Sciency Words: Eccentricity

Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s word is:


According to the dictionary, eccentricity means deviating from the established norms, especially in an odd or whimsical manner.  I’m sure we’ve all met eccentric people at some point in our lives.  In high school, I was involved in musical theatre, so I got to see eccentricity up close and personal.  But did you know actors and actresses aren’t the only ones who behave eccentrically?  The term can also be applied to the behavior of planets.

The established norm for a planet’s behavior is to orbit a star.  To be more specific, planets are supposed to circle stars, as in their orbits should be perfect circles.  The less circular a planet’s orbit is, the more eccentric it is said to be.  In fact, a planet’s eccentricity can be quantified by measuring just how un-circular its orbit is.

One of my planets in Super Planet Crash has a highly eccentric orbit. Can you guess which one?

One of my planets in Super Planet Crash has a highly eccentric orbit. Can you guess which one?

The thing is, according to Kepler’s laws, no planet has a perfectly circular orbit.  The “established norm” is a myth.  They all travel along slightly ovular paths, sometimes coming a little closer to the Sun, sometimes moving a little farther away.  So it turns out that, just like people, all planets are at least a little eccentric.

P.S.: Today’s post is related to a series on Planet Pailly about sciency video games.  To find out more, click here.  To start playing Super Planet Crash, a game where you can see eccentric planets in action, click here.

Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • I really like the idea of a column or series or blog on scientific words and their meaning. I applaud the effort but on this one it was rather short and not really an explanation at all.

    If you want us to understand the word, you didn’t really explain the word.
    The word has its origin in medieval Latin . where greek and Latin terms were mixed and used to describe new concepts : eccentricus < Greek ékkentr (os)menaing out of center (Ec means center – same family of words as Ego (self center) + Latin -icus

    The word is not exclusive to astronomy but used in many sciences ,first and foremost of course in MATH (a nifty thing that Kepler guy used to calculate stuff…)

    If you you want to explain the meaning or subject of a word, you explain a "term" not a "word". explaining a word is done in a dictionary, explaining the term is done in an encyclopedia

    Here are a few examples:
    Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" – Sciences : psychology, psychobiology, criminology and cognitive science.

    Eccentricity (graph theory) of a vertex in a graph

    Eccentricity (mathematics), a parameter associated with every conic section

    Eccentric (mechanism), a wheel that rotates on an axle that is displaced from the focus of the circle described by the wheel

    (Mechanics physics,engineering

    Eccentricity vector

    Horizontal eccentricity, in vision, degrees of visual angle from the center of the eye
    (Optics – part of Physics )
    Orbital eccentricity, in astrodynamics, calculated from orbital state vectors as an absolute value of eccentricity vector or using other methods based on orbital energy and angular momentum

    Eccentric anomaly, the angle between the direction of periapsis and the current position of an object on its orbit

    Eccentric contraction, the lengthening of muscle fibers (medicine,biology)

    Eccentric, in astronomy, a type of deferent, a circle or sphere used in obsolete astronomical systems to carry a planet around the Earth or Sun

    Eccentric position of a surveying tripod to be able to measure hidden points
    (engineering, surveying, trigonometry)

    Wow ….all those are scientific terms but since you invites us to " to help us all expand our scientific vocabularies together"

    I gladly took your invitation and added something to the subject.
    Should you want to know what Kepler actually did and how he came to his conclusion, never hesitate to ask..
    Also should you come across "sciency term" that you like to know more about..You can actually ask the research desk of your local library (slower than Google but at least as good) or you could if you want send me a mail.

    It is meant as constructive critique.I really applaud your effort to open the world of science to everyone.


    • Personally, I think this article was fine (in fact I personally picked it out of the back catalogue of old sciency word articles. We’re fast forwarding to the current articles next week).
      Sure he could have expanded it to cover various different uses of the word, but I’d prefer to read little “info-bites” than an expanded treatise. My brain is childlike and impatient.

      • It was meant as a constructive critique.I was not pointing out the length, dictionaries manage to explain a word with even less text. I lauded the idea and applauded the decision to include such a section in the line up of topics.
        Ernie and Bert explain words to childlike minds all over the world in very short skit. And this is exactly the flavor of the piece as it started with those Sesame street intro phrased: “Let’s do it together.”
        Nothing wrong with that, but the article failed to explain the word and very little to explain the subject.
        I do not expect a scientific paper and I repeat the effort is great and the execution has a lot going for it; not the least enthusiasm.
        But am I to remain silent? Is critique, and discussion not the idea behind a comment section?
        I listened to your pod-casts, a great percentage of content could be classified as critique,personal opinion and the entire format appears based on discussion.
        Was I mistaken to assume discussion is welcome?
        (Perhaps the “Grumpy Pants honor” is given to anyone disagreeing ?)

        • Just adding my own personal opinion to the record. All opinions are of course welcome.