I usually don’t like making grandiose statements ahead of myself, like “Astrology is totally unscientific”, because I prefer leaving the benefit of the doubt until I check the claim. In the case of Astrology, however, there’s no use pretending.

Astrology isn’t science. It makes baseless predictions, relies on overly-generalized statements and has a false basic premise*.  You can read this online from various other sources, and there isn’t much use for me to reiterate the points made.

Picture by joelwillis via Flickr (Creative Commons 2.0)

What I am going to do is test the basic premise.

* Phil Plait, “The Bad Astronomer”, has a great analysis of Astrology that goes over all the above, and more, as does the skeptic dictionary and the Astronomical Society of the Pacific among many, many others. You can also watch Australian Skeptics’ Richard Saunders brief live argument with an Astrologer.

Note: For your convenience (and due to popular demand), I added an automatic tool where you can measure the force applied by any object at any distance. Test it yourself!

Click here to open the Force Calculator! (opens as a new window).

The basic premise of astrology

Astrologers claim that the positions of the planets and “Zodiac” signs (constellations of stars) at the moment of our birth – and generally throughout our lives – affect our personality, mood and affairs.

I will not get into the so-called  “metaphysical” effects, a mishmash of misunderstood physical theories (quantum physics, dark matter, dark energy, etc) with some pseudoscientific new-age unfalsifiable claims (from “fate” and “luck” to “planetary energies”, whatever that means). What I will do is treat the claim that astrology has merit in science. Many astrology-believers think that since the planets exert gravity, they might affect our brains, and therefore our moods.

Many people give the moon as an example. The moon’s gravity is known to affect tides – a powerful force we can witness. Many take this as proof that the planets’ gravity is affecting our bodies. On its face, the claim makes sense.

We are going to examine it.

Gravity, the force of masses

Any two objects with mass exert gravitational force on one another. That force is related to the masses of the objects and the distance between them by the formula:

\(F= G \frac{M, m}{r^2}\) \(left( G=6.67\cdot 10^{-11} \frac{\mbox{m}^3}{\mbox{kg} \cdot \mbox{s}^2} right)\)

Where G is the universal constant of gravitation, M and m are the masses of the objects and r is the distance between them.

Since we think of planets as incredibly big objects, the idea that their gravity affects our bodies sounds reasonable. But to a newborn, there are other “massive” objects around that exert the same type of force as the planets. They might be much smaller than the planets, but they are much closer, too. If the position of planets at the moment of our birth defines our personality, so should the positions of objects in the delivery room.

This is a testable claim.

The test: planets vs. delivery room

We are going to compare two forces, those coming from the planets and those coming from objects in the delivery room, to reach a conclusion:

  • If the forces from the objects in the delivery room outweigh those from the planets, then astrologers should, at the very least, ask the weights and positions of the people in the delivery room when they calculate your chart.
  • If, however, the forces of the planets are substantial, then astrology might have some scientific merit. This is what we are about to check.

OMG! Math! Panic!


We are about to calculate physical forces so there is some math involved, but you can choose if you want to see it or not. Yes, I’m that considerate.

If you want to go over my math so you can repeat it yourself, add to it (items I missed?) or criticize me (peer-review away, mathematicians) you can reveal the calculations by clicking the “Show the Math” links.

Otherwise, just continue reading the solutions only. Those are useful too.


One more note: Forces are directional (vectors), but in this case, since we want to calculate the maximum possible force, we will treat them as if they are “lined up”, and therefore calculate them numerically.

What about the mother?

Right, the mother is also in the room, and her body also exerts a gravitational force on the baby. However, The baby is inside the mother, and in her midsection. He is, almost literally*, in her center of mass. For all intents and purposes the mother’s gravity “cancels out” from all directions and there’s no use adding her into the calculation.

* Physicists, stay calm, think “spherical chicken in a vacuum” and bear with me here.

On we go.

The delivery room

Since my intent is to calculate the most basic hospital delivery room, I put in the most basic items that should be found in one. There are likely many more people and pieces of equipment in and outside the room, but the goal of these calculations is a “conservative estimation.”

Therefore, I will ignore the size of the hospital, other people walking by and other large machines that exist in the building. See “Conclusion” for more about those.

Here’s a list of what should be the most basic elements in a delivery room:


  • A doctor (obviously)
  • A nurse
  • OB tech (whose job is to help the doctor and nurse during the actual birth)
  • The partner (assuming the mother has one)


The Calculation

In the following section I will calculate the force exerted on the baby from each of these elements by estimating their weight and mass and their relative distance.

I will assume average-sized staff (75-85 kg), leaning towards the thinner side, to keep my estimate conservative. I will also assume that the baby is level with their midsections (i.e., their centers of mass) which will allow me to ignore their height in my calculation.

Show the Math

The Doctor

The doctor stands directly in front and above the baby before it is born. If anything affects the baby, he is it.

  • Mass = 82 kg
  • Distance from baby = 0.3 m (30 cm)
\(F_{doctor}=G\frac{82 kg \cdot 3.6 kg}{(0.3 m)^2}=(6.67\cdot 10^{-11}\frac{m^3}{kgcdot s^2}) \frac{295.2 kg^2}{0.09 m^2}=2.19\cdot 10^{-7} \frac{m\cdot kg}{s^2}\)

The force exerted by the doctor’s gravity = \(2.19\cdot 10^{-7} N\)

The Nurse

  • Mass = 75 kg
  • Distance from baby = 1 m
\(F_{nurse}=G\frac{75 kg \cdot 3.6 kg}{(1 m)^2}=(6.67\cdot 10^{-11}\frac{m^3}{kg\cdot s^2}) \frac{270 kg^2}{1 m^2}=1.8\cdot 10^{-8} \frac{m\cdot kg}{s^2}\)

The force exerted by the nurse’s gravity = \(1.8\cdot 10^{-8} N\)

The OB Tech

This person will be standing next to the instruments, monitoring the delivery. He will likely be a bit further away than the doctor and nurse.

  • Mass = 80 kg
  • Distance from baby = 3 m
\(F_{OB Tech}=G\frac{80 kg \cdot 3.6 kg}{(3 m)^2}=(6.67\cdot 10^{-11}\frac{m^3}{kg\cdot s^2}) \frac{288 kg^2}{9 m^2}= 2.13\cdot 10^{-9}\frac{m\cdot kg}{s^2}\)

The force exerted by the OB Tech’s gravity = \(2.13\cdot 10^{-9} N\)

The Partner

  • Mass = 80 kg
  • Distance from baby = 0.5 m
\(F_{Partner}=G\frac{80 kg \cdot 3.6 kg}{(0.5 m)^2}=(6.67\cdot 10^{-11}\frac{m^3}{kg\cdot s^2}) \frac{288 kg^2}{0.25 m^2}= 7.68\cdot 10^{-8}\frac{m\cdot kg}{s^2}\)

The force exerted by the partner’s gravity = \(7.68\cdot 10^{-8} N\)

Bed or Birthing Chair

  • Estimated mass: 276 lbs = 125.19 kg
  • Estimated distance: 0.05 m (5 cm)

(Source: http://www.spinlife.com/Drive-Medical-600-lbs.-Bariatric-Full-Electric-Frame/spec.cfm?productID=82578 this isn’t a birthing bed, but it’s close enough for an estimate)

\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 125.19 kg}{(0.05 m)^2}= 1.2\cdot 10^{-5}\frac{m\cdot kg}{s^2}\)

The force exerted by the bed’s gravity = \(1.2\cdot 10^{-5} N\)

Heart Monitor

  • Estimated mass: 25 kg
  • Estimated distance: 1 m
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 25 kg}{(1 m)^2}= 6\cdot 10^{-9} \frac{m\cdot kg}{s^2}\)

The force exerted by heart monitor’s gravity = \(6\cdot 10^{-9} N\)

Scale (to weigh the baby)

  • Estimated mass: 3.6 kg
  • Estimated distance: 3 m

(source: http://www.egeneralmedical.com/detecto-digital-baby-scale-scale-71170.html this is a small version, good enough for our calculation, but it’s worth noting most hospitals will carry a much larger one, on wheels, obviously weighing much more).

\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 3.6 kg}{(3 m)^2}= 9.6\cdot 10^{-11} \frac{m\cdot kg}{s^2}\)

The force exerted by the scale’s gravity = \(9.6\cdot 10^{-11} N\)

Blood pressure monitor, Stethoscopes and other random small items

There are a LOT of items in a delivery room, and I am very likely to forget a whole bunch of them. We will estimate, though, a total of 5 kg of extra random items like more chairs, the blankets and sheet, stethoscopes, blood pressure monitors, picture frames, and anything else that might exist in a room and didn’t add into the calculation. This is a very very conservative calculation.

I will take the average distance of all of those random items as 4 meters.

  • Mass = 5 kg
  • Average distance from the baby = 4 m
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 5 kg}{(4 m)^2}=7.5\cdot 10^{-11}\frac{m\cdot kg}{s^2}\)

The force exerted by the random items’ gravity = \(7.5\cdot 10^{-11} N\)

Total Maximum Force

So, to summarize (and, for those of you who cared not for the mathematics, to state in the first place):

  • The Doctor = \(2.19\cdot 10^{-7} N\)
  • The Nurse = \(1.8\cdot 10^{-8} N\)
  • The OB Tech = \(2.13\cdot 10^{-9} N\)
  • The Partner = \(7.68\cdot 10^{-8} N\)
  • The Bed = \(1.2\cdot 10^{-5} N\)
  • Heart Monitor = \(6\cdot 10^{-9} N\)
  • Scale = \(9.6\cdot 10^{-11} N\)
  • Other Small Objects = \(7.5\cdot 10^{-11} N\)
Show the Math
  • From people: \(2.19\cdot 10^{-7}N + 1.8\cdot 10^{-8} + 2.13cdot 10^{-9}+7.68\cdot 10^{-8} N = 3.1593\cdot 10^{-7}\)
  • From objects: \(1.2\cdot 10^{-5}N + 6\cdot 10^{-9}N + 9.6\cdot 10^{-11}N + 7.5\cdot 10^{-11}N=1.2006171\cdot 10^{-5}\)

Total Force: \(1.232cdot 10^{-5} N\)

The Planets

EDIT: I have recalculated the forces from the planets. It seems that during the initial calculations I made a rather small (but recurring) conversion error, and due to vigilant commentors, it was properly corrected. You should note, though, that the total force after this re-examination didn’t change. My calculation was fine, I just had a problem with how I wrote it out in the process (in the math part). Apologies.

Now, astrology claims that the planets exert a force on the baby, and their different locations change that force ever-so-slightly to somehow affect the baby’s personality traits.

The idea that the planets exert a force, even on the baby, is true. Whether or not it is canceled out or overwhelmed by other forces is a different issue.

Our next step, then, is to calculate the maximum force that can be exerted from the various planets, and combine them to get the maximum possible force exerted by the planets.

Show the Math


  • Mass: \(0.3302\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 77,300,000 km (\(7.73 \cdot 10^{10} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 \mbox{kg} \cdot 0.33\cdot 10^{24} \mbox{kg}}{(7.73\cdot 10^{10} m)^2}=1.33\cdot 10^{-8}\frac{m\cdot kg}{s^2}\)

Maximum Force by Mercury = \(1.33\cdot 10^{-8} N\)


  • Mass: \(4.85\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 38,000,000 km (\(3.8 \cdot 10^{10} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 4.85\cdot 10^{24} kg}{(3.8\cdot 10^{10} m)^2}=8.06\cdot 10^{-7}\frac{m\cdot kg}{s^2}\)

Maximum Force by Venus= \(8.06\cdot 10^{-7} N\)


  • Mass: \(0.642\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 54,600,000 km (\(5.46 \cdot 10^{10} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 0.642\cdot 10^{24} kg}{(5.46\cdot 10^{10} m)^2}=5.17\cdot 10^{-8}\frac{m\cdot kg}{s^2}\)

Maximum Force by Mars= \(5.17\cdot 10^{-8} N\)


  • Mass: \(1899\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 893,000,000 km (\(8.93 \cdot 10^{11} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 1899\cdot 10^{24} kg}{(8.93\cdot 10^{11} m)^2}=5.72\cdot 10^{-7}\frac{m\cdot kg}{s^2}\)

Maximum Force by Jupiter = \(5.72\cdot 10^{-7} N\)


  • Mass: \(568\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 1,195,000,000 km (\(1.195 \cdot 10^{12} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 568\cdot 10^{24} kg}{(1.195\cdot 10^{12} m)^2}=9.55\cdot 10^{-8}\frac{m\cdot kg}{s^2}\)

Maximum Force by Saturn = \(9.55\cdot 10^{-8} N\)


  • Mass: \(86.8\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 2,580,000,000 km (\(2.58 \cdot 10^{12} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 86.8\cdot 10^{24} kg}{(2.58\cdot 10^{12} m)^2}=3.13\cdot 10^{-9}\frac{m\cdot kg}{s^2}\)

Maximum Force by Uranus = \(3.13\cdot 10^{-9} N\)


  • Mass: \(102\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 4,400,000,000 km (\(4.4 \cdot 10^{12} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 102\cdot 10^{24} kg}{(4.4\cdot 10^{12} m)^2}=1.27\cdot 10^{-9}\frac{m\cdot kg}{s^2}\)

Maximum Force by Neptune = \(1.27\cdot 10^{-9} N\)


I am including it in because astrologers do, too.

  • Mass: \(0.0125\cdot 10^{24}kg\)
  • Minimum Distance from Earth: 4,200,000,000 km (\(4.2 \cdot 10^{12} m\))
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 0.0125\cdot 10^{24} kg}{(4.2\cdot 10^{12} m)^2}=1.7\cdot 10^{-13}\frac{m\cdot kg}{s^2}\)

Maximum Force by Pluto = \(1.27\cdot 10^{-13} N\)

The force from all the planets combined

All of the forces above were calculated as if the planet is in its closest position to the Earth. The chances that all planets together will be in such positions are incredibly small. This doesn’t usually happen, and the resultant combined force is much smaller. However, we can still calculate the maximum theoretical force that can be produced by all planets combined on the newborn baby.

Here they are:

  • Mercury = \(1.21\cdot 10^{-8} N\)
  • Venus = \(8.06\cdot 10^{-7} N\)
  • Mars = \(5.17\cdot 10^{-8} N\)
  • Jupiter = \(5.72\cdot 10^{-7} N\)
  • Saturn = \(9.55\cdot 10^{-8} N\)
  • Uranus = \(3.13\cdot 10^{-9} N\)
  • Neptune = \(1.27\cdot 10^{-9} N\)
  • Pluto = \(1.27\cdot 10^{-13} N\)

(Before you protest about Pluto, read this: there are many problems with including Pluto in the calculation of gravity – the least of which is his “partner” Charon, who’s of similar mass. However, Astrologers calculate Pluto into their maps, and so I thought it would be appropriate to include the force it exerts, too.)

Show the Math
\(1.33\cdot 10^{-8}N + 8.06\cdot 10^{-7}N + 5.17\cdot 10^{-8}N + 5.72\cdot 10^{-7}N + 9.55\cdot 10^{-8}N + 3.13\cdot 10^{-9}N + 1.27\cdot 10^{-9}N + 1.27\cdot 10^{-13}N=1.5442\cdot 10^{-6}N\)

Total Force = \(1.54297\cdot 10^{-6}N\)


So, what do we have?

  • The combined forces of the delivery room = \(1.232\cdot 10^{-5} N\)
  • The combined forces of the planets = \(1.544\cdot 10^{-6} N\)

Difference =\(\frac{1.232\cdot 10^{-5} N}{1.544\cdot 10^{-6}} = 8.01\)

The forces from the delivery room are 8 times bigger than the combined force from the planets, and we have calculated a very conservative estimate.

Proponents of the claim might jump out of their seats and claim the forces are extremely close. They seem close (if a factor of 8 is considered close) but we have to remember a few important issues that show conclusively that the forces from the planets are minuscule compared to the forces exerted on the baby from his immediate surroundings:

  • The planets do not, ever, line up where they are all as close to Earth as our calculation asserted. The realistic force from the planets is lower.
  • Our estimates for both the distances, the amount of people and their weight was very conservative. In reality, hospitals have a lot more people and staff, much more equipment in the room and directly outside of it.
  • Hospitals are huge places. If planets as far as a few billion kilometers exert force on our newborn baby, the MRI machine (that weighs 50-60 times the weight of the doctor, nurse and OB Technician combined) at some floor below, and the CT machines somewhere in the hospital should be taken into account as well. Those would dramatically increase the difference between the two forces.
  • And, one of the most notable point of all: We ignored the Earth’s gravity!

We ignored the Earth’s gravity!

To be fair, I ignored the Earth’s gravity in both cases, for a very good reason: it absolutely trumps both. Since it is also coming from the ground, and the other forces are spatially distributed, my goal was to show that even without gravity, the difference exists, and is indeed noticeable.

But the Earth’s gravity is important here.

The Earth isn’t a perfect sphere; its radius varies from 6357 km to around 6378 km.

Assume the baby is 6360 km from the center of the Earth.

Show the Math
\(F=6.67\cdot 10^{-11}\frac{m^3}{kg s^2}\frac{3.6 kg \cdot 5.974\cdot 10^{24} kg}{(6.36\cdot 10^{6} m)^2}=35.46 \frac{m\cdot kg}{s^2}\)

In this case, the force exerted on him by gravity would be \(35.46 \mbox{N}\)

As you can see, this is \(10^6\) times more than the forces exerted by the occupants of the delivery room, and \(10^7\) times more than the force exerted by the planets together. It’s a powerful force, gravity.

And there’s more. The Earth’s gravity isn’t constant. It varies across the surface of the planet (as the radius varies). We usually use the average rounded number for the gravitational acceleration (\(9.806 \mbox{m}/\mbox{s}^2\)) but in different locations on the Earth, the number varies.

If the claim astrologers make is that the force from other planets affect a baby’s personality – and we’ve seen how small that force is! – then the change in the Earth’s gravitation should have an effect too. In this case, Astrologers should consider the location and elevation of your birth as well as the date and time, to calculate the variations in the Earth’s gravity.

The next time an Astrologer offers to calculate your chart, you should reminder them about that.

One more thing: The Labor Itself

We didn’t include this part in the initial calculation, but this is definitely something that we should take into account, since this is likely to be quite a powerful force.

A baby doesn’t just “walk out” of the womb, it is pushed out by the mother’s muscles. If you see any TV shows at all, you know that at the moment where the baby – and doctor – are ready, the doctor will ask the woman to “Push!!” resulting in the baby’s head being pushed out (if all is well) and the doctor assisting the baby the rest of the way.

This “push” and the movement out of the woman’s womb also exert force on the baby. On top of that, there is usually a large amount of time during which the woman’s body exerts force on the baby before it actually comes out. This would apply pressure on his body; obviously, it’s not enough to harm the baby, but it definitely exists. And labors can be long… long and tedious processes. Ask your mother how long she was in labor.

So for a large number of hours (36 is the average!) the baby is subjected to pressure from the mother’s contractions, and then to the force that pushes him or her out of the womb.

So.. why don’t Astrologers ask how long your labor lasted?


There are many things that are plain false in the claims that Astrologers make, and many blogs and sites covered the reasons why. Now, though, you could see for yourselves how the basic premise – that planets’ positions, affect the personality trait of a newborn baby – is just silly.

If the planets’ positions affect the baby’s personality traits, so should the Doctor’s position, the OB Technician, the position of the heart monitor, the CT machine down the hall and the size of the hospital and the amount of people in it.

So, unless Astrologers are willing to take these components into account when they produce your “Chart”, it seems their claims are plain silly.

And you should tell them that.

Do you have more objects to test?

Now you can. Due to popular demand, I’ve prepared a small tool to help you calculate the force from object at any distance. Play with it, and share your findings in the comments!

Click here to open the Force Calculator!

(opens in a new window).



Once again, thanks goes to:

  • Capn_Refsmmat, for some language issues, for his mastery of the LaTeX plugin and for his math peer-review.
  • Daniel Grrrrrr for his English support and patience. Lots of it.
  • UnintentionalChaos (from ScienceForums.net) for some math peer-review and clarity correction issues.