
I had said in the comments: "I know it’s out of focus, but why? And why is the sun a full circle? It was only a quarter of the way up."
Oddly, some people were talking about the "circle of confusion" (a complicated subject in photography) and others were talking about the song by Cyrkle, "Red Rubber Ball" (because, as Paul Simon wrote in the lyrics, "The morning sun is shining like a red rubber ball").
Several commenters usefully brought up bokeh — which I understand — but, as I said in the comments, "I thought the camera would do that in the parts of the photo that were not deemed to be the subject. I've aimed the camera at the sun like this thousands of time and I've never seen it 'decide' that the sun is the subject and everything else should be gently fuzzed — especially with this additional effect of completely reshaping the sun, showing a circle for something that was visible as less than a half circle."
Kylos responded: "It’s actually the opposite. It’s focused on something nearby instead of the sun. My experience is that shooting into the sun can cause autofocus to get confused. Possible a lens flare or glowing dust particle caused the autofocus to think the subject was inches from your camera. Because the sun is also completely out of focus, it’s circle of confusion ends up projecting on top of the darker horizon."
That pointed me at what I declare to be the answer: "Maybe an insect flew by, got focused on, then went off camera as the image was captured. That's my theory! The phantom gnat!"
Interesting to wear a buzz cut in 1966. From the Wikipedia article on Cyrkle:
