A Calculation Anyone Can Check
One of the oldest skeptical arguments against astrology is also one of the few that can be settled with a single equation. Newton’s law of universal gravitation states that the gravitational force between two objects is:
F = G × (m₁ × m₂) / r²
where G is the gravitational constant (6.674 × 10⁻¹¹ N·m²/kg²), m₁ and m₂ are the masses of the two objects, and r is the distance between their centers. Everything in this equation is measurable. There’s no interpretation involved, no symbolic layer, no question of which framework to apply. You plug in numbers and get a number out, in newtons, a unit of force.
The argument goes: if planetary positions at birth influence a person through some physical mechanism, gravity is the obvious candidate — it’s the only force that operates at planetary distances with any appreciable strength. So: how does the gravitational pull of a planet at the moment of birth compare to the gravitational pull of, say, the obstetrician standing next to the newborn?
Let’s actually do it.
Mars vs. the Obstetrician
Take a newborn with a mass of 3.5 kg. Take Mars, with a mass of about 6.39 × 10²³ kg, at its average distance from Earth — roughly 2.25 × 10¹¹ meters (225 million kilometers).
F(Mars) = (6.674 × 10⁻¹¹) × (6.39 × 10²³) × 3.5 / (2.25 × 10¹¹)² ≈ 2.95 × 10⁻⁹ newtons
Now take an obstetrician with a mass of 80 kg, standing half a meter from the newborn.
F(obstetrician) = (6.674 × 10⁻¹¹) × 80 × 3.5 / (0.5)² ≈ 7.48 × 10⁻⁸ newtons
Dividing the second by the first: the obstetrician’s gravitational pull on the newborn is about 25 times stronger than Mars’s, at Mars’s average distance. Even at Mars’s closest approach to Earth (around 7.8 × 10¹⁰ meters, which happens only during certain oppositions), the obstetrician’s pull still exceeds Mars’s by roughly a factor of three.
This is the calculation behind the familiar version of this argument, and as far as it goes, it’s correct. Mars is a relatively small planet — about one-tenth of Earth’s mass — and even at planetary distances, its gravitational pull on a small object is genuinely tiny compared to a person standing nearby.
Where the Soundbite Breaks Down
The problem is that “Mars” is doing a lot of work in this argument, and the argument doesn’t generalize the way it’s usually presented to. Try the same calculation with Jupiter.
Jupiter’s mass is about 1.898 × 10²⁷ kg — roughly 3,000 times more massive than Mars, and about 318 times the mass of Earth. Its average distance from Earth is about 7.78 × 10¹¹ meters (5.2 times farther than the Sun, roughly 778 million kilometers — more than three times Mars’s distance).
F(Jupiter) = (6.674 × 10⁻¹¹) × (1.898 × 10²⁷) × 3.5 / (7.78 × 10¹¹)² ≈ 7.33 × 10⁻⁷ newtons
Compare this to the obstetrician’s 7.48 × 10⁻⁸ newtons. Jupiter’s gravitational pull on the newborn is roughly ten times stronger than the obstetrician’s — despite Jupiter being 778 million kilometers away. Even if the obstetrician were standing close enough to be in physical contact (say, 0.3 meters), Jupiter’s pull would still exceed theirs by a factor of more than three.
This is worth sitting with, because it inverts the usual takeaway. The “your doctor’s gravity matters more than the planets” argument is true for Mars. It is not true for Jupiter or Saturn, whose enormous masses more than compensate for their distance. If gravitational force were actually the mechanism astrology proposed — and if “more gravitational force = more influence” were the relevant logic — Jupiter and Saturn would have a stronger physical claim to influence than most objects in the room at any birth, including the people in it.
Why This Doesn’t Rescue the Astrological Claim
Having shown that the simple “your doctor outweighs the planets” soundbite doesn’t hold for the gas giants, it would be a mistake to conclude that this validates astrology’s claims about Jupiter and Saturn. The reason is that gravitational force — the quantity in Newton’s equation — is not the quantity that could plausibly affect a human body even if it were large. What would matter, if anything did, is tidal force: the difference in gravitational pull between one side of an object and the other, which is what causes actual physical effects like ocean tides.
Tidal force falls off with the cube of distance (1/r³) rather than the square (1/r²), because it depends on the gradient of the gravitational field across the width of the object, not the field strength itself. This changes the picture dramatically, because it punishes distance much more severely.
The Moon, despite its small mass (7.35 × 10²² kg — less than Mars), is close enough (about 3.84 × 10⁸ meters) that its tidal effect dominates every other celestial body by an enormous margin. Calculating the tidal acceleration gradient (proportional to 2GM/d³) for the Moon versus Jupiter at their respective distances:
Moon: 2GM/d³ ≈ 1.73 × 10⁻¹³ (in SI units, per meter of separation) Jupiter: 2GM/d³ ≈ 5.38 × 10⁻¹⁹
The Moon’s tidal gradient is roughly 320,000 times larger than Jupiter’s, despite Jupiter being far more massive — because tidal force cares about distance so much more than raw gravitational force does. Every other planet, including Jupiter and Saturn, is even further behind the Moon on this measure than this comparison suggests, because they’re all considerably farther from Earth than the Moon is, and the cube in the denominator punishes that distance severely.
Now compare the Moon’s tidal effect to the obstetrician’s. Using the same formula for the obstetrician at 0.5 meters:
Obstetrician: 2GM/d³ ≈ 8.54 × 10⁻⁸
The obstetrician’s tidal gradient exceeds the Moon’s by a factor of roughly 500 million. On the metric that actually corresponds to a physical mechanism with documented real-world effects (the tides), every planet — including the giants whose raw gravitational force exceeds a nearby person’s — is utterly negligible. The Moon, the only celestial body whose tidal effect is large enough to move oceans, is still hundreds of millions of times weaker than the tidal pull of a person standing next to a newborn.
What “Doing the Math” Actually Shows
The honest conclusion from running these numbers isn’t quite either of the soundbites that circulate. It isn’t simply “your doctor’s gravity beats the planets” (true for Mars, false for Jupiter and Saturn on raw force). And it isn’t “well, actually, Jupiter’s gravity is stronger than your doctor’s, so maybe astrology has a point” (true on raw force, but raw gravitational force isn’t a mechanism that could plausibly affect a developing fetus or newborn in any way — what would need to act on a body is a differential force, and on that measure, every planet loses badly, including to furniture).
What the math actually demonstrates is that “gravity” as a mechanism for planetary influence doesn’t survive contact with the equations regardless of which version of the argument you run. Either the force is too small to matter (Mars, on raw force; every planet, on tidal force) or it’s a force without the right kind of structure to plausibly act on a body (Jupiter and Saturn’s raw gravitational pull, which — unlike tidal force — doesn’t create any differential effect across an object, just a uniform acceleration that a body in free space wouldn’t even register as a force at all, the same way you don’t feel the Earth’s gravity pulling you and the room you’re in at slightly different rates).
This is, in its way, a cleaner result than the simplified version usually offered. The simplified version invites a “gotcha” rebuttal — “well Jupiter is bigger than your doctor” — that happens to be numerically correct and therefore can feel like it reopens the question. The complete version, run through to tidal force, closes it more thoroughly: there’s no formulation of “gravitational influence” — raw force or tidal force, near planets or far ones — under which any planet’s pull on a person comes anywhere close to mattering relative to the objects already in the room.
What’s Left for “As Above, So Below”
None of this settles whether celestial positions correlate with anything in human life through some other mechanism — non-gravitational, non-causal, or something not yet understood. The gravitational argument was never a complete refutation of astrology as a whole; it was always a refutation of one specific, often-claimed mechanism. What this calculation does is close off that mechanism cleanly, in both directions, for anyone tempted to reopen it with “but Jupiter is huge.”
Jupiter is huge. It’s also nowhere near close enough for that to matter, by the measure that would actually need to matter. The math doesn’t care which planet you pick.