I guess I’m getting to be an old curmudgeon (Hey Kids – get off my lawn!) but there are some irritants in life that just seem to capture my attention, no matter how trivial they may be. So, if this post applies then use it; if you find it is off the wall please excuse it as an artifact of my advanced age.
A number of years ago my daughter, who was very interested in amateur astronomy, did a science fair project of the libration of the moon. Here are a couple of pictures from her report:
Everybody knows that the moon shows only one face to the earth – we can never see the far side (please don’t call it the ‘dark side’). It is “tidally locked” with the earth. But maybe not quite.
It turns out that the moon’s orbit is not perfectly circular, there is some eccentricity in its monthly ellipse around the earth. If you hear about ‘supermoons’ or ‘micro-moons’ you know that there is an apogee and perigee in the lunar orbit. Not only does the moon come closer and farther away by a fraction every month but orbital mechanics dictates that it slows in orbital velocity at apogee and speeds up near perigee. So, based on when you look it is possible to see a little of the ‘far side’ depending on where the moon is in its orbit.
Similarly, the moon does not orbit the earth at the equator, but its orbit is inclined about 5 degrees. This means that sometimes the moon is a little north, and sometimes it is a little south of perfectly in line with an earth-based observer.
Put together, it is possible to see about 9% of the far side of the moon, in pieces, at various times.
The spot on the edge of the moon that is tilted the most toward an earthly observer is called ‘the libration point’.
Have you heard that term before? I bet you have but in a different context.
Joseph-Louis Lagrange (1736-1813) was an Italian mathematician who played a large part in the development of the metric measurement system (SI) in post-revolutionary France. He also studied orbital mechanics involving three bodies (e.g. sun/earth/moon) and mathematically proved there are locations around such an orbit which are gravitationally stable. These points are called Lagrange points in his honor. There are typically 5 such points and I will leave it to the student to research their locations.
As you can see Lagrange points and Libration points are quite different and literally have nothing to do with each other.
But if you read any number of popular media stories – and even several NASA technical papers – there appears to be confusion and the terms are used interchangeably. This is so widespread that some dictionaries have started changing the definitions to keep up with what appears to be popular usage.
Unfortunately, the curmudgeon in me realizes that this erroneous usage has become so common that it will be hard to change usage in popular literature.
But at least you now know the difference. And you, like me, will stop when you hear some ‘expert’ (never an astronomer) mixes the terms and think about how much ignorance is being displayed.