Is that what we should call a blue moon on the day of a lunar eclipse that also happens to be a supermoon? The moon during the eclipse turns a deep red or, most often, orange- depending on what atmospheric conditions back on Earth are like. And blue and orange make... well, brown.
This Wednesday, January 31st, there will be a weird co-incidence of three events regarding the moon:
The first, and most ordinary, is that the moon will be a "supermoon." Now don't get >too< excited about this. Those in the astronomy biz (and I count myself amongst them) know that the moon, like every other body in the solar system, orbits in an ellipse and is sometimes closer to its parent body and sometimes further away.
A supermoon is simply a full moon that occurs when the moon is relatively close to Earth. So, it appears a bit bigger than usual- about 8-10% bigger than average. That's a difference so minor that, if it weren't pointed out to us, we probably wouldn't notice it. And, if we are being honest with ourselves, even if it >were< pointed out to us, we might not be able to visually detect a difference.
Moreover, they're not rare at all. Supermoons, depending on how you define them, happen a few times a year.
In other words, it's only kinda' cool.
THE (not really) BLUE MOON
The second thing that's happening is a bit rarer. In fact, it occurs only once in a blue moon. Indeed, it >is< a blue moon!
A blue moon has nothing to do with the appearance of the moon. It's a trick of our calendar. It is, simply, a second full moon within a calendar month.
Our month is based loosely on the lunar cycle of phases which takes about 29 and a half days to go from full moon to full moon. Our months (or, should I say, "moonths") are a generally a day or two longer than that because to pad out the year. So, with 30 or 31 days to work with, it is possible, albeit uncommon, for a two full moons to occur within a given calendar month.
In fact, they occur a little more than once every three years on average. And, that is, by my reckoning, about once in a blue moon.
Alas, the moon will probably not turn blue.
A LUNAR ECLIPSE!
For skywatchers, the most exciting of the three coincidental events that occur on January 31 is a Total Lunar Eclipse.
If you happen to be in the right spot on Earth (which, this time around, is the western USA and Canada or in the Pacific) you will see the shadow of the Earth slowly take a bite out of the moon. The bite will grow larger and larger until the moon is covered.
The moon, however, will not totally disappear. From our vantage point on Earth, we will see the as a dimly glowing red or orange ball.
For those of us in the Eastern half of the USA, the sight will not be quite as dramatic. We will catch the start of the eclipse just as the moon sets in the west and the sun is rising in the east. We will miss the deepest eclipse.
Imagine, however, the view from the moon itself. At total eclipse, an observer on the lunar surface would see, when looking back at Earth, a great orange or red ring of fire in the sky- all the sunsets in the world >at the same time< as the sunlight streams through our atmosphere. Everything around that observer would be glowing in the eerie light of this ring.
The sight would be astonishing.
Perhaps someone alive today will witness this sight.
Until then, if you happen to be in the right spot, get up early and have a look... and enjoy the co-incidence!