A blue moon is when there is a second full moon in the month.
I went out on a moonhunt this evening and by chance found the perfect spot where the moon shone directly along the Mersey. And because I have been working between two different cameras I could not remember how to focus this one manually - and by the time I had figured it out, a cloud bank had come over. So I did get a picture but it was not the one I went out for.
Ian