Built on Facts

Astrology and Gravity

Astrology has never so far as I know proposed a scientific mechanism by which the planets should affect the lives of us mortals. I suppose this is fair enough, you wouldn’t expect magic to work by normal laws. If the position of Jupiter says you should spend the rest of the week face down in the mud, well there’s no rule saying that Jupiter’s going to wreck its vengeance on you with any of the four known fundamental forces.

Of course there’s no indication that Jupiter or any other planet has anything to do with your personality at all by virtue of its position during your birth, or indeed that astrology is anything other than a way to turn confirmation bias into money. But let’s run some numbers, and let’s pretend that its hypothetical mystic force has something to do with good old fashioned gravity. We’ll do this because it sets up a classic intro problem where the student compares the gravitational influence of Jupiter compared to the gravitational influence of the doctor performing the delivery.

First we need the equation that gives the gravitational force between two objects:


G is the gravitational constant, M is the mass of the planet Jupiter (or the delivery doctor), m is the mass of the baby, r is the distance between them. But we’re not interested in the force, we’re interested in now the two forces compare to each other. Since G and m are the same in both cases, we can just compare the ratio of the masses of the affecting bodies to their distances.

The mass of Jupiter is about 1.9e27 kilograms, and the mass of a doctor might be around 80 kilograms. The distance to Jupiter typically is somewhere in the vicinity of 7.5e11 meters, and the distance from the emerging kid to the doctor might be taken to be… I dunno, half a meter.

Dropping the factor of Gm, I get the proportional influence of gravity from Jupiter to be 3,377 kg/m^2. Th influence from the doctor is 320 kg/m^2. Don’t worry, the units aren’t important. Neither are the numbers, only how the compare to each other. We see that Jupiter has about ten times the gravitational attraction. Which is pretty small nonetheless. The influence of a smaller and more distant planet like Saturn would be much less.

So Jupiter beats out your doc, but barely. At least in terms of gravitational pull… in terms of your future I suspect the good doctor is more important.


  1. #1 andyb
    January 14, 2009

    (assuming a spherical doctor 🙂

  2. #2 Chris P
    January 14, 2009

    I am still inclined to believe that there is some correlation between bad driving and the phases of the moon. Last week was seriously bad so I look up and sure enough it’s a full moon.


  3. #3 Chris P
    January 14, 2009

    I am still inclined to believe that there is some correlation between bad driving and the phases of the moon. Last week was seriously bad so I look up and sure enough it’s a full moon.


  4. #4 Uncle Al
    January 14, 2009

    1) Princess Diana retained one of the most extensive expensive stables of soothsayers in England. How did that work out vis-a-vis their continued employment?

    2) OK, we’re scientists. Conceive, gestate, deliver, and suckle a baby on the moon as an astrological control case. Also covers Schumann resonance vs. human EEG frequency spews,


    None of that NASA Hubble vs. Keyhole crap, either – send a woman not a re-engineered man to do the heavy lifting. If you want good statistics in a condensed PERT chart, send Mormons. (Gingham dresses in 1/6 gee – whoa!)

  5. #5 Max Fagin
    January 14, 2009

    Of course (playing devils advocate here) astrologers will say that these calculations don’t show anything, because the planets are not the CAUSE of astrological effects, anymore than a stop sign is the CAUSE of a car coming to a stop.

    Many astrologers claim that the planets are merely correlated with an unknown force, and it is that unknown force that directly causes the personality traits which astrologers claim to predict.

  6. #6 Peter
    January 14, 2009

    To a first approximation, astrological effects are based only on the date of birth as any cursory review of newspaper “horoscopes” will show. Now the magnitude of solar gravitation varies seasonaly since the Earth orbit is not a perfect circle, but the effect is very small and is symetric about the apogee. Since the published Horoscopes have no trace of such symetry it is doubtfully that the magnitude of the gravitational force is the significant variable.

    So calculating the relative magnitude of Jupiter ‘s gravitation vs the good Doctor is likely missing the mark.

    So that leave direction… Now Astrologers clearly are concerned with direction – they speak of which constellation the sun appears to be “in” at birth (one’s “sign”) But since first order analysis totally ignores the daily rotation of the earth, the direction of import is not the direction of the gravitational force relative to the birth process but the alignement of that force with an specific inertial frame of reference fixed on the stars.

    So what ever the mechanism, it requires a preferred inertial reference frame which despite repeated efforts, we have not been able to discern. In fact all four fundemental forces seem to be invariant with respect to interial refence frames so looking at any of them vis a vis Astrology appears pointless. So one can only conclude that science has not yet discovered the mechanism responsible for Astrological determinism! “There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy”.

  7. #7 andy.s
    January 14, 2009

    I think that the ‘mechanism’ has been pretty much forgotten. IIRC, it’s from medieval theo/cosmology. As the newborn’s soul descends from heaven (believed to be just past the sphere of fixed stars, outside the orb of Saturn), it acquires impurities as it passes through the celestial spheres of each planet.

    The type of impurities is influenced by the position of the planet, kind of like dials on a machine.

  8. #8 Matt Springer
    January 15, 2009

    Actually I think astrology is considerably older (Babylonian, maybe?), though for all I know the first proposed “mechanism” may have been medieval. Its antiquity results in the sun not actually always being in the appropriate constellation for that date due to the precession of the equinoxes.

  9. #9 braden
    January 15, 2009

    Bill Nye addressed astrology in an episode on pseudoscience in his short-lived “Eyes of Nye” show. According to the episode, the Babylonians (I think? Maybe the above comment tainted my memory. =P) defined one’s astrological sign as the constellation the sun rose in front of at the time of your birth. Of course, this was 2000 years ago, and since then the Earth has rotated such that we’re all off by a sign (an astrological sign, not a minus sign! 😉 ). So by virtue of my birth I am, according to astrologers, a Gemini, although by the original definition it seems like I ought to be a Cancer – I wonder how the astrologers would wiggle out of this one!

    (For those interested in the aforementioned episode: http://www.youtube.com/watch?v=ossSSyndFag)

  10. #10 Arjen Dijksman
    January 15, 2009

    Are there any astrologers that invoke gravitational influence of jupiter at the birthdate of a newborn child? I guess such astrologers wouldn’t gain much credibility among peers. If I were astrologer, I would try to have it built on facts😉 I would look for parameters that shape the personality, especially in the 9-12 months before and after his birth, for example the mother’s hormonal and metabolic influence during gestation, food composition, sleeping periods, light, temperature, weather… Such parameters are seasonal and periodic and are often in phase with terrestrial, solar, lunar and planetary periodicities. If I would invoke any influence of jupiter, it would be the 11-12 year-periodicity, which we find also in the solar activity cycle (and in the chinese zodiac cycle). Astrology is still an unexplored scientific field…

  11. #11 Pseudonym
    January 16, 2009

    When is the last time that an Astrologer actually looked at the sky to see what was up there? I’d wager some time in the mid-1700s.

  12. #12 andy
    November 9, 2009

    Astrology was known in India roughly 2000+ years ago, I would say earlier, but I would not have any known proof because a lot of invaders burned and destroyed a lot of things in India. Remember that around at the time people didn’t have electricity and TV, some didnt have a roof so they looked at the stars and found things happening based on their positions. I don’t know if anyone attributed it to gravitational forces (where was Newton when you needed him?), but they realized the affect of planets and stars and documented it. Knowledge was probably lost when Turkish invaders burned down the Nalanda university in 12th century – wikipedia states that “Its collection was said to comprise hundreds of thousands of volumes, so extensive that it burned for months when set aflame by Muslim invaders”, however knowledge of how to calculate to infinite years has remained with certain people, who publish annual almanacs relating to the same. The knowledge of “WHY” is probably lost forever. I assume it is gravity because of lunar effect on tides.
    am also too curious for my own good

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