A photo of parhelion

18/09/2011

Driving from Zagreb to the island of Krk on Friday afternoon, 16th of September 2011, somewhere in the region of Gorski Kotar, I spotted an instance of the atmospheric phenomenon known as “mock sun”, “sun dog” or, more scientifically, “parhelion“. It is a reflection of the sun in tiny ice crystals that constitute high altitude cirrus clouds. A perhelion looks like another, smaller and dimmer sun on either or both sides of the sun, at the same level and not very far from it (exactly 22 degrees away from it). Here is a picture that I asked my wife to make with my mobile phone.

The parhelion is indicated here:

There is a late doxographic account in Aetius which informs us that the reputable presocratic philosopher Anaxagoras of Clazomenae “explains the so-called parhelia” in a similar fashion in which explains the rainbow, which he takes to be “a reflection of the sun’s radiance from a thick cloud” (Aet. III.5.11= 59 A 86 Diels). A more detailed account is found in Aristotle’s Meteorology and I invite the reader to check the accuracy of this account at the picture above:

Parhelia and sun-rods always appear beside the sun,  and not either above or below it or opposite to it; nor of course do they appear at night, but always in the neighbourhood of the sun and either when it is rising or setting, and mostly towards sunset. They rarely if ever occur when the sun is high, though this did happen once in the Bosporus, where two mock suns rose with the sun and continued all day till sunset. (Meteor. III.2 372a12-16, tr. Lee, slightly modified)

I suppose that the curiosity of the Bosporus parhelion was not that it was double – that occurs relatively often – but rather (i) that it lasted the whole day, including (ii) when the sun was high in the sky. A bit later, in Meteor. III.6 377b28-a12, Aristotle gives his detailed explanation for the characteristic appearance of parhelia, notably why they “occur at sunset and sunrise, and neither above nor below the sun, but beside it, neither very close to the sun, nor very far off”. No need to go into his obscure explanation here, though.


Double rainbow

06/09/2010

In his work Meteorologica (III.2, 371b33-372a3), Aristote wrote:

Not more than two rainbows occur at the same time. Of two such simultaneous rainbows each is three-coloured, the colours being the same in each and equal in number, but (i) dimmer in the outer bow and (ii) placed in the reverse order.

On 31 August 2010 Zagreb saw some drammatic weather, including heavy nimbostratus clouds which started to tear apart on the western horizon at the sunset. Here are two photographs I made from the window of my study, facing east, which verify Aristotle’s points (i) and(ii). I became aware of these two facts only after reading the quoted passage.


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