My understanding is that it is the SECOND full moon in a calendar month that is the "Blue Moon" this week's is just a plain full moon. As it occurs about every 3 years or so it ain't all that rare either.
My mistake, just realised that full mooon was 31st July not 1st August. mea culpa. :apologetic:
If anyone is interested in calculating when full Moons are (or new or 1st ¼ or 3rd ¼) I can provide a formula.
It's not very complex, but does go on a bit (as do most astronomy formulae). All rather nerdy of course.
If I'm bored enough one evening I'll put it into a spread-sheet to calculate phases for the next few of years, highlighting when blue moons occur.
Interestingly it's one of the few lunar events that's based on the full moon - most calculations are based on the new moon.