In the Adelaide region of South Australia, the arrival of autumn was signalled by the heliacal rising of a star the local Aboriginal people called Parna. The appearance of Parna just before sunrise warned the Aboriginal people that the annual autumn rains (Figure 1) would soon arrive and that they needed to build large, waterproof huts. Local place names illustrate this: a hilltop campsite south of Adelaide was named Parnangga, which meant “autumn rains" and referred to the appearance of Parna in the morning sky. The identity of Parna has remained a mystery, as the identity of the star in Western terms was never given.
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| Figure 1: Autumn thunderstorm over Adelaide. Image from bom.gov.au |
But if we do a little research, we can figure out the most likely candidate for Parna. First, we look at the autumn rains. The autumn rains occur around the end of March and beginning of April. According to the Bureau of Meteorology, the average monthly rainfall in the period from 1977 to 2010 increased from 19.9mm during the summer months (December-February) to 40.9mm in April, after which it surpasses the monthly average of 45 mm and increases throughout the winter to reach a peak of 79.7mm in June (see Figure 2). The March average (24.9mm) is just above the summer average (19.9mm), showing that the increase in rainfall during April rises significantly from that in March. This suggests that Parna would rise just before dawn in mid-March.
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| Figure 2: The average rainfall for Adelaide between 1977-2010. Taken from Hamacher (2012). |
Therefore, Parna is most likely a bright star (probably 1st or 2nd order magnitude) that rises just prior to sunrise in mid-March in the southeastern sky. This leaves only one obvious candidate.
The bright star Fomalhaut, in the constellation Piscis Austrinus, rises at dawn on 15 March. At sunrise, it is ~ 22 degrees above the horizon at an azimuth of ~ 112 degrees, corresponding to the southeasterly direction. The star Fomalhaut is the best candidate for the star Parna as it meets the criteria set out and is the only bright star in that region of the sky visible at sunrise (see Figure 3).
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| Figure 3: The star Fomalhaut visible in the southeastern skies of Adelaide just before sunrise. Image from Stellarium Astronomical Software. |
Reference
Hamacher, D.W. (2012). On the Astronomical Knowledge and Traditions of Aboriginal Australians. PhD Thesis, Department of Indigenous Studies, Macquarie University, pp. 79-82.