When do you call a full moon »blue moon«?

When do you call a full moon »blue moon«?

When do you call a full moon »blue moon«?

Everyone knows that about once a month is full moon … but, strangely enough, full moon is always on a different day each month and always at a different time. The reason is that the moon month is shorter on the average, than the calendar month. Without being to mathematical, one can say that a moon cycle lasts approx. 29.5 days. This is how the time of full moon spreads irregularly throughout the calender months and it happens (about every 2.5 years) that two full moon dates fall onto the same month, one at the beginning and one at the end of the month. This second full moon is called »blue moon«.

Februrary presents a special case, as it is the only month with 28 or 29 days that is shorter than the moon month and simultaneously surrounded by two months with 31 days. When no full moon occurs in February, there will be a full moon at the beginning and end of January, as well as at the beginning and end of March. Thus, a blue moon in January, no full moon in February and another blue moon in March. The next time this will happen, will be in 2018 (recurs approx. every 19 years):

• Full moon on 2nd January 2018 and 31st January 2018
• No full moon in February 2018
• Full moon on 2nd March 2018 and 31st March 2018

But why is it called blue moon in the first place? Particularly, when it is not at all blue. Various explanatory models and hypothesis exist of its origin. On one hand, history speaks quite frequently about full moon appearing in a special bluish light shortly after a volcano eruption. On the other hand, the phrase »once in a blue moon« is used to describe a rare event. Yet, this could derive from an Old English poem of 1528, which states:

»Yf they say the mone is blewe
We must believe that it is true.«

It is reported that initially, blue moon was referred to the fourth full moon in a season, then later the second full moon within the calendar month. And this is the way it remains today.

The next blue moon will be on 31st August 2012, after the full moon on 2nd August 2012 will have already taken place [finally an opportunity to use future perfect]. We would point out that the blue moon will not look differently to any other full moon (and particularly not blue), but will only be special in our thoughts.

If you do not want to miss this date, simply register to receive our free full moon memo and you will be reminded via e-mail.