Blue Moon??

S&T may have retracted it but they are happy to live with
it now.
check thier website for the story.

2 Full Moons in any one calendar month is my understanding too. Once every 2.7 years.

Seanie.

I recall doing a radio interview and I looked up all the various definitions. For the sake of an easy explaination I plumped for the "two full moons in the same calendar month" as published by S&T.

Another reference (possibly the same one) also referred to the case where there are two such instances in the same calendar year - it is the second instance that is called the "blue" moon.

Also, I found some data that suggested this happens every two to three years.

Dave,

There was an interesting graphic the other day which had year on the vertical axis (ex. 2000, 2001, etc) and month on the horizontal axis with a small circle depicting months with two full moons.

You would be surprised how often they happen. The reference above to 2.7 yrs seems quite accurate and, in fact, on the basis of my memory of the graphic, I would not be surprised if it is even more often than once every 2.7 yrs.

Anthony.

PS. This graphic is similar to the one I had seen the other day also on the S&T website: www.skyandtelescope.com/observing/object...5141.html?page=2&c=y ... and indeed the 2.7 yr average seems dead on.

Taking calculations to three decimal places

Synodic period of moon (time from full moon to full moon) = 29.531 days.

Earth year 365.243 days.

Number of synodic periods in one year = 365.243 / 29.531 = 12.368

That is twelve with a bit over.

How long does it take for those bits to round over into a whole number (an extra full moon)

We take the remainder 0.368 and take the reciprical ( divide it into one)

1/0.368 = 2.716 years

Any excuse to use my calculator...

Mark