A friend: “Hey, did you see that blue moon last night?”
Me: “Uh, yeah, I went outside and took a peek.” (Even if I didn’t.)
Look, don’t misunderstand. I’m all for anything that gets people outside and looking at the sky, and if overblown media stories about “super” moons and “blue” moons accomplish that, then it’s a good thing. But let me explain why I don’t get all that excited about it.
- Full moons are right up there with clouds as enemies of astronomical observing. Not only are they so bright that they wash out anything but the brightest stars and planets, they really aren’t much good for lunar observing, either. With the sun directly over the middle of the lunar disk, there are no shadows, and therefore no relief. Compare these two images of the lunar crater Hipparchus, which is near the center of the moon as we view it from Earth. This first one is taken when the moon is nearly full, almost completely illuminated.
Now look at this image taken near first quarter, when we see the lunar disk as half-illuminated, with the sun casting long shadows over Hipparchus (which is the central crater). I’ve rotated this image to give it roughly the same orientation as the first one. Look at how much more detail is visible. Can you even tell you are looking at the same object?
- Full moons occur every 29 ½ days. They aren’t rare.
- The so-called “super” moon occurs when the moon is near perigee, its closest approach to Earth. The difference between perigee and apogee (point in its orbit when it is most distant from Earth) is significant.
Because the moon is closer and therefore appears bigger, it is brighter. A full moon coincides with perigee about every 14 lunar phase cycles—roughly once a year. Not rare.
- For most people, a “blue” moon is the second appearance of a full moon in the same calendar month, although that definition resulted from a misinterpretation of the original meaning. Normally there are 12 full moons in a year, three per season or quarter. These are traditionally given names such as Harvest Moon, Hunter’s Moon, etc. When the occasional “extra” full moon appears in a given quarter, the third such is designated a “blue moon” so that the fourth can retain its traditional name. The original definition leads to a less frequent occurrence, the last such being August 20, 2013 and the next being in May, 2016. The modern and looser definition will let us claim a blue moon in July, 2015. So “once in a blue moon” may not be as rare as you thought!
So what IS a rare event that you get excited about, smarty pants? Well, how about an event I observed on June 5, 2012 (albeit briefly, through clouds) that will next occur in 2117? Now that’s rare!