Definition of Terms

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.

About waynehale

Wayne Hale is retired from NASA after 32 years. In his career he was the Space Shuttle Program Manager or Deputy for 5 years, a Space Shuttle Flight Director for 40 missions, and is currently a consultant and full time grandpa. He is available for speaking engagements through Special Aerospace Services.
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10 Responses to Definition of Terms

  1. Sean says: Wikipedia lists them as synonyms, I don’t know what to believe!

  2. Beth says:

    Well said, Wayne. Like any engineer, I prefer my terms and usage to be as precise and correct as possible. Not always achievable, but always a worthy goal.


  3. Steve says:

    There is a reason that I take a few minutes before technical meetings to set some ground rule definitions before proceeding. Some roll their eyes while others sit quietly in their thankfulness that some other guy swallowed his pride and made such a basic affirmation that everyone else was wondering.

  4. Dan Adamo says:

    I’m sure a vast majority of my astrodynamics colleagues would agree with your admonition, Wayne. Another important aspect of Lagrange point terminology I see lost all too often is context. What 3-body system are we talking about when an author launches into a discourse about “L2”? If it’s the Lagrange point over the Moon’s farside, it should be termed “Earth-Moon L2” or “EML2” IMHO.

    BTW, one of the foundational Space Age texts on the restricted 3-body problem in which Lagrange points are derived is Victor Szebehely’s “Theory of Orbits” published in 1967. Szebehely refers to Lagrange points as “equilibrium” points because acceleration of the massless third body is zero at these locations when expressed with respect to a frame uniformly rotating with the first and second bodies about their common center of mass. And, to further add to the terminology chaos, Szebehely places the second equilibrium point BETWEEN the first and second body. Under this convention, the Lagrange point over the Moon’s farside would be “EML1”. To be clear, I do NOT advocate using Szebelhaly’s naming convention.

    And I would be remiss in not correcting your implication that the Moon’s geocentric orbit plane is inclined 5 degrees with respect to Earth’s equator. That angle is the inclination of the Moon’s orbit with respect to Earth’s heliocentric orbit plane, the ecliptic. This small angle, is one reason why the Sun cannot shine very far into polar craters on the Moon. So, because Earth’s equator is inclined 23.5 degrees with respect to the ecliptic, the Moon’s orbit plane can be inclined to Earth’s equator by as much as 23.5 + 5 = 28.5 degrees or as little as 23.5 – 5 = 18.5 degrees. These variations play out as the Moon’s orbit plane precesses over an 18.5-year period. Currently, the Moon’s equatorial inclination has passed its minimum value and has increased to 22.9 degrees. When at maximum inclination in a few years, the full Moon nearest winter solstice will shine from nearly overhead at JSC’s latitude (29.5 degrees N).

  5. Paul Schermerhorn says:

    C’mon Wayne, we both know that the Moon has never been liberated.

  6. rangerdon says:

    Now, if only we can dustbin crude spaceships….

  7. Karen Johnson says:

    This reminds me of our time at JSC and the efforts to clarify (and break the habit of using) “zero gravity,” and “nominal.”

  8. I had never heard of libration points before reading this, but it’s hard to see how anyone could confuse them with Lagrange points. They’re totally unrelated.

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