Hubble's Universe Unfiltered

  • October 10, 2013

    The Naming of Comets

    by Frank Summers


    The poem below is an adaptation of T. S. Eliot's poem "The Naming of Cats" from "Old Possum's Book of Practical Cats." This book served as the basis for the musical "Cats." Unless you are an Eliot or broadway afficianado, the verse below will make immeasurably more sense if you read the original first. That work is under copyright, and can not be printed here, but can be found elsewhere on the internet. Notably, the "Look Inside" feature includes the entire poem as the first work in this version of the book (at the time of writing). Also, the performance of the poem in the film version of "Cats" is available as a YouTube video.



    The Naming of Comets
    by F. J. Summers

    The naming of comets is a scientific matter,
    It isn't just one of your late night games;
    You may think at first I'm from a mad alma mater
    When I tell you, a comet must have three different names.

    First of all, there's the name that the public uses daily,
    Such as Lovejoy, Kohoutek, West or McNaught,
    The first name was Halley, or in America "hailey,"
    The name of the discoverer, or so you've been taught.

    There are compound names to handle the muddle,
    When two or more observers have made their claims,
    Such as Hale-Bopp, Shoemaker-Levy, or else Tempel-Tuttle,
    But most of them sensible astronomer surnames.

    But I tell you, a comet needs a name that's particular,
    A name that's specific, for a catalog, not fun,
    Else how can the solar wind push its ion tail perpendicular,
    Or spread out its dust tail when it passes the Sun?

    Of names of this kind, let's take C/2012 S1,
    Where a "P/" says periodic, but a "C/" says its not,
    Codes for the year, the half-month and the count and you're done,
    Now a name that belongs to only one comet you've got.

    But above and beyond there's still more naming jargon,
    And those are the names from a scientific source,
    With orbit and origin thrown into the bargain,
    These groups of comets are found in research discourse.

    When you notice a comet with a profound eccentricity,
    It's a "New" comet, I tell you, from the far Cloud of Oort,
    And a Kuiper Belt comet can achieve synchronicity,
    With the comets, with the orbits, with the periods that are short.
    The families of Jupiter,
    Encke and Chiron,
    Halley and External, cometary class names.



    Since poetic license, and adherence to T. S. Eliot's original form, can sometimes confuse a scientific message, here's a prose interpretation of the comet naming verses.

    The comet names most people know about are the ones honoring people, telescopes, or institutions. For example, Comet Lovejoy was discovered on November 27, 2011, by Terry Lovejoy. Comet Catalina-NEAT was found by both the Catalina Sky Survey and the Near-Earth Asteroid Tracking program (NEAT). However, Comet Halley (pronounced "hal-ee" or "hall-ee," but definitely not "hail-ee") is unusual in that Edmund Halley did not discover the comet, but predicted its return. Only a few other comets are named after the people who calculated their orbits. Details of who gets credit can be found in the IAU Comet-naming Guidelines.

    Using common names gets rather confusing in the era of space missions and dedicated observing programs. NEAT has discovered dozens of comets, while the Solar and Heliospheric Observatory (SOHO) has discovered a couple thousand. Keeping track of those requires a more catalog-style name. The scientific designation includes the type of comet and when it was discovered such that Comet Lovejoy is also known as "C/2011 W3."

    The two most common prefixes are "P/" for a periodic comet that completes an orbit in less than 200 years, and "C/" for a comet only seen during one passage by the Sun and not expected to return for a long time. Comet Lovejoy has an orbital period of about 622 years, and gets the "C/" prefix.

    The year of discovery is straightforward, but the letter following it is not. The twenty-four half-months of the year are labeled from A to Y, skipping "I" because it can be easily confused with the number one. Comet Lovejoy's discovery date of November 27, 2011, gets the designation of "2011 W."

    Finally, the "3" at the end of Comet Lovejoy's designation simply says that it was the third comet discovery assigned during that half-month period. The full naming description, including other prefixes and some special cases, can be found in the Cometary Designation System resolution.

    The third "name" referenced in the poem above is that of the various classes in which astronomers have grouped comets. The traditional split has been into long-period comets (more than 200 years to complete an orbit) that come from the Oort Cloud and short-period comets (less than 200 years) that derive from the Kuiper Belt. In recent years, classification is based on an orbital characteristic called the "Tisserand parameter," but the two main groups still remain.

    Within the short- and long-period groups, several sub-groupings are defined, often named after a proto-type. Comet Halley is a long-period comet that has been captured into a short-period orbit (75.3 years) and is the standard for the "Halley-type" comets. Comet ISON appears to be on its first pass from the Oort Cloud through the inner solar system and is called a "New" comet. The orbits of short-period comets can be dominated by interactions with Jupiter (Jupiter family), or exist outside (Chiron-type) or inside (Encke-type) that region. A description of comet classes can be found in this 1996 paper on "Comet Taxonomy," though these definitions do adjust and new ones can be added over the years.

    Whew! That's a lot of comet naming details just to explain a somewhat whimsical poem. However, I do very much enjoy combining the scientific and the literary in this fashion. In any case, this poem serves as a great way to test the size of the Venn diagram intersection of astronomy enthusiasts and musical lovers.