Can anyone tell me why....

[Doug Sundseth]

That definitely looks like an internal reflection (lens flare). Whenever you have a bright* light source in your frame, you're likely to get some sort of ghosting or flare. This can be significantly increased if you have a filter (especially an uncoated filter) on the front of the lens. (And it's a reason I only put filters on for effects – they don't provide much protection anyway.)

* Bright relative to the rest of the frame. It need not actually be very bright in absolute terms.

[BryanC]

It's sometimes called "ghosting". There's a few things involved here, but ultimately, it's due to the filter. You have a bright light source, (the moon), in a high contrast situation, (dark sky). Sensors have their own UV type filter covering them that is reflective. The light of the moon is coming in, reflecting off the sensor, back out through the lens and then is reflecting off the back of your filter, and back into the camera. The reflection gets inverted during this trip, so there's your tell-tale sign. The reflection is easily visible because it's set against the dark sky.

[monstermum]

Originally Posted by BryanC

It's sometimes called "ghosting". There's a few things involved here, but ultimately, it's due to the filter. You have a bright light source, (the moon), in a high contrast situation, (dark sky). Sensors have their own UV type filter covering them that is reflective. The light of the moon is coming in, reflecting off the sensor, back out through the lens and then is reflecting off the back of your filter, and back into the camera. The reflection gets inverted during this trip, so there's your tell-tale sign. The reflection is easily visible because it's set against the dark sky.

WOW! This is a great, clear explanation, thanks Brian. I get it. I like the effect but I would rather have it there on purpose.
I appreciate all the answers!

mm

[3bayjunkie]

I think its interesting that you exposed for the dark side of the moon. Cool take on the photo