terameters (Tm, 1012 m) be used for planetary- and solar system-scale distances, making Earth’s radius 6.37 Mm and its distance from the Sun 150 Gm. For larger distances, he suggests petameters (Pm, 1015 m) and exameters (Em, 1018 m) within our galaxy, and zettameters (Zm, 1021 m) and yottameters (Ym, 1024 m) for extragalactic distances. In this regime, Proxima Centauri is about 40 Pm (instead of 4 light-years) away, the Milky Way is 1 Zm across, and Andromeda is 20 Zm away. The proto-galaxy UDFj-39546284, one of the earliest and most distant objects detected to date, would sit at a distance of 126 Ym.


Some astronomical distances using SI units. Note: Distances not to scale. (Credit: Keith Atkin, Astronomy & Geophysics)

Atkin expands his argument to other units of measurement — why use kilograms or solar masses (the Sun’s mass, 1.98 × 1030 kg, is equal to 1 solar mass, and the masses of other stars can be measured on this scale, such that Betelgeuse is almost 8 solar masses) when SI prefixes and grams will do? He suggests that the unofficial prefix besa (1033) be used, such that the Sun is 1.98 Bg and Betelgeuse is 15 Bg. Similarly, the electron volt (eV), which measures the energy of particles in both astronomy and physics, could instead be expressed in terms of attojoules (aJ, 10−18 joules).

He also argues that units such as arcminute and arcsecond should be done away with, though admits that “the degree is presumably here to stay.” Atkin proposes measuring all angles in terms of either decimal degrees or radians, rather than using degrees, arcminutes, and arcseconds, as astronomers do today.

Will Things Change?

But change is difficult, and often a slow process, to boot. In his conclusion, Atkin admits that “as a colleague recently observed: ‘Andromeda will be a good deal closer to the solar system than it is now, before we read of its distance in zettametres.’” His colleague is likely correct, especially given examples such as the public and professional pushback to the recent demotion of Pluto as a dwarf planet under the updated definition of the word planet.

It’s worth noting, too, that units such as the AU and solar mass relate distances and objects to those we understand closer to home — explaining that a star is 8 solar masses immediately makes it clear that it is eight times as massive as our Sun, while stating that Jupiter is 5.2 AU from the Sun paints the clear picture that it is five times Earth’s distance from our star. Light-years, too, are a relatively intuitive unit, even if they are unique to astronomy, and speak both to distance and the time light requires to reach us.

However, the ultimate goal of science is to understand and describe the universe around us. Scientists constantly strive to test, improve, and update their understanding, and standardizing the units that pervade astronomy, Atkin writes in closing, “will surely benefit understanding and communication within astronomical circles and between astronomy and related sciences.”


[This post originally appeared on]

MORE ABOUT: cosmology, solar system
  • Uncle Al

    A relativistic universe has four distinct distances: luminosity (inverse square), angular diameter, parallax, and proper motion. No two of them need agree to maintain consistency. Only local (“touching”) clocks can be synchronized.

    Fifty years of physical theory are defective, for none of it predicts anything empirically verifiable. It is baby steps from :”defective” to “curve-fitted” (parameterized!) to “wrong.” Look outside achiral isotroptic vacuum that is both postulated (being “beautiful”) and universally falsified (baryogenesis).

    The whole of it can be falsified at the postulate level by one hour in a brightspec(.)com microwave spectrometer. I’ll only tell how if you ask. (Ticklers! 40,000:1 signal to noise ratio. Everything is better with N-14 hyperfine coupling.)

    • bwana

      Is this somehow associated with the article!?

  • Ronald Stepp

    ROFL… the guy seriously thinks that gigameter, terameter, and so forth is a good substitute for what we’ve been using? The only reason most of the people who aren’t scientists even bother looking at the occasional science article is BECAUSE we have awesome cool terms like light year, AU, and so forth (I’ll give you mega parsec, that always bugged me too), but really, does he think everyone is an engineer?

    • 7eggert

      While AU might be a good unit to measure planetary orbits in the sol system, it’s useless outside – unless we happen to discus another one-solar-mass star.

      Parsecs – completely useless except for confusing readers, training to deal with stupid units and for having the unit “Attoparsec per Microfortnight”.

      Light years – maybe a good intragalaxial unit, but even for telling the distance to Andromeda, it’s just crap. As soon as you have those odd units and numbers above 1000, you’re better off with SI units.

      mm Hg – good for measuring blood pressure using a vacuum pump and mercury – literally. Otherwise: Crap. Don’t use that for Venus atmospheric pressure.

      • Uncle Al

        Venus’ surface is about 470 °C and 92 atm. pressure CO2 versus critical constants 31.10 °C and 72.9 atm. The whole planet is a supercritical carbon dioxide extractor! Condensation in the highlands must produce the most interesting accessible vapor transport accumulation in the solar system.

        What does Mars have, rusty sand and perchlorate?

        • 7eggert

          A comparatively more survivable environment. Maybe Mars will be our launch base for mining iron, generating oxygen in the process and not sitting in an as much as 11.2 km/s of gravity well. Then we’ll refuel our generation space vessel on titan, …

          • dan_2000

            Not for humans. Those who go there will die quickly. The human circadian cycle will never adjust to the low gravity and daily/yearly cycles of Mars.

            The gravity on Mars is only 3.8 m/s^2 compared to 9.8 m/s^2 (~10 m/s^2) on earth at sea level.

          • 7eggert

            The human circadian cycle does adjust to the ISS well enough to survive some minutes, and the natural daily cycle of humans is just one mars day.

            Naturally humans could not give birth while living more than 5000 km above sea level, but thanks to genetic mutations, some people can.

        • dan_2000

          OK. so we have a surface pressure on Venus of 9.2 MPa (~10 MPa) and a critical pressure of 7.3 MPa compared to a normal earth pressure of 100 kPa. Why didn’t you say so in the first place?

    • dan_2000

      After reading your comment, the word interloper comes to mind. That is a person who has no connection with the field of astronomy making a comment about how astronomers should modernise their way of measuring. Just because Americans want to hang back into the past and continue to become technically behind the rest of the world, doesn’t mean that the rest of us need to follow suit.

      If you only look for articles on science hoping to see if they are still using fake units in order to get your jollys, then you needn’t bother to look anymore. Go back to reading the comics or something else related to Ludditism, such as the American west of two centuries ago.

  • OWilson

    We cannot get a global average temperature published daily, but no problemo with the daily temperature of your back yard!

    Instead we get the “third most warmest for this particular month in this decade!” From the Government.

    As Johnny’s audience used to say, “But how hot WAS it!” Lol!

    I’m always skeptical about changing common sense language to newspeak!

  • Not_that_anyone_cares, but…

    Were one to completely redefine a measurement package wouldn’t this be the place to get away from ten and go to 64 or something better.

  • Andrew Worth

    Sometimes it’s helpful for astronomers to speak the same language as the rest of us, on this plant we routinely use numeric values from billionths (nano-) to billions (giga-) rarely do we need to go into even the trillions unless we’re talking about the US national debt. What we do is change the unit of measure to keep the numbers manageable. Using meters, kilometers, astronomical units and light years nicely covers the whole universe and keeps the numbers in the lay mans comfort zone.

    • dan_2000

      No, it is not. It is up to you to learn their language. You should smarten up, not have them to dumb down.

      • Ronald Stepp

        How old are you, again?

        • dan_2000

          Old enough to keep learning and moving forward. The world doesn’t stop just because you become an old fuddy-duddy. It moves forward with you or without you.

      • bwana

        More B.S. from a scientist that no one will understand or listen to…

  • Nate

    I’ll agree with most of this, but there ARE benefits to using the light-year (or light-hour/minute/second): when working such large distances this gives you distance AND time in a single value. Some event was observed at 100 ly, well then it occured 100 years ago. Maybe this doesn’t matter for pure observation, but for communication within our solar system it does. Expect a 2.6 second delay for replies from the Moon, and 6-44 minutes from Mars depending on orbital relations

    • Mike Richardson

      This, exactly. The light-year can be expressed in terms of kilometers or miles, and helps convey the interrelationship between time and space. Perhaps if we expand into space, and the term “year” becomes something that not everyone experiences as 365 days, we will need a more universal measure of distance. But it would still make sense to base it on the speed of light, which seems pretty universal in this part of the universe, for the foreseeable future.

  • OWilson

    As long as they let the usage evolve gradually, lest you get “A disaster investigation board reports that NASA’s Mars Climate Orbiter burned up in the Martian atmosphere because engineers failed to convert units from English to metric”.

    “Hey I thought you said, inches?” :)

    Or as part of the official NASA “Muslim Outreach” program, as the former Director put it, “to reach out to the Muslim world and engage much more with dominantly Muslim nations to help them feel good about their historic contribution to science, math and engineering.” How about “فعل” too? :)

    Not to mention revising all the astronomy text books, and that (parenthesis) stuff that attempt to reminds you of the “old” measure, but often gets it wrong! :)

    • dan_2000

      Are Americans still using textbooks? How backwards can you get? In a modern 21-st century world, there is no need for outdated textbooks? Everything can be read on a computer screen and computer “books” can be updated daily if desired. If the “old” measures are wrong, then good, they should be. It would serve any person right to get a wrong answer when using old fake units.

      • OWilson

        ‘More haste, less speed’, or ‘Fools rush in i…’ is appropriate here! :)

        When Canada’s Trudeau government decided almost overnight by government fiat (not an election issue) to switch over to metric, the far reaching regulations created havoc in the building, manufacturing industries, mandating that all goods, including tools, parts, lumber must be sold in metric units only.

        But most infrastructure was built in imperial, 2 X 4 struts, 4 x 8 panels, 12 X 12 tiles, railway lines, assembly lines, containers, bottles, boxes and box cars, etc. and the new metric measurements resulting in a fiasco.

        (Canada’s biggest trading partner was the U.S. and the flow of goods and parts was severely affected.

        The government eventually had to put a moratorium on ihe mandatory regulations and allow quantities to be sold in traditional measure. We thankfully never had to use the term, “First down and 9144 Centimeters to go!” Lol!

        Now Imperial measure is not illegal!

        But we are still vulnerable to misunderstandings, like, “NASA’s Mars Climate Orbiter burned up in the Martian atmosphere because engineers failed to convert units from English to metric”.

        Not to mention over-regulation which results in ingredients listed in metric AND imperial and in TWO languages, yet, on a tiny tiny pill container

        (Try checking out the ingredients and dosage when you are old and shaking with back pain! Lol!)

        • dan_2000

          The metrication of Canada was not over night it was implemented via a 4 phase 10 year plan. Investigation, Planning, Scheduling and Implementation. Each part of the economy was divided in sectors and each sector coordinated their own pace of metrication. The metrication plan was on schedule and moving quite smoothly until Joe Clark became Prime Minister.

          Unlike Canada, Australia, New Zealand, South Africa and others in the commonwealth successfully completed metrication and never looked back. As a result these countries have strong stable economies. Canada’s mistake is it put its eggs all in one basket and depends too much on the US. Canada will learn the hard way that Donald Trump is no fan of Canada and will have no problem cutting trade off with Canada. Then Canada will have no choice but to sign a trade deal with Europe which will demand only metric products be traded.

          Of course a metricated football game becomes First down and 10 m to go. Only a moron would make such a stupid remark as you did.

          At least in the US all medication doses are now in metric only and there are often two languages as well, Ingles y Español.

          • OWilson

            When they can put a man on the moon, maybe, just maybe, we’ll let them tell us how to measure!

            We did it 50 years ago!

            Why not Islamic measure?


          • OWilson

            I lived through it and it was a fiasco in the building and manufacturing areas. Imagine trying to fit a metric extension onto an existing building, or converting land registry records from acres and square feet.

            Like Obamacare,. it was “planned” and implemented by bureaucrats, who had no knowledge of business or manufacturing!

            The government were quite unprepared for the fall out, but surged ahead anyway, against the people’s wishes, (which is one reason why totally incompetent “conservative” Joe Clarke, found himself Prime Minister, one day)

            They also figured nobody would notice the usurious Federal Taxes on gasoline by the cheaper liter! :)

            As Trudeau and his enforced ill conceived mandatory metric gave you Joe Clarke, so Obama, and his ill conceived mandatory Obamacare, gave you Donald Trump!

            History repeats! :)

  • bwana

    Just what we bloody well need! Another set of astronomy measurement units! Light-years (LR) and astronomical units (AU) are just fine, thank you!!

    • dan_2000

      Are they? Are you an astronomer, a scientist, an engineer? Those who have to work in these fields are handicapped from growing in knowledge when antiquated policies, including units, are promoted because old Luddites and interlopers can’t keep up. Comments like yours should only be taken seriously by those who want to lag behind. For those that want to move forward, your comments will be ignored.

      • bwana

        I’m Engineer and a Scientist, and comments like yours are simply a distraction from getting work done. I’m quite sure you will be the one ignored, at least until the world agrees that metric units are universal.

        • dan_2000

          If you are a Scientist and an Engineer and you are that easily distracted from the use of proper SI units, I would highly question your credentials.

          • bwana

            There is a distinct difference between proper SI units and units that mean something to the average person. If you wish to be high and lofty, by all means talk in units only Scientists and Engineers understand. If you wish to be understood by the average person, who in all likelihood pays your salary, use units that are easily comprehended.

  • Lorie Franceschi

    I read the guys article, and thought it was so bad if he was trying to be serious that I was laughing before I got even half way through it. The lay person that is taught either miles, feet and inches or kilometers, meters and centimeters, understands what some one that is describing how far Earth is from the Sun understands what they are talking about.
    We know the light travels at about 300 kilometers a sec or 186000 miles a sec. Changing it now is not productive, even if it is done slowly. If we start teach our youth the first year they start to have an understanding of math, they have to be taught both, until all of the people in the fields are long dead a buried. This article is a joke.

    • dan_2000

      If you are American, your comment is expected. If you are a teacher, you should be ashamed. Teachers should want to teach the latest and newest technology quick and not wait for all of the Luddites die out. That may be the American way to do it slowly but technological changes occur fast and the more you lag, the farther behind you become. But thankfully it isn’t the way of the developed world and the main reason many countries are now 30 years ahead of the US in technological development. So, yes, encourage Americans to make slow changes and help the rest of the world move even farther ahead of you.

      BTW, the speed of light is 299.792 458 Mm/s, using proper SI symbols. Learn the rules of SI units and symbols and use them properly.


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