Greatest libration of the year

Greatest liberation of the year, did you say?

Alas, no. “Libration” is a word that the spell-checker doesn’t know, and underlines with a squiggly red line. But there is such a word, and it means the “rocking” of the Moon. This, even if not in the same class as liberation for excitement, is quite an interesting phenomenon. But again I’m going to disappoint you: though it is about to happen, you won’t be able to see it (this time). Read on!

Latin libra means a balance, whose beam and pans oscillate up and down according to the weights placed on them. The constellation Libra looks like such a pair of scales. Does the Moon then rock or sway, like a balance, kelly-doll, or cradle?

It appears to keep the same face to us all the time as it goes around us, but those who examined it carefully discovered that the scenes at the Moon’s limb (its visible edge) subtly change. A certain mountain peak may be just visible; but sometimes it is gone, and at other times other peaks beyond it prick into view.

Those peaks are in Luna Incognita, the Unknown Moon. A shiveringly evocative name for a territory beyond a world’s visible edge, a land that we thought we could never see but that steals into view – a tantalizingly foreshortened view – at these magical moments!

Or so it was; I first learned of it from an astronomy journal that used to have an article on observations of Luna Incognita. That was before satellites were sent around the Moon and showed us the whole of its far side all too easily.

One mountain range that showed a little of itself on the Moon’s western edge (formerly called the eastern edge – let’s not get into the reasons for that now) was suspected as being part of a circle surrounding a “sea,” like one of the huge dark “seas” which mark the near side. The suspected invisible basin was named (by Julius Franz in 1906) Mare Orientale, “eastern sea.” And lo! when spacecraft photographed the far side, there was indeed such a huge Mare Orientale, the Moon’s largest and most bull’s-eye-like impact basin.

Why does the Moon librate? One reason is that it goes around us in a plane inclined at 5 degrees to the plane in which we travel (the ecliptic), so sometimes we look slightly “up” and sometimes slightly “down” to it, and therefore see more at its south or north poles. Another is that it goes around us in an ellipse, not a perfect circle, so that it is moving sometimes faster and sometimes slower along its orbit, and we sometimes see more at its east or west sides. A third reason is that you, yourself, are being carried around by Earth’s rotation, so that at sunrise and sunset you look toward the Moon at angles left or right of where you would be looking if you were at Earth’s center.

The combined effect is that the area of Moon visible from Earth is not just 50 percent: a further 9 percent shows, bit by bit, over a period of 30 years. That band of territory, maybe a hundred miles wide, is Luna Incognita, an abolished kingdom rather like Lotharingia or Tannu-Tuva.

There is always, somewhere around the Moon’s limb, a point where libration reaches deepest into Luna Incognita. Sometimes it hardly reaches at all (the Moon’s face then being its average hemisphere), other times more deeply, and every couple of weeks or so it reaches a maximum, then shrinks back. One of these maxima is the greatest libration of the year. This year it happens, by my calculation, on September 15 at 20 hours Universal (Greenwich) Time, in other words, the night between Sep. 15 and 16. The view at a certain point on the edge of the Moon reaches 9 degrees beyond its average edge. (The maximum can sometimes be more than 10 degrees.)

Since such a thing isn’t a sharp peak, the exact moment, even the exact night, doesn’t make much difference. But:

What does make a difference is the Moon’s-edge position where it happens. A large libration-view is no use to us if it is in a part of the Moon’s edge that is in darkness.

Here is an enlarged bit of one of the charts from my Astronomical Calendar 2014. The Moon is shown at 8 times its real size.  Libration is indicated by the red tabs sticking out from the Moon: they are at the points where libration is greatest, and are longer if the libration is greater. And as you can see, on the 15th and 16th the center of libration is on the northeastern quadrant. And as the Moon is at Last Quarter, its western side is illuminated by the Sun, a quarter of the way around the sky to the left.

Look toward the spot where the red tab is and your eyes will be aimed deep into Luna Incognita, but you will not see it: it will be part of the black sky.

Never mind, there are other reasons to go out and look at the Moon, after it rises in the east about 10 (on the 14th) or 11 or midnight. It reaches the top of its arch around the map of the sky, in the constellation Taurus, after sailing past the Pleiades and in front of the Hyades stars (which my picture has to show in front of it because the Moon is enlarged). The line across the map is the ecliptic. The point of libration is on the northern edge of the Moon, because it is south of the ecliptic and we are in this sense looking “down” at it.

Libration was the thing to talk about now because now is its maximum for the year. I intend to mention it again at a date when it is nearly as great and can be seen.

I can’t help also mentioning that I started this way of marking libration with a red spot, which Sky & Telescope borrowed, at first with acknowledgment. And that, besides “liberation,” libration should not be confused with “libation,” as happened once in the pages of Science News. Libation can cause wobbles of another kind, being the swigging of wine.