Types of Blue Moons Explained

The Next Blue Moon Date

That Blue Moon may not really be...!

More Blue Moon Information @ Seasonal Blue Moon

So, what is a blue moon, really? Seems that this term is more of an enigma than most of us ever realized. While there are some songs about the blue moon, most seem to really be about being sad and lonely. There are a lot of cafes and book stores out there with blue moon in the name. I'm not at all sure why, other than it seems to be a cool name.

There are a few stories about the moon turning blue in the evening sky (or appearing to turn blue) as the result of an enormous wild fire in Canada in September, 1950. There is another similar story of the blue moon event occurring in the 1880s due to volcanic smoke in the sky on an island in the Pacific Ocean. From the sounds of it, there have been, on occasion, the appearance of blue moons from time to time.

But the term seems to have been attached to a familiar and common event in only the last 30 years. It does not even involve the color blue. When we can view a Full Moon two times in one calendar month, then that is also referred to as a "blue moon." Some individuals claim that the second calendar Full Moon was based on the phrase "once in a blue moon." Yet, in truth, that seems strange since the "two full moons in a calendar month" happens about once every 2.5 years.

An even rarer event is when we have two calendrical blue moons in one calendar year. This unusual dual blue moon situation last occurred in 1999. Blue moons occurred in January and March, with no Full Moon at all in February. Now that's not the first time it has occurred. It seems to occur about 19 years apart. But that time frame is not set in stone. I went back in time looking for other occurrences. The years that I found it to occur were 1809, 1847, 1866, 1885, 1915*, 1961** and 1999. The future years are 2018, 2037 and 2094. But, as you can quickly ascertain, it is not like clockwork.

Now, we have to be realistic here. When it comes to the time of day and Full Moons, a lot depends on the "time zone" we are using when determining that event. If a Full Moon occurs on the first day of the month or on the last day of a month, there is the possibility that the occurrence will happen on the day before or day after based on a different time zone. I used GMT (Greenwich Mean Time) for the above years to create a sense of consistency.

It is possible to have two full moons in any calendar month with the exception of February since there are normally only 28 days in that month, (except during a leap year when there are 29 days in February). Unless I'm wrong (and yes, that is conceivable) I do not think it is possible for a second Full Moon to occur in the month of February. If a blue moon were to occur in February, then that would truly be a "once in a blue moon" event! I was not able to find any February blue moons in my 300 year search of unusual moon events. If anyone can find a February blue moon, please feel free to send me an email with the year and time zone that it occurred in.

The average number of days between Full Moons is 29.53059 days. This is called a lunation or synodic month. However, that is not an "exact" and it is possible to be a little shorter or a little longer in time depending on other factors. Now, if your math is up to snuff, you begin to see that it is possible to have 13 full moons in a calendar year since the 13 Full Moons would occur in approximately 354 days while there are 365/6 days in a year. This explains why we end up with an "extra" Full Moon to deal with once in a while in a given year.

Now that is the basic and recent definition of the blue moon. However, as with any story, there are other perspectives and information in defining and describing the meaning of the blue moon. Therefore, this page is a work in progress and will be updated as more information is researched and discovered. Several years ago, there was a great article written by Philip Hiscock, "The Folklore of the Blue Moon" that could be found online at the Griffith Observatory website. But it is no longer available. The link takes you to the last grab by the Wayback Archives of the article, and definitely give it a read.

My favorite resource is no longer maintained, but you can still read the archived pages of moon facts and information at Keith's Moon Facts.