Joule’s “Last Glimpse”


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Green Flash, Treasure Island, Florida – Photo by Robert McCoy

The first part of last month I finally saw my first green flash. As an avid sunset watcher I have been on the lookout for this optical phenomenon for years. So you can imagine how excited I was to actually see one. My only lament was that I didn’t get a photo of it.

Thankfully, my fellow jetty bum, Robert, caught a shot of it which he graciously agreed to let me share with you. Though there are different instances when these can be witnessed, all of them are the result of the refraction of light through the earth’s atmosphere (which acts like a prism).

The flattened oval in the photo above is known as an interior-mirage flash which occurs when the surface is warmer than the air above it (usually at sea level). In 1869, James Prescott Joules (a physicist) described it thusly; “at the moment of the departure of the sun below the horizon, the last glimpse is coloured bluish green.”

Categories: Observations

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