Tuesday, May 10, 2011

Comets, Comets, Comets!

About once every five years or so, our skies are graced by a brilliant visitor, stretching across the sky like a "fuzzy star". These are comets - "dirty-snowballs" that become visible to us as they approach the sun. Comets are like icy asteroids... they are composed primarily of relatively low-density ice and dust. Most comets can be found in the spherical shell of debris in the outer solar system called the Oort Cloud and in a belt of debris beyond the orbit of Neptune, called the Kuiper Belt.  Astronomers believe that up to one trillion (that's a million-million) comets populate the Oort cloud, which extends up to one light year from the sun... nearly a quarter of the distance to the nearest star system (Alpha Centauri). In deep space, a comet looks much like an asteroid - there's no tail and it's not particularly bright. But if a nearby star or massive object passes close by our solar system, its gravity can knock the comet from its slow orbit and send it reeling towards the sun.  As it gets closer and closer, the ever-increasing intensity of the solar wind begins to vaporise the icy dust on the surface of the comet, which trails behind the comet. This is the tail of the comet and its most recognisable feature (Figure 1). The tail can stretch for millions of miles, littering the solar system with trails of debris, called meteoroid streams, which are the source of annual meteor showers as the earth passes through these dirty stretches of our cosmic neighborhood.

Figure 1: Comet Hyakutake: Great Comet of 1996.
Image courtesy of NASA.

Contrary to popular belief, the tail always points away from the sun, even as it moves away (because the solar wind always radiates from the sun). Some comets originate in the solar system closer to the sun and have rather short periods, which can be seen in the skies at regular intervals. The most famous of these periodic comets is Comet Halley, which appears in our skies every 75-76 years. As time progresses, these comets become lose more and more debris, shrinking smaller and smaller until the solar wind completely vaporises them. Other comets are either ejected back into deep space or fall to their deaths directly into the sun. 

Throughout history, comets have been seen with fear and fascination, frequently attributed to the world of celestial beings or as omens of death and destruction. When Westerners began studying Aboriginal communities in the early 1800s, they noted that the Aboriginal people viewed extraordinary or unusual natural events with great dread. The unexpected arrival of a bright comet often triggered fear and were associated with death, spirits, or omens – a view held by various cultures around the world. Such views include those of the Tanganekald of South Australia who perceived comets as omens of sickness and death, the Mycoolon of Queensland who greeted comets with fear, the Kaurna of Adelaide who believed that the sun father, called Teendo Yerle, had a pair of evil celestial sisters who were “long” and probably represented comets, and the Euahlayi of New South Wales saw comets as evil spirits that drank the rain-clouds causing drought, with the cometary tail representing a large thirsty family that drew the river into the clouds. The Moporr clan of Victoria described a comet as Puurt Kuurnuuk - a great spirit, while the Gundidjmara of Victoria saw a comet as an omen that lots of people will die.

Aboriginal people in the Talbot District of Victoria likened comets (called “Koonk cutrine too”) to smoke, where “too” means “to smoke”. This is similar to a report from Cape York Peninsula, where an Aboriginal community saw a comet as the smoke of a campfire.  Similarly, the Aboriginal people of Bentinck Island in the Gulf of Carpentaria called a comet burwaduwuru, which means "testicle with smoke".  A common view among Aboriginal communities of the Central Desert links comets to spears.  A Pitjantjatjara man named Peter Kunari described comets as the manifestation of a being named Wurluru who lived in the sky and carried spears that he occasionally threw across the heavens (a possible reference to meteors?). A similar association is shared by the Kaitish.  The Rainbow Serpent, a much-feared evil spirit found in the Dreamings of many Aboriginal groups, was sometimes associated with comets. Some speculate that the origins of the Rainbow Serpent lay in transits of Halley’s comet, which was seen every 76 years, reinforcing stories handed down by Kuku-Yalanji law-carriers and custodians of the Bloomfield River, Queensland.

In 1899, Spencer & Gillen described a form of evil magic called Arungquilta, which involved meteors and produced comets and was used to punish unfaithful wives in Arunta communities. If a woman ran away from her husband, he would summon men from his group and a medicine man to perform a ceremony intended to punish her. In the ceremony, a pictogram of the woman was drawn in the dirt in a secluded area while the men chanted a particular song. A piece of bark, representing the woman's spirit, was impaled with a series of small spears endowed with Arungquilta and flung into the direction of where they believed the woman to be, which would appear in the sky as a comet (bundle of spears). The Arungquilta would find the woman and deprive her of her fat. After the emaciated woman died, her spirit appeared in the sky as a meteor.  In 1907, Carl Strehlow cited a nearly identical ceremony. However, in Strehlow’s account, the man felt pity for his wife and decided to revive her by rubbing fat into her body. As she healed, the comet faded from view. In some Arrernte and Luritja communities, comets are spears thrown by an ancestral hero to make his wife obedient to him. To some Arrernte clans, a comet was also a sign that a person in a neighbouring community had died, usually because of infidelity, and pointed to the direction of the deceased.  A similar description is given about the Karadjeri of coastal Western Australia, but is instead attributed to meteors. Given the two accounts by Spencer and Gillen of the same ceremony, it is possible they are confusing comets with meteors.

Table 1: Aboriginal words for comets from across Australia. Click on the image to zoom.

Next week: Historic Comets in Aboriginal Astronomy

For more information (click on link for PDF of paper):

Hamacher, D.W. and Norris, R.P. (2011). Comets in Australian Aboriginal Astronomy. Journal of Astronomical History & Heritage, Vol. 14, No.1, pp. 31-40.

See also:  Comets Triggered Aboriginal Tales of Doom, by Stuart Gary, ABC Science (18 October 2010)

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