Astrology’s “obstetrician strawman” is no strawman

So, one among the dozens of ridiculous claims made by Ed Kohout in this thread was that the claim famously posited by Carl Sagan in Demon Haunted World, that the obstetrician in the room imparts more gravitational force on a newborn baby than does Pluto, is a strawman. Edit: To clarify, he referred to gravitational and tidal forces as proof that the planets have an effect on human lives (which we, of course, understand and can measure!), and handwaved away the Sagan quote preemptively as though it was a strawman caricature of the actual astrological arguments about gravity (which he didn’t, by the way, expand on). This strikes me as a bit of a Courtier’s Reply, and the fact is, the argument about gravity actually knocks gravity out as a potential vehicle for whatever influences are claimed about the planets’ influences — especially given that these influences are purportedly equally strong/subtle for any of the planets. The Sagan quote about Pluto’s gravity being less than the obstetrician’s is a sound-bite form of a knockout argument for one of the four fundamental forces.

Well, Ed said Jupiter instead, but I’m willing to crunch some numbers to see how right he is. For these calculations, since I’m no math genius, I’m using this Newtonian gravity equation calculator. Yeah, yeah, Newtonian physics have been superceded by relativity, but I’m not about to try to calculate this out using relativity, that would be ridiculous. Newtonian physics hold in this case anyway.

I’ve expanded out all the numbers from scientific notation to straight digits. Average baby weight is 3.4kg, so let’s go with that. As some commenters helpfully point out, this assumes the force from the center of a spherical mass, so assume a spherical baby and a spherical obstetrician. Because gravitational calculations are wibbly with oddly-shaped objects.

Jupiter at closest approach to Earth, weight rounded up:

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 1900000000000000000000000000 kilogram
distance between objects (r) = 628743036 kilometer

gravitational force (F) = 0.000001090388427237 newton

Well that’s not a very big number, is it? Even despite my using the closest approach and rounding up the weight significantly. Still, it might be more than an obstetrician, we’ll see. However, the claim is generally about Pluto, not about Jupiter, so let’s see its force.

Pluto at closest approach to Earth (actually, even closer than closest approach, distance rounded down):

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 12500000000000000000000 kilogram
distance between objects (r) = 4300000000 kilometer

gravitational force (F) = 0.0000000000001533723634397 newton

Much, much smaller. I bet a pencil next to the baby would exert more force than that.

10g Pencil, 1cm away

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 10 gram
distance between objects (r) = 0.01 meter

gravitational force (F) = 0.00000002268684 newton

Wow. Yup, a pencil next to a baby exerts more gravitational force than Pluto. Less than Jupiter, but more than Pluto.

Okay, now how about the obstetrician?

Heavy (average male) obstetrician 1 meter away:

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 86 kilogram
distance between objects (r) = 1 meter

gravitational force (F) = 0.0000000195106824 newton

Wow, a result! Jupiter exerts more gravitational force than an obstetrician a meter away from the baby! But obstetricians aren’t generally a meter away from a baby while delivering it, are they? They’re normally actually touching the baby during that all-important moment of birth (why it’s all-important and conception isn’t, I’ll never know!). But being 0 meters away will dwarf any planets, so let’s say the obstetrician’s only 10cm away instead.

Heavy (average male) obstetrician 10cm away

object 1 mass (m1) = 3.4 kilogram = 3.4 kilogram
object 2 mass (m2) = 86 kilogram = 86 kilogram
distance between objects (r) = 0.1 meter = 0.1 meter

gravitational force (F) = 0.00000195106824 newton

But let’s give the astrologers a fair chance, and put someone lighter in the position of obstetrician.

Light (average female) obstetrician 10cm away:

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 70 kilogram
distance between objects (r) = 0.1 meter

gravitational force (F) = 0.0000015880788 newton

Nope, obstetrician’s still exerting more force.

Meanwhile, how much force does Earth exert on this baby?

Earth, assuming baby is 1m off ground:

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 5970000000000000000000000 kilogram
distance between objects (r) = 1 meter

gravitational force (F) = 1354404348000000 newton

(That’s for 1 meter from the centre of the Earth. I did it wrong.)

Earth, assuming baby is on surface of the planet:

object 1 mass (m1) = 3.4 kilogram
object 2 mass (m2) = 5970000000000000000000000 kilogram
distance between objects (r) = 6378 kilometer

gravitational force (F) = 33.295022252823 newton

Let’s recap.

Jupiter:            0.000001090388427237 newton
Pluto:              0.0000000000001533723634397 newton
Pencil:             0.00000002268684 newton
Male OBGYN, 1m:     0.0000000195106824 newton
Male OBGYN, 10cm:   0.00000195106824 newton
Fem OBGYN, 10cm:    0.0000015880788 newton
Earth:            33.295022252823 newton

What was all this about a strawman again?

Astrology’s “obstetrician strawman” is no strawman

28 thoughts on “Astrology’s “obstetrician strawman” is no strawman

  1. 2

    I’m willing to accept your claim that you’re no math genius. You’ve certainly proven that you’re no physics genius.

    You calculated that a 3.4 kg baby, one meter above the ground, would weigh about 300 trillion pounds (I converted from newtons to a unit more intuitive to me). By your logic, if you raised the baby to two meters above the ground, it would weigh a mere 75 trillion pounds.

    For gravitational calculations, the relevant distance is not the distance of closest approach between outer surfaces (such as one meter above the ground). It’s more like the distance between the two centers of gravity.

    The gravitation formula applies to point masses. For extended objects, every piece of each object is attracted to every piece of the other object, and you have to add up all the force vectors.

    Nevertheless, the gravitation formula works well enough for extended objects if they are sufficiently far away (like Jupiter). It also works if the objects are spherically symmetrical and non-overlapping.

    The obstetrician could approximate a sphere by curling up into the fetal position, but that posture is more likely for an onlooking father. In practice the obstetrician will adopt a more complex shape. If he or she is bending over the mother, the gravitation from the head will be partially canceled by the gravitation from the feet.

    Gravity gets complicated for extended objects. You can’t even just attribute all the mass to the center of mass and do the calculations from there. For example: if the earth were a hollow sphere, and spherically symmetrical, you wouldn’t experience any terrestrial gravity in the interior. All the gravitation from any part of the surrounding shell would be exactly canceled by opposing gravitation from the other side.

    I think I’m inclined just to take Carl Sagan’s word for it.

  2. 3

    Being that I’m a computer nerd, and didn’t realize the application of the formula needing to be from the center of the mass, I can correct the record. If I need to include the “assume a spherical cow” joke in the calculations, I can. The point is less about the gravity of the Earth than it is about the fact that the Earth by far dwarfs other planets. No need for acrimony. It’s illustrating that our astrologer friend has less of a clue about this stuff than even I do.

  3. 4

    As mentioned above, the distance between two objects is really the distance from their centers of mass. Therefore, the actual value of the force exerted on a newborn is (G)*(3.4 kg)*(5.98e24)/(6.37e6)^2, or 33.3 N, where G is a constant. This is a much smaller answer than what you got, although still thousands of times bigger than Jupiter’s.
    And mck9, you can treat most objects, such as people, as point masses. A hollow Earth is simply a rare exception to that rule. Hope that clears this up.

  4. 5

    A convenient way to sanity-check calculations involving force is to remember that 1 Newton is approximately the weight of 1 apple.

    (Or you can remember that the acceleration due to Earth’s gravity, ~10 ms^-2, is also the relationship between mass in kg and weight in Newtons, so a 1kg mass weighs about 10 N. (actually 9.8 and change, but for estimating, 10 is close enough.))

  5. 7

    It’s easier than that. Relativistic effects are irrelevant because all the speeds involved are far too low. So classical stuff is sufficient.

    Gravitational force increases linearly with mass (assuming the baby doesn’t change mass during birth), and decreases with the square of the distance between centres of mass. Fortunately, cuz Jupiter is so far away, we can treat it as a lumped mass. See the wikipedia page on the universal law of gravity for the actual equation.

    Assuming the obstetrician weighs about 80 kg, Jupiter is about 2.4×10^25 times more massive.
    Assuming the centre of mass of the obstetrician is about 1m from the baby, Jupiter is about 5.9×10^11 times further away. The doc isn’t that far away, so we can’t use a lumped mass here, but it doesn’t matter, as we’ll see below.

    Unfortunately, this means that Jupiter exerts about 400x more gravity on the baby than the obstetrician does. Treating the doc as a distributed mass wouldn’t change this in any significant way.

    Of course none of this matters, because there is no evidence at all that gravity influences the personality or mind or whatever or a person. Indeed, there is plenty of evidence that gravity has no effect on personality/mind. (Consider astronauts, who aren’t different when in space than when they’re on earth, even though gravity has a huge effect. Consider people who live at high altitudes versus those who live elsewhere. Tibetans, for instance, are not noticeably different from Netherlanders.)

    Burden of proof is on the astrologer: show conclusively that there is a *causal* relation between any quantity or quality of a celestial object and a measurable feature of a human. If they can do that, then any self-respecting scientist would have to accept it.

    Ball’s in their court. It’s going to be a long wait.

  6. 8


    And mck9, you can treat most objects, such as people, as point masses. A hollow Earth is simply a rare exception to that rule. Hope that clears this up.

    Yes, you can treat people as point masses, provided that you’re willing to get the wrong answer.

    The example of a hollow Earth is admittedly an artificial example. For a more realistic example, imagine yourself sitting down with the baby in your lap. It’s probably possible to bend over and adjust your limbs so that your center of mass coincides with the baby’s center of mass. If you merely apply the gravitation formula to the two centers of mass, you would conclude that the gravitational attraction between you and the baby is infinite. That’s the wrong answer, no matter how attracted you are to the baby in other respects.

    For nearby objects, especially if they’re nowhere near spherical, shape matters. More precisely, the distribution of mass matters, and the closer the objects, the more it matters. Knowing the total masses and the distance between the centers of mass simply is not enough to give you the right answer. In most cases it is enough to provide a good approximation. This is not one of those cases, depending on the obstetrician’s placement and posture.

    Consider two simplified models of the obstetrician’s mass distribution. First, a sphere, 18 inches in diameter; second, a vertical cylinder, six feet tall, with the same volume and mass as the sphere, and centered at the same distance from the baby. Both the sphere and the baby are three feet off the floor.

    Assuming a spherical baby (maybe this is a baby Volvox?), we can calculate the attraction of the spherical obstetrician with the familiar inverse square law.

    Without doing the math, we can see intuitively that the cylindrical obstetrician will exert less of an attraction. First, the top and bottom are farther away than the sphere was. Second, the attractions to the top and bottom have vertical components, and correspondingly reduced horizontal components, according to the rules of trigonometry. The vertical components, of course, cancel each other out.

    For a longer cylinder, say, six thousand light years, the attraction will be reduced further. Almost all of the mass will be very far away, and almost all of what little attraction remains will be deflected into vertical components and canceled out.

    Shape matters.

    And Jason: I intended no acrimony. I just don’t want you to embarrass yourself by making this argument elsewhere without correcting it. The fact that you calculated an astronomical weight for a newborn should have been a clue.

  7. 9

    It should have, you’re absolutely correct. If I’d have known how much weight a newton represents (at Earth’s surface, one apple roughly, as stated earlier in the comments), I would certainly have caught it. The facts remain though– Sagan said Pluto, astrologers claim Pluto has as much influence as any other planet, and even Jupiter’s weight would be swamped out by any nearby object in the room of appreciable mass.

    Now that I know better, I will certainly be careful in making the argument in the future. The argument itself stands though. There’s no way gravity’s responsible for the effect astrologers have never bothered to show as actually happening. Every astrologer I’ve met has basically assumed there’s something to the practice first (presupposes an effect, perhaps?) and picked a superficially plausible mechanism afterward. Gravity’s out, for the reasons laid out in this post and comments. Electromagnetism’s out too, since many of these bodies don’t have electromagnetic fields. I don’t know what else they could point to, but they haven’t even shown there’s any tangible influence to begin with, so I don’t even see the point of them arguing with us on the intertubes except to muddy the waters enough to protect their revenue streams.

  8. 10

    Here’s the best part:

    The gravity of, say, Jupiter isn’t pulling on the baby like someone pulling on her arm. It’s attracting every part of her with the same gravity. It’s also pulling her mother, and the obstetrician, and the floor and the ceiling and the whole Earth. Nothing is holding the baby back from that (tiny!) acceleration.

    In other words, the baby, like the rest of us, is in free fall toward Jupiter. (Though nobody’s actually going to end up at Jupiter, because we’re also in free fall toward the Sun and the Sun wins.) If you let gravity accelerate you, you don’t feel that force at all. Like an astronaut in orbit, you are “weightless.” (A shuttle orbit is still close enough to Earth that the astronaut’s true weight – the force of gravity on them – is something like .8 or .9 what it is on the surface… but they don’t feel it, because they’re not resisting it: they’re in free fall. That’s why physicists flinch when the floating-around-in-space thing is called “zero gravity” – it has nothing to do with how strong the gravity is, you could be in free fall inside the event horizon of a black hole.)

    The whole planet Earth is in free fall toward every single other astronomical body in the universe – in other words, there’s nothing holding us back from being accelerated toward them by gravity. So from the point of view of anyone on Earth, there’s just no way the gravity of any such body could have any effect at all on any of us. The doctor’s gravity would, in theory, be felt, because he’s holding on to her and preventing her from falling toward his center of mass. The gravity of everything from the Moon on up is effectively zero – absolutely undetectable by any earth-local experiment.

    Of course, we could talk about the tidal effect, which is when gravity pulls different amounts on different parts of the same object, because its not all at the same distance from the planet or whatever in question. Tidal effect can’t be ignored in free fall, because it’s actually stretching you – any solid object will always resist. But the tidal effect goes as one over the cube of the distance, which means you add another eleven zeroes or so after that decimal point for Jupiter (twelve zeroes for Pluto) and the ghost of Isaac Newton starts laughing his ethereal ass off – the baby’s grandma three states away has more gravity than that.

  9. 11

    Astrologers don’t need to show a causal connection between, say, the stars and personality. It could be that the zodiac signs correlate with something else which has some causal influence on personality. Like the seasons. I wouldn’t be surprised if babies born in the northern temperate zone would have personality differences dependent on the time of year they were born. A baby born in early winter wouldn’t get out much in its first few months of life, and might develop a more introspective personality or something.
    Of course, the more detailed kinds of astrology that use the exact minute of birth and look at the positions of all the planets just has to be nonsense, and there is not the slightest reason to believe it. But maybe we should retain an open mind about the possibility that month of birth has some statistically detectable effect on personality in the temperate zones of the planet. Of course, if there is such an effect, it would be caused by normal physical causal channels.

  10. 12

    @ Lou Jost:

    I seem to remember that at least one huge study has looked for something like that and failed to find it, but setting that aside…

    The summer-autumn-winter-spring seasonal cycle is pretty much just a sinusoidal variation in temperature and precipitation – a smooth curve, in other words, from hot and dry in the summer to cold and wet in the winter and back again. From such a simply-shaped cause, you’d expect to see a very simple effect. To take your example, you’d get introverts born in the winter, and extroverts born in the summer, and in-between sorts of people born in the fall and spring, and that’s it. You wouldn’t get twelve distinct personality types, you’d get maybe two to four types in varying degrees – for example, you’d expect an Aquarius (February) to be just like a Scorpio (November) except more so, or maybe vice versa. Rather than clear boundaries between signs, you’d expect a fuzzy transition period of several weeks on the calendar where astrology would be much less accurate.

    You would also expect other stimuli to cause similar variations in people, if they were also linked to the proposed mechanism. In our example, you’d predict that Canadians would be way more introverted than Americans, and Mexicans would be more extroverted, because of their relative climates. You’d expect that in the southern hemisphere, every star sign would exchange its astrological meaning with the sign across the circle (Aries act like Libras, Taurus act like Scorpios, etc.) And you’d expect that in places like India or equatorial Africa that don’t experience the same cycle of four seasons, astrology would either fail or work entirely differently.

    And I’ve never seen any astrologer claim anything that looks remotely like any of that. No matter what mechanism an astrologer proposes, he or she never draws any novel conclusions from it. If astrology really worked by gravity, the most important thing in a horoscope would be the moon by a huge margin, followed by the sun, with tiny tiny effects from the bigger or closer planets like Jupiter and Mercury; the Zodiac would be entirely irrelevant. If astrology worked by magnetism, the big thing would be the sun, then the gas giants, then maybe Venus, but no Mercury, Mars, Pluto, or Zodiac, and astrologers would track sunspots and ask for the longitude of your birthplace. (Bonus points for being born under the Northern Lights.) If astrology worked by electricity then it would only be effective for people whose mothers gave birth while balanced on top of Van de Graff generators.

    A rational person hypothesizing that birthdate has an effect on personality only has to find a statistical correlation, not a causal connection. An astrologer has to show that their predictions have features characteristic of things that are caused, at all, by anything, before I even care enough to ask what specific cause they might have. Astrology has this whole body of weird-ass claims that don’t even have the basic shape of something you could link to a mechanism. A physically plausible astrology wouldn’t even look like astrology; it would have to be fundamentally different.

  11. 13

    Hey, forget the obstetrician. How about the several hundred tons of hospital? The position of the delivery room relative to the center of mass of the hospital has got to factor in….

  12. F

    I’m just in love with this:

    assuming baby is on surface of the planet

    It will probably be even more entertaining if taken completely out of context.

  13. 15

    @Robert, yes, we should expect any such effect to be more or less continuous with time., but there are lots of natural phenomena that behave quite differently from one month to the next, so I don’t think you can safely rule out a weak effect month-to-month. People act differently in December than in January. In many places May is much different than June.

    Of course these seasonal effects, if they existed, would be mashed up in the southern hemisphere. Because there are more people in the northern hemisphere, the northern hemisphere pattern (if such a thing existed) would still be a statistically valid generalization about humans overall.

    Just to make myself clear, I am not defending astrology (which is pure bunk). I am making a philosophical point. The simple monthly version of astrology could theoretically make statistically valid predictions even if its proposed causal mechanism is nonsensical, because its alleged causal mechanism is correlated with something else (seasons) that could have causal effects. So unlike most skeptics, I would not want to say it is IMPOSSIBLE that the simple monthly predictions of astrology could be statistically valid (though I don’t think there is any evidence that they are valid). When we get to the more detailed predictions using planets, etc, those really would be impossible, because there is no valid causal explanation from within astrology, and there aren’t any non-astrological causes that are correlated with these.

  14. 16

    Actually, the gravitational force Jupiter (or Pluto) causes roughly the same acceleration of the head of the baby, the toes of the baby, the hospital and the Earth itself. You want to look at the tidal force (the difference in attraction between the head and toes of the baby) and not the absolute gravitational attraction.

    Similarly, the gravitational force exerted by the sun is quite considerable, however, since it pulls at both the earth, our heads and our toes in roughly the same way, we don’t get pulled up at noon and weighed down at midnight.

    The tidal force decreases as the third power of the distance (as opposed to the second power, for the attracting force), and therefore the tidal influences of the planets are a LOT smaller than the forces exerted by a nearby pencil.

  15. 17

    Wow… my tl;dr has been ably summarized by someone who shares my first name. That was trippy – for a second I thought I’d posted again without knowing it.

    Though interestingly, since everything around us is not attracted equally by the gravity of a nearby obstetrician, we’re actually comparing the tidal gravity of the planets to the direct gravity of any nearby pencils, doctors, or grandparents. Which is just another reason that the planets can’t win.

  16. 18

    My problem with his argumentation is that he’s giving Courtier’s Replies with every comment, only he’s also appealing to one of the fundamental forces, with precious little specificity. It’s explicitly not a strawman because he’s referred to tidal and gravitational forces as proof that the planets have an effect on humankind. He only called it a strawman to handwave away the argument. Preemptively, to boot.

    So I proved the argument to show that we can knock gravity right out. Regardless of HOW he says gravity does the astrological magic, gravity ain’t the mechanism.

  17. 20

    You DO realize that it isn’t a response to a charge of building a strawman argument to prove that your argument is CORRECT, right? An accusation of strawmanning implicitly accepts that the argument may well be right, but says that it is irrelevant because it doesn’t address the actual argument that is being made, but instead attacks a caricature. What you need to do to prove your argument not a strawman is show how it MATTERS that the gravitational force on the baby is less than that of the doctor by what astrologers argue. Which you have not done here.

  18. 24

    @Robert B.

    Haha, I hadn’t read your comment yet when I posted that (i’m suspecting mine was held in moderation a bit since it was my first post here, and I’m quite sure I read most comments at time of posting.)

  19. 25

    […] The fourth force, gravitation, describes the attraction between objects with mass. It is far weaker than electromagnetism. It operates over large enough distances to plausibly provide a mechanism for astrology, assuming that the only objects we care about are the Sun and the Moon, which together account for nearly all the tidal and orbital forces our planet experiences. It has been dismissed at length in this post and contributors’ comments. […]

  20. 26

    Jason struggles:

    The facts remain though– Sagan said Pluto, astrologers claim Pluto has as much influence as any other planet, and even Jupiter’s weight would be swamped out by any nearby object in the room of appreciable mass.

    I just found out about this silly blog post, and again I would like you to show me who the astrologer is that does gravitational calculations for clients!!! YOU CAN’T, fool, and thus the straw man is still lingering outside your cabin door.

    Now that I know better, I will certainly be careful in making the argument in the future. The argument itself stands though.

    LOL! I love how you spent all of this time laying out the math, poorly at that, to show us that, by golly, we don’t walk the surface of Jupiter or Pluto because Earth’s gravity is so darn strong at the surface! In any event, planetary gravity is still infinitely more powerful than the empty space around that planet that is infinitely bigger in size, and so according to you such a math should prove that the “effect” of empty space on humans must mean almost everything because the amount of space around the birth mother is way bigger than the ob gyn.

    There’s no way gravity’s responsible for the effect astrologers have never bothered to show as actually happening.

    Gravity IS an effect of mass and energy, not the other way around, so no, gravity cannot be responsible for the effect that causes it in the first place.

    Astrology is the study of CYCLES and SYNCHRONY in nature, not a “proof” of how the universe works, nor does it need to understand any of these “forces” that are at work. Can you not grasp that?

    Now, have you considered angular momentum, and if not, when will you?

    Also, you do realize that all the planets have vastly different compositions and vibratory signatures, magnetic fields, energy output, etc.?

    You continue on, wounded, but pride intact:

    Every astrologer I’ve met…

    How many is that? 3? 300? More conjecture on your part that points up your ignorance, as you then show:

    … has basically assumed there’s something to the practice first (presupposes an effect, perhaps?) …

    Have you not read anything I’ve written? The connection is that WE and THE PLANETS are both part of the same natural world, and that the natural world is highly sympathetic within itself and patterns are known to propagate; ancient sciences are all grounded in this assumption. It simply is foolish to assume that our life rhythms (our species is how many Venus/Mars cycles old?) are completely independent of our environment.

    We live on Earth, and yes, Earth is the primary influence on life (we have blood pressure that equals the planet’s air pressure, we are adjusted to the gravitational constant, etc), and no astrologer would deny that, and in fact the Earth/Sun nodes are considered to be the most personal points of the chart. Earth, though, does not exist in a vacuum, but instead a local dynamic group of planets that exert much force upon it, and determine its cyclical harmonics.

    You see, your position is exactly that of the religion nutters who think human beings are divine and unfettered by the environment unless it is the will of god.

    … and picked a superficially plausible mechanism afterward.

    On this, I do agree, but some of the elements were quite excellent and have been expounded upon in modern times.

    Gravity’s out, for the reasons laid out in this post and comments. Electromagnetism’s out too, since many of these bodies don’t have electromagnetic fields.


    I don’t know what else they could point to, but they haven’t even shown there’s any tangible influence to begin with, so I don’t even see the point of them arguing with us on the intertubes except to muddy the waters enough to protect their revenue streams.

    Yes, charlatans all, sitting back, laughing, counting all their riches with giddy laughs, and hoping to god people don’t demand statistical proof!

    Side-splitting, you are, Jason. Did an astrologer break your heart once?

  21. 28

    That’s not his game, gravityswings @27. Ed Kohout runs away for a while, then when he decides something’s monumentally lacking in his life, returns to boost his ego by mistreating people on the internet. He’s a thundering boor, and I only let his posts through because they prove just how much “nothing” there is in his philosophy.

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