Wednesday, February 27, 2013

Arcturus: Food and Seasonal Change

Aboriginal people have been using the stars as indicators of seasonal change for thousands of years.  In the following series of blog posts, I will show you how this was done in some depth. Today, we begin with the brightest star in the constellation Bootis, a red giant called Arcturus.

Yolngu - Arnhem Land

Our first example comes from the Yolngu people of Arnhem Land.  When the star Arcturus appears in the dawn sky (just before sunrise), the Yolngu people begin harvesting the corms of the spike-rush. The spike-rush (Eleocharis dulcis), also called the rakia, water chestnut, or gulach (Figure 1), is a grass-like sedge with an edible tuber, which can grow to a height of over 1.5 m and is found across northern Australia. The plant had numerous uses for Aboriginal people, which included making baskets or fish-traps from the grass-reeds or eating the carbohydrate-rich tubers. The growth rate of the spike-rush is dependent on the level of rainfall, with the tubers growing when the water saturates the top several centimeters of soil. Because Arnhem Land has a monsoonal climate with wet and dry seasons, the tubers are generally harvested during the wet season when the rains have provided sucient rain for the growth of the tubers.

Figure 2: The spike rush, also called the rakia. 
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Approximately 90% of the annual rainfall in northern Australia (~0.9-1.2 m) occurs during the summer months of November to April. Therefore, the tubers of the spike-rush are harvested after the start of the wet season, roughly in late November when Arcturus is about 10 degrees above the horizon at sunrise.

One of the interesting aspects about linking calendars with the stars is that the stars are gradually moving over time.  Stars move with respect to each other as they orbit the centre of our galaxy (which we call stellar proper motion). As they orbit, their distances to us change, which affects their brightness. Stars also change their position with respect to the horizon as the earth wobbles on its axis due to precession.  This means that the rising and setting positions of the stars at certain times of the year will gradually change over time.  Because of the effects of precession, Arcturus would not have been a useful indicator star for the harvesting of the spike-rush over 7,000 years ago.  This means that prior to that time, Aboriginal people would have used a different star.  We do not know what star they used, but prior to 7,000 years ago, the bright star Antares in the constellation Scorpius would have been suitable. This is an example of predictive cultural astronomy.

Wergaia - Western Victoria

Our second example comes from the Wergaia people of western Victoria. In Wergaia traditions, Marpeankurrk was a woman who lived in the mallee scrub.  One day, she was in the bush looking for her food. Her people were starving. It had not rained for a long, long time. Rivers and billabongs had disappeared - the bul-rushes had shrivelled up and died! She lifted up logs but could find no lizards or snakes. She looked around and saw that there were also no grass seeds or fruit to eat.

After walking for many hours she saw a wood ant's nest. So desperate was she that she went to it and opened up the nest with her digging stick. In the nest she saw thousands of larvae. She put one in her mouth and ate it. The larvae were delicious! She collected all that she could and hurried back to her people. The larvae of the wood ant saved the people. It soon became their favourite food. When Marpeankurrk died she went up into the sky and became the star Arcturus.
When Marpeankurrk appears in the northern evening sky, the Wergaia people begin collecting the larvae of the bittur (wood ant).  The larvae (Figure 2) were an essential food source during the winter months of August and September.

Figure 2: The wood ant and larvae. 
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When Arcturus sets just after the sun, the larvae are gone and summer (Cotchi) begins. Arcturus is ~30 degrees above the northern horizon at sunset as seen from this region at the beginning of August and sets just after sunset by the beginning of October - the beginning of the summer months. The northerly position of Arcturus in the Victorian sky (due to the higher latitude of Victoria) means that Arcturus remains relatively low, never exceeding 33 degrees above the horizon.  Because of this, Arcturus will rise and set over the course of 10 hours, less than a full winter's night.  But as with the Yolngu, Arcturus would not have been a useful star for denoting this period more than 7,000 years ago.

This shows us that Aboriginal sky knowledge gradually changes as the cosmos evolves.

Next: Capella and the Beehive Cluster


Hamacher, D.W. (2012). On the Astronomical Knowledge and Traditions of Aboriginal Australians. Doctor of Philosophy thesis, Department of Indigenous Studies, Macquarie University, Sydney, Australia, pp. 76-78.

A Celestial Calendar: Questacon