Undecennial

Undecennial (pronounced uhn-duh-seh-nee-uhl)

(1) Occurring or observed every eleventh year.

(2) As “undecennial magnetic period”, the Sun’s solar cycle.

1858: The construct was undec-, (from the Latin undecim, (eleven), the construct being unus (one) + decem (ten)) + -ennial.  The -ennial suffix was from the Latin -enniālis, the construct being annus (year (and figuratively “time, season, epoch”)) + -ium (the suffix used to form abstract nouns) + -ālis (suffixed to nouns or numerals creating adjectives of relationship).  It was a combining form denoting years.  The Latin undecentesimus was from ūndēcentum (ninety-nine; 99).  In Roman numerals, 99 was written as XCIX, the construct of which was XC (90: 100 minus 10) + IX (9: 10 minus 1) thus XC (90) plus IX (9) equals XCIX (99).  The fear of the number 11 is described as hendecaphobia.  The alternative adjective (and non-standard noun) is undecennary (once every eleven years) and the adjective in Portuguese is undecenal.  Undecennial & undecennary are adjectives and a (non-standard) noun; the noun plural is undecennials.

Centennial (every hundred years; commemoration of an event that happened a hundred years earlier) is the best known of the words suffixed with “-ennial” but there are fun constructs with meanings not immediately obvious including demisesquicentennial (75 years), the construct being demi- (half-of) +‎ sesqui- (one-and-a-half) +‎ centennial (of 100 years) and quadranscentennial (twenty-fifth anniversary (now often called “silver jubilee)), the construct being quad, from the Latin quadrans (quarter) + -ennium (a variant of annus) + -ālis (the “quad” thus a reference to the four 25 year quarter-centuries in a century).  Unfortunately, sexennial (pertaining to a period of six years; taking place once every six years) (the construct being sexennium (a period of six years) + -al) means just a “six year period or cycle” although in August 2024, in Boston Massachusetts there was the Sexennial: A Sex-Positive Variety Show.

In modern use, there’s also been some re-purposing.  The first use of “postmillennial” was to describe the world after the year 1000 and it has been used of things Pos-2000 but it was also adopted in the nineteenth century by certain Christian sects to describe the doctrine the Second Coming of Christ will take place after the millennium; the antonym was premillennial (pertaining to the belief the Second Coming will take place before the millennium.).  In the 21^st century, it’s used also of “Generation Z”, the one following the “Millennial Generation”.  Premillennial seems not to be used in this context (that would be the (Baby) Boomers).  The construct of the adjective perennial was the Latin perenn(is) (lasting through the whole year or for several years, perennial; continual, everlasting, perpetual”) + the English -al (the adjective-forming suffix imparting the meaning “of or pertaining to”.  It’s familiar from its use in botany where it describes plants active throughout the year, or having a life cycle of more than two growing seasons (and thus used sometimes in the sense of “appearing again each year; annual”) but is used also (sometime loosely) of waterways and such.  In figurative use, “perennial” is used widely (and loosely) of just about anything (art, music, politics et al) with the quality of or tending to “continuing without cessation or intermission for several years, or for an undetermined or infinite period; never-ending or never failing; perpetual, unceasing”.

Images from NASA’s Solar Dynamics Observatory highlight the appearance of the Sun at solar minimum (December 2019, left) versus solar maximum (May 2024, right).  These images are in the 171-angstrom wavelength of extreme ultraviolet light, which reveals the active regions on the Sun that are more common during solar maximum.

The Sun’s 11-year cycle was first detected in 1843 by German apothecary & amateur astronomer Heinrich Schwabe (1789–1875). Schwabe noticed a pattern in the number of sunspots that appeared on the Sun's surface over time.  In 1825 Herr Schwabe obtained his first telescope and between then and 1867 (on every day the skies were clear) he recorded the size & shape of sunspots and it was in 1838 he first suspected the phenomenon might be cyclical, his initial findings suggesting a ten-year cycle.  The discovery was wholly serendipitous because he wasn’t interested in sunspots (then thought random events) but was one of a number of astronomers searching for “Vulcan” a speculative planet in an orbit between Mercury which theories suggested should exist because its presence would account for the otherwise inexplicable peculiarities in Mercury's orbital path.  As a theory, the science was sound because earlier the same math had been used correctly to predict the existence of Neptune, based on calculation which determined the gravitational influence required to explain disturbances in the orbit of Uranus.  Over the decades, sightings of Vulcan had been reported but all quickly were discounted and the search continued until Albert Einstein’s (1879-1955) theory of general relativity (1915) was confirmed and Mercury's variation from the orbit predicted by Newtonian physics was understood to be a manifestation of the curvature of space-time induced by the mass of the Sun.

Because of Vulcan’s predicted proximity to the Sun, it would have been very difficult to observe with the telescopes of the nineteenth century, the only plausible method being to view it during its transit in front of the Sun.  The reason Herr Schwabe kept notebooks with almost daily sketches of the Sun and its spots was that he wanted to ensure he would never confuse a spot with the passing Vulcan and mush have be surprised when he noticed the suggesting of a cyclical pattern.  In 1843 he published his initial findings which indicated sunspot activity appeared to peak every ten-odd years and which his paper attracted little interest, it did inspire a Swiss professional astronomer to begin his own regular observations and these, combined with Herr Schwabe’s earlier drawings confirmed the sun’s undecennial pattern.  The use in 1858 of “undecennial” to describe the solar cycle seems to have been the first use of the word in English.

Visible light images from NASA’s Solar Dynamics Observatory highlight the appearance of the Sun at solar minimum (Dec 2019, left) versus solar maximum (May 2024, right). During solar minimum, often the Sun is "spotless".

The Sun's eleven-year cycle (the solar cycle) is driven almost wholly by changes in the body’s magnetic field dynamics.  The Sun’s magnetic field isn’t as stable as that of Earth which, although subject to some ongoing movement, retains its essential polarity for at least hundreds of thousands of years.  Deep within the Sun there exists a layer called the convection zone (where hot plasma rises, cools, and sinks) and these interactions, over time, cause the Sun’s magnetic field lines to twist and tangle.  Things are influenced also by differential rotation, the Sun rotating faster at its equator than at its poles (the equatorial regions taking some 25 days to complete a rotation, the polar regions around 35.  What this does is “stretches and wind up” the magnetic field lines, resulting in what astronomers describe as “a twisted, complex magnetic environment”.  All this combines to produce the “solar maximum & minimum”: Every eleven years the “twisted & tangled” magnetic field lines stretch to the point where suddenly they “snap”, creating a realigning process in which they are “straightened out”.  During solar maximum, the Sun has many sunspots (regions of intense magnetic activity), solar flares, and coronal mass ejections.  When the cycle resets to solar minimum, these activities reduce as the Sun's magnetic field temporarily stabilizes.  The other obvious effect of the undecennial magnetic period is the periodic polarity flip:  Every 11 years, the Sun’s magnetic poles reverse, north becoming south and vice-versa, something which happens on earth every few hundred-thousand years.

Quantum Tech Club's chart of the solar cycle: This cycle of low-high-low sun activity operates on a cycle of about eleven years though there are always variations, the length of each cycle not exact and the volume of activity also varies.  The previous Solar Cycle (24) was classified "not particularly active" and the current cycle (25) was predicted to be similar but it turned out to be more vibrant.  So, while the numbers bounce around, the undecennial pattern remains constant. 

For cultural reasons, an eleven year cycle sounds somehow strange to us and we’re unaccustomed to such things being associated with prime numbers although in entomology there are insects with no aversion to primes.  In entymology, there are insects with no fear of the number 17.  In the US, the so-called “periodical cicadas” (like those of the genus Magicicada) exist in a 17 year life cycle, something thought to confer a number of evolutionary advantages, all tied directly to the unique timing of their mass emergence: (1) The predator satiation strategy: The creatures emerge in massive numbers (in the billions), their sheer volume meaning it’s physically impossible for predators (both small mammals & birds) to eat enough of them to threaten the survival of the species. (2) Prime number cycles: Insects are presumed to be unaware of the nature of prime numbers but 17 is a prime number and there are also periodic cicadas with a 13 year cycle.  The 13 (Brood XIX) & 17-year (Brood X) periodic cicadas do sometimes emerge in the same season but, being prime numbers, it’s a rare event, the numbers' least common multiple (LCM) being 221 years; the last time the two cicadas emerged together was in 1868 and the next such even is thus expected in 2089.  The infrequency in overlap helps maintain the effectiveness of the predator avoidance strategies, the predators typically having shorter (2-year, 5-year etc) cycles which don’t synchronize with the cicadas' emergence, reducing chances a predator will evolve to specialize in feeding on periodical cicadas. (3) Avoidance of Climate Variability: By remaining underground for 17 years, historically, periodical cicadas avoided frequent climate changes or short-term ecological disasters like droughts or forest fires. The long underground nymph stage also allows them to feed consistently over many years and emerge when the environment is more favorable for reproduction.  Etymologists and biological statisticians are modelling scenarios under which various types of accelerated climate change are being studied to try to understand how the periodic cicadas (which evolved under “natural” climate change) may be affected. (4) Genetic Isolation: Historically, the unusually extended period between emergences has isolated different broods of cicadas, reducing interbreeding and promoting genetic diversity over time, helping to maintain healthy populations over multiple life-cycles.

Lohanic undecenniality: Lindsay Lohan at eleven year intervals: 2002 (left), 2013 (centre) and 2024 (right).

A “year” as defined (one orbit of our world around the sun) on Earth is a standard measure and on this planet it makes complete sense but in other places (such as the Sun) it’s just an abstraction although we map “years” onto many remote places, vast distances best understood as expressed in “light years” although cosmologists for many purposes prefer the parsec (a unit of astronomical length, based on the distance from Earth at which a star would have a parallax of one second of arc which is equivalent to 206,265 times the distance from the earth to the sun or 3.26 light-years.  Its lineal equivalent is about 19.1 trillion miles (30.8 trillion km)).  It takes Pluto 248 Earth years to make its orbit of the Sun so that’s the length of one Plutoian year, meaning that between being discovered in 1930 and the humorous cosmic clerks at the International Astronomical Union (IAU) in 2006 voting re-classify Pluto as a dwarf planet (on the basis that the icy orb failed to meet a set of criteria which the IAU claimed had been accepted for decades), not even on year had there passed.

So it’s only on Earth one of our “years” is of direct relevance and we tend to measure anniversaries with the numbers we prefer (1, 2, 5, 10, 20, 25, 50. 100, 250, 500, 1000, 10,000 etc) (21 is a special case) but this meaning nothing to the physics of the Sun and even here there have been cultures in which some things have tended to the undecennial.  In India, the Kumbh Mela (or Kumbha Mela) is one of the great pilgrimage festivals in Hinduism (the pre-Covid gathering in 2019 said to be the largest (peaceful) assembly of people ever known) and although it is celebrated in what tends to be a twelve year cycle, because of the complexity and regional distribution of some celebrations, there have been times when things have happened at an eleven year interval.  Among the indigenous peoples of North America (notably the Hopi), there were also reports from anthropologists of ceremonial cycles based on natural and astronomical cycles that can approximate an eleven year pattern due to environmental changes or social cycles, although it doesn’t appear the intervals ever assumed a precise, recurrent eleven-year pattern.  Certain ceremonies were linked with observations of the sun (and other celestial bodies), aligning closely with solar maximums in some cases.