Expert: Paul Wagner - 12/8/2009

Question
QUESTION: When was the last time there was a blue moon on New Year's Eve heralding in a new decade.

I know there is a blue moon every 2-3 years.  I also know that blue moons fall on New Year's Eve every 20 years or so; but how often do they fall on New Year's Eve at the beginning of a new decade?

When was the last blue moon on a New Year's Eve that heralded in a new Decade?  When will be the next one?

ANSWER: Hi BIll

I am not going to do all the hard work for you, but here is a link to all the full moons in the past 110 years...and you can check for the last one on December 31...which, by definition, will be a blue moon.  I thought it might have been in 1980, but that was only the 29th or so...

The next one, as you might know, is in about three weeks, on New Year's Eve.  And then next time that happens is in about 29 years.  

here's the link:

http://home.hiwaay.net/~krcool/Astro/moon/fullmoon.htm


and do a little exploring on that site--it's great source of all kinds of information about the moon!

Paul Wagner

---------- FOLLOW-UP ----------

QUESTION: The answers  you provided Paul are the ones that I already knew as I thought I explained.  The real question, perhaps "the hard work" you refer to is when was the last one heralding in a NEW DECADE which means was it:
December 31st 1809, December 31st 1819, ..... December 31st 1909.  Do you understand the question?

Unless the answer(s) is/are December 31, ???9 then it isn't an answer to my question.  I am not sure how to calculate it and I don't think there are any records I can look out.  There is NO Blue Moon on December 31st heralding in a NEW DECADE in the past hundred years.
Bill Fassler

Answer
Hi Bill

Thanks for the note---and yes, I obviously wasn't clear on my last answer.  The date would have to be 12/31 at the end of a decade.  

The reason this info isn't readily available is that the definition of a Blue Moon that you are using is relatively recent--only in the last 150 years or so.  Before that, people didn't track these because the definition didn't exist.  So there seems to be little point in looking back farther.  

The math to calculate this would seem relatively straightforward.  The lunar cycle is 29.53 days, and the length of the year is 365.2425 days.  So if you start at December 31 of this year, you should be able to work back to an answer except that you have to take into account leap years.  That makes the whole thing really complicated.  And I am sorry, but I don't have a solution for that.  I know it wasn't 1809.  But there was one on December 31, 1819--at 3:36 GMT.  I don't know if there has been one since...

If you take the data from the lunar chart from the link, and plug it into a Julian Calendar converter, you can check the other dates since 1819.  You will probably want to use an excel sheet for this.  

One other thought---technically, the first decade started on January 1 of the year 1 A.D., so every decade thereafter should start on January 1 of a year ending with the digit 1.  I know this is not a common application, but if you are really trying to research something technical, it is worth noting.  

That said, I have managed to write a lot here and not definitively answer your question.  Apologies.  If you do get this one figured out, please let me know.

Paul Wagner