Re: Questions about two Blue Moons

Hi Timothy,

To research your question I did a web search on "blue moon" and came up 
with lots of interesting companies like the Blue Moon Detective Agency and 
Blue Moon Records. The name is certainly a very popular and evocative one, 
hence our interest in the topic.

Amongst the other websites I found a very useful and informative website 
about Blue Moons, complete with a Blue Moon Calendar.

http://www.obliquity.com/astro/bluemoon.html

This site reminded me of a fact I had forgotten. The Moon's phases (and 
lunar eclipses) repeat on a 19 year cycle (the Metonic cycle), so your 
pattern of two Blue Moons either side of a no-full-moon February should 
have occurred in 1980 and 1961. However, because of a slight (2 hour) 
difference between the length of a Metonic cycle, and the exact number of 
days, plus the complication of leap years, the pattern isn't quite regular 
and 1980 got missed out. (In fact, in 1961 the second Blue Moon was in 
April, not March! The exact pattern last occurred in 1915!)

There will be no Blue Moons next year (2000). The pattern of two blue moons 
only occurs once in each Metonic cycle, so will recur in 2018 and 2037, but 
by the time 2056 comes, the two hour difference per Metonic cycle between 
cycle length and exact number of days will have shifted one of the full 
moons away. The website I referred you to has a very good discussion of the 
pattern of recurrence of these Double Blue Moon years, but it doesn't have 
a special name for them.

http://www.obliquity.com/astro/blue3.html

Take the time to read the pages on that website, they will tell you lots of 
little details I don't have space for here, including a full list of Double 
Blue Moon years for several millennia!

Nick