On Friday, much of the world will have a “blue moon.”
The moon won’t actually be blue in color. The expression “once in a blue moon” is a statement about the rarity of an event, not about the moon’s color.
The expression “once in a blue moon” existed long before anyone tried to define exactly what a “blue moon” is. I believe that someone tried to come up with something about the moon that rarely happens, and got the idea that it might be an “extra” full moon in a given period of time.
In most years we have 12 full moons, one in each month. But because the lunar month is slightly shorter than the calendar month, some years, such as 2015, have 13 full moons. The synodic month (full moon to full moon) averages 29.530589 days, which is shorter than every calendar month in the year except February. Those extra 1/2 or 1-1/2 days accumulate over the year, causing the date of full moon to migrate through the calendar month.
To see what I mean, here is a list of full moon dates in 2015:
If we look forward to 2016, we see the date of full moon continuing to move forward through the month until it falls on the December 14 at the end of the year.
Full moon is an instantaneous event, when sun, Earth and moon fall close to a straight line. It occurs at the same instant everywhere in the world, whether the moon is above or below the horizon. The full moon on July 31 occurs at exactly 10:43 Coordinated Universal Time (UTC), the time that astronomers and pilots use. It is equivalent to Greenwich Mean Time (GMT), the local time in Greenwich, the suburb of London England where the Royal Observatory was established by King Charles II in 1675.
Universal Time always uses the 24-hour clock, no “a.m.” or “p.m.” The day starts at midnight (0h), noon is 12h, and the last minute of the day begins at 23h 59m.
The time zones across the world are all measured from Greenwich and expressed in the number of hours they differ from Greenwich time. In most of eastern North America, we are currently on Eastern Daylight Time (EDT), which is obtained by subtracting 4 hours from GMT or UTC, so full moon occurs at 6:43 a.m. EDT.
As we move west across North America, we have to subtract more hours from UTC to get the local time. For example, on the west coast, we use Pacific Daylight Time, which subtracts 7 hours from UTC. Full moon occurs at 3:43 a.m. PDT.
So, where did the idea come from that this extra full moon should be called a “blue moon”? The idea seems to have originated in the Maine Farmers' Almanac, which started listing the quarter of the year in which 4 rather than 3 full moons occurred. In 1946, Sky & Telescope published an article which misinterpreted the older definition and defined a blue moon as the second full moon in a month. This definition was forgotten until the popular board game Trivial Pursuit used it as one of its trivia questions in 1986. This made this rather arcane definition mainstream.
So, when you look at the “blue moon” on Friday morning, don’t expect to see any different color. Just be aware that it is a byproduct of the many compromise we make so that our calendar works well most of the time.