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Aries and Obstetrics

May 3, 2013

Somehow it took it until yesterday for me to come across this amazing video  Tim Michin’s Storm.

If you are yet to see it, watch it now.

With this fresh in my mind, I remembered a simple calculation from when I first learnt about Fermi problems; what detectable effect does the position of Jupiter have on people here on Earth? Now I don’t mean to poke fun at astrology by pointing out how it directly contradicts science… Well actually, I do. I’m going to presume that nobody reading this actually believes that the position of the stars and planets can have an effect on your luck or personality, so this is just an example of a silly calculation backing it up.

There are four fundamental forces of nature; gravity, electromagnetism and two nuclear forces that are only really significant at the subatomic level. I think it goes without saying that the dominant force that a planet exerts on us must be the gravitational one, right? We know it’s gravity that keeps us from floating into space, not electromagnet-boots! Man I want me some of those…

So let’s calculate the relative gravitational effects on a newborn baby between two things: The constellation Aries and the obstetrician. The gravitational force between two objects is proportional to the product of their masses divided by the square of the distance between them. The constant of proportionality is called Newton’s gravitational constant, however we will omit this from the formula since we only care about the relative difference between the obstetrician and Aries. So in some scaled units (where the constant is equal to 1) the force between two masses (M and m) separated by a distance r, is given by


In both cases the mass of the baby will be the same, so for simplicity we can say the mass of the baby is 1 and we’ve just changed our units again; really, all that’s important is the quantity \frac{M}{r^2}. So let’s make some estimations about the constellation Aries! I couldn’t find the exact mass of particular stars in Aries, but if we want to grossly over-estimate the mass of Aries (because we want to give Astrology a fighting chance) then we’ll assume that all 40 stars in Aries are as heavy as the heaviest known star ever, which is 400000000000000000000000000000000kg (or 4\times 10^{32}kg in scientific notation). The very closest (and thus the most gravitationally influential) star is about 100000000000000000 metres away (10^{17} metres). So a gross over-estimation would be to assume all of these stars are not only as heavy as the heaviest star, but also are situated at ‘the edge or Aries’ closest to us. So the total mass of the 40 stars is at most M=1.6\times 10^{34}kg and they’re no closer than r=10^{17} metres; in our fictional units of force, the force on the newborn due to Aries is

F=\frac{1.6\times 10^{32}}{(10^{17})^2}=0.016.

Let’s now compare this to the gravitational effects of the obstetrician. Let’s say the obstetrician is around 75kg and his (or her) centre of mass is around 0.5 metres from the baby during delivery. So in the same units, the force on the baby due to the obstetrician is about


Notice that the gravitational effect of the obstetrician on the baby is almost 2000 times larger than the gravitational effect of Aries. Although you probably didn’t need mathematics to convince you that astrology is bullshit. So this mathematics was a little pointless; some might say it was just a bit of mathsterbation, really.

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