Dark Ring Around the Sun: Altocumulus Cloud Shadow?

[Alec Jones]

Hi Everyone,

I recently took this picture of what appears to be a dark ring around the sun,

http://theboltonhalotriangle.blogspot.c ... hadow.html

At first glance, it appears to be an altocumulus cloud casting a shadow on to some sort of projection layer. The effect was visible to the eye, not just photographically. I took one or two other photographs at the same time, separated by a few seconds, but none of them show this dark ring. What is striking is that the ring is so even and regular. While I have seen cumulus clouds cast shadows frequently, this is the first time I have seen Ac clouds do the same. Can the effect be explained purely in terms of perspective?

Would anyone like to comment on what they think this dark ring is and what might have produced it? Thank you in advance,

Regards,

Alec