Geology Through Literature:
Hans Christian Andersen's: The Comet (1869)
And we have finally reached the ninth and final entry of Hans Christian Andersen's geological references.
The Comet (1869)
Comets
Now there came a comet with its shiny nucleus and its menacing tail. People from the great castles and people from the poor huts gazed at it....
But a little boy and his mother still stayed inside their room. The mother believed ... that her son would soon die. The little boy lived many more years on earth. Indeed he lived to see the comet return sixty years later.
...
"This is the time to look at the comet," cried their neighbors....
The boy saw the bright ball of fire, with its shining tail. Some said it was three yards long, while others insisted it was several million yards long—such a difference.
In general a comet is is a body of ice, rock, and organic compounds that can be up to several miles in diameter. "Comets are thought to originate from a region beyond the orbits of the outermost planets. Scientists believe that gravitational perturbations periodically jar comets out of this population, setting these "dirty snowballs" on orbital courses that bring them closer to the Sun. Some, called long-period comets, are in elliptical orbits of the Sun that take them far out beyond the planets and back. Others, called short-period comets, travel in shorter orbits nearer the Sun" (Nasa.gov).
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| Comet 153P/Ikeya-Zhang which has the longest known tail at over a billion kilometers. Image courtesy of NewScientist. |
Although many comets are known to have a return period, much of the comets with known return periods had only been discovered fairly recently. There are two types of comets based on their orbital periods; known as long-period comets, which are comets with orbital periods greater than 200 years, and short-period comets, with orbital periods less than 200 years. Even the short-period comets can be broken up into Halley-Type comets, which have orbital periods between 20 and 200 years, and Jupiter-family comets, that have orbital periods less than 20 years.
Besides Halley's comet, which was determined in 1705 by Edmund Halley to return at a set amount of time (75-76 years), there aren't many comets that have been identified with periodic return periods. To date, only ~300 periodic comets have been identified, with less than 10 identified by the publication of this story (1869).
The problem with identifying these comets is that typically only bright comets are easily identified in the sky, especially by the naked eye. These bright comets, often called "Great Comets", however usually need to be fairly large, pass closely to the sun, and pass closely to earth. Although these bright comets are bright enough to be seen by the human eye, their close approach to the sun often can spell disaster for the comet, reducing it to space dust. So many of the Great Comets are one time affairs. Halley's Comet being the exception of a bright comet that continues to have a periodic return.
Within the story Andersen seemed to take the appearance of a bright comet and a periodic comet and mixed them with his own tale of a man that would live another 60 years. To date there is only one comet with an approximate return periods of 60 years, and that was only discovered in 2015. This comet, C/2015 F5 (SWAN-XingMing), has a return period of 60.9 years and was only barely visible due to its mostly lack of a tail.
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| The Great Comet of 1861 as painted by E. Weiss. Image courtesy of Wikipedia. |
The Great Comet of 1861 (C/1861 J1) would probably be the closest great comet to the time of publication. Although, as I said that many of the Great Comets are destroyed, it appears that this comet was linked with other pervious comets and had been determined to have an orbit of 407 years. As more time passes and more of the historical records are analyzed it will likely come that even more of the previously thought one-off comets will be determined to be linked, and/or have a periodicity themselves.
Andersen makes note of the length of the comet tail, in which I assume is a difference between looking at in in the sky (three yards long) and thinking of it in its real life length (several million yards long). In real life, the tail is created when solar winds and solar radiation pressure blows ionized gasses and debris off of the comet. This causes the tail to always be pointed away from the sun. Since the tail is a product of the sun, the closer to the sun that the comet gets, the longer and brighter that the tail gets. And there are actually two tails on a comet, the blue tail is formed from the ionized gasses blowing off the comet ball while the white/pink tail is from the small dust particles.
Typical visible comets can get a tail that can reach up to 150 million kilometers, much more than even the high-end length noted in the story. And as can be seen in the top picture here, the length of the tail can even get to over a billion kilometers in length.
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