Why astrology can’t be explained by gravity

Edit: I’m aware I left the squared notation out of the first equation, but it wasn’t left out of the calculation, just what I put into word to display the actual equation. The answers are still correct.

Over the years I’ve had many an argument/flamewar about many things, but primarily issues of skepticism and pseudoscience because that’s what I’m interested in.

When astrology comes up, one defence I’ve heard many, many times is that “The moon controls the tides, and people are mostly made of water, then maybe gravity is why the stars have such an effect on our personalities!”

I’m going to go ahead and explain why that’s simply not possible.

The stars are so far away, and gravity is such a profoundly weak force (You may not think it, but try jumping into the air some time. You just overcame the entire gravitational field of Earth, all 59,736,000,000,000,000,000,000,000,000 kilograms of it) that the effect the stars have on us gravitationally, is negligible at best, and completely overwhelmed by the gravitational effect of other people simply walking around us.

For example, I’m supposedly a Taurus which means I’m stable, committed, persevering and a bunch of other hokum if you believe in that sort of crap.

If we take the 10 closest stars in the Taurus constellation which, mind you aren’t even the stars that make up the shape of the constellation, you have a range of spectral class stars from A through to M, at distances ranging from 31 to 94 light years.

List of 10 closest stars in Taurus, with distances in light years and spectral class.

Stars in the classes A, G, F, K and M have masses ranging from the following:

  • A – 1.4 – 2.1 Solar Masses (SM)
  • F – 1.0 – 1.4 SM
  • G – 0.8 – 1.2 SM
  • K – 0.5 – 0.8 SM
  • M – 0.5 – 10 SM

And for the sake of this argument I’m simply going to assume average masses for these stars for the equations, so A = 1.75 SM, F = 1.2 SM, G = 1 SM, K = 0.65 SM, M = 5 SM.

 Also for simplicities sake, I’m going to assume the above stars constitute a point mass at the averaged distance of their combined distance from earth with the combined masses of the average sized star of that spectral class. Here goes.

The combined mass of the 10 stars above comes to 15, putting it at 2.98365*10³¹(15*1.9891*10³º) kilograms at a distance of 64.2 LY ((31+45+48+52+55+65+69+90+93+94)/10=64.2)

And finally, the average weight of a baby at birth is 3.4kg (That seems a lot to squeeze out of a vagina. I feel sorry for women)

Now we know the numbers we need to apply newton’s law of universal gravitation, seen here

Where F is Force, G is the gravitational constant (6.67428*10-¹¹ m³ kg-¹ s-²), m¹ and m² are point masses 1 and 2, and r² is the radius squared, so the equation will look like this:

That’s 0.01835 pico-Newtons. The -pico SI prefix denotes a factor of 10^-12, or a trillionth of a something. That’s tiny.

Now, how about the gravitational force of that person you’re coming out of when you’re born? Let’s say, assuming point masses, a distance of about a foot, or .33 of a metre separates you when you’ve been born and your mum (Who in this example isn’t fat and is around, say 60kg)is holding you or something. How would that look?


1.25*10^-7 N is 125000 piconewtons, so the gravitational force exerted on a baby by its mother holding onto it is 6 million times stronger than the gravitational force exerted on that same baby by the 10 closest stars in the constellation Taurus.

Your move, Astrology.


One Response to Why astrology can’t be explained by gravity

  1. KiwiTT says:

    Excellent article. Will link to it if I ever have someone mention it debates.

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