Diana Hsieh posts this story, about supposedly educated people. I’d like to think it’s apocryphal, but sadly, I’ve had too many similar experiences to think so. She, and husband Paul, via whom I found the link, say it’s good for a laugh, but I don’t find it very funny.

About 6-7 years ago, I was in a philosophy class at the University of Wisconsin, Madison (good science/engineering school) and the teaching assistant was explaining Descartes. He was trying to show how things don’t always happen the way we think they will and explained that, while a pen always falls when you drop it on Earth, it would just float away if you let go of it on the Moon.

My jaw dropped a little. I blurted, “What?!” Looking around the room, I saw that only my friend Mark and one other student looked confused by the TA’s statement. The other 17 people just looked at me like “What’s your problem?”

“But a pen would fall if you dropped it on the Moon, just more slowly.” I protested.

“No it wouldn’t,” the TA explained calmly, “because you’re too far away from the Earth’s gravity.”

Think. Think. Aha! “You saw the APOLLO astronauts walking around on the Moon, didn’t you?” I countered, “why didn’t they float away?” “Because they were wearing heavy boots,” he responded, as if this made perfect sense.

As the piece points out, this was a philosophy major, who would have presumably had a class or two in logic. There was a time that philosophy majors could, and would have been expected to understand physics, because physics and science itself was in fact an outgrowth of philosophy (it was called “natural philosophy”). That day seems, sadly, to be past. But how can anyone this appallingly ignorant be considered well or broadly educated?

And even worse, he didn’t realize how ignorant he was–he probably thought himself well enlightened on the subject, and more than competent to lecture to his lesser undergraduates. He was “don’t know squared” (which is sadly, for obvious reasons, often the case).

Equally sadly, I have a similar story from the aerospace industry itself. Back when I worked at the Aerospace Corporation, a couple decades ago, I was fresh out of school, and sitting in a meeting with more senior people, discussing a conceptual design for a new military geostationary satellite. The subject was how to provide orientation. The two traditional choices were spin stabilization (many of the Hughes communications satellites used this technique) and active reaction control, which was more accurate, but limited the lifetime, due to depletion of propellant.

I (or someone, but I think it was me) suggested using gravity gradient stabilization (that is, taking advantage of the fact that a non-spherical satellite will naturally orient itself in the local vertical position, due to differential tidal forces between the line of the orbit and the small distances of the appendages from that line). The response of one of the supposedly experienced engineers was, “There’s no gravity gradient at geosynchronous altitude.”

I was a little surprised. “Oh, you mean there’s not enough to do the job?” (I was thinking that perhaps he’d already considered it, and run the numbers.)

“No, there is no gravity gradient effect that high–it only applies in LEO.”

Note that he wasn’t making a quantitative argument, he was making a qualitative one. Low orbits had gravity gradient, high ones did not.

Being much his junior, I didn’t want to get into an argument about it, but my boss, who was also attending, happened to be Vladimir (Val) Chobotov, author of books on orbital mechanics and a reigning expert on the space debris problem, so I figured he’d speak up. He didn’t.

Walking back from the meeting with him, I asked him what that was all about. It turned out that I was right, but he hadn’t thought it worth getting into it with him in the meeting. We later wrote up a paper suggesting it.

What happened? Sometimes even engineers don’t always apply good scientific principles. In this case, I suspect that he was an airplane guy who’d migrated into the space business (as often was the case in the beginning decades in the space industry), and had never really learned the fundamentals of orbital mechanics, or the underlying principles. Instead, he’d probably taken a space systems design course, and been given a lot of engineering rules of thumb, one of which was, no doubt, that gravity gradient can be used in LEO, but not in GEO.

And that’s not a bad rule of thumb, as long as you understand where it comes from. Gravity gradient is indeed much less at twenty thousand miles altitude than at two hundred miles, and for most satellites could be considered, for practical purposes, to be non-existent. But we weren’t talking about most satellites–we were looking at a new concept, much larger than anything previously deployed in GEO, with long booms and appendages that might, in fact be used for G-G stabilization. But because he didn’t understand the physics, he mistook a rule of thumb for natural law, even though the law of gravitation says that the earth’s gravity extends out to infinity, though it drops off as the square of the distance. As evidence that it works much farther away than GEO, consider an object over ten times as far again (the original subject of this post), the Moon.

The Moon’s rotation rate is exactly the same as its orbital period. As a consequence, it always shows the same face to the earth–we never saw the “back” side of it until we sent the first probes in the 1960s. Isn’t this an amazing coincidence, that the two rates would coincide so that the view from earth was always the same?


The moon is in what’s called a tidal lock, another way of saying that it’s stabilized by the gravity gradient. It’s not perfectly spherical–it’s a little unbalanced, and one side has a little more mass than the other. Over the eons, gentle but persistent gravity gradient torques have oriented it into its present state and stabilized it there, always with the heavy side either facing away, or toward the earth, and thus it always presents the same view in the sky.

And of course, had we wanted to have a discussion of the issue in that meeting, that’s exactly the example I would have used then.

But to get back to the original topic, this to me is another example of C. P. Snow’s two cultures (well described in Pirsig’s Zen And The Art Of Motorcycle Maintenance): the liberal arts types who are ignorant of mathematics and science (and often perversely proud of the fact), and the scientists and engineers who have to actually make things work.

[Update at 12:14 PM PDT]

For those who didn’t get enough spacecraft dynamics in this post, go check out this little discussion of Explorer I from Professor Hall, who’s back from his motorcycle trip to Montana.