The argument surfaces regularly in defenses of astrology: the moon is massive, it pulls the oceans into tides twice a day, and the human body is mostly water — so why wouldn’t the moon’s gravity affect us too?
It’s an intuitive argument. It sounds physically grounded. And it is wrong, in a way that is worth understanding carefully — not because the physics is obscure, but because the specific error it makes reveals something interesting about how gravitational effects actually work.
The argument conflates two different things: gravitational force and tidal force. They’re related but not identical, and the distinction between them is precisely what determines whether the moon does or doesn’t affect ocean tides. Once you understand the distinction, it becomes clear why the moon creates ocean tides but exerts no meaningful tidal effect on human bodies — and why, in the specific calculation that matters, your obstetrician does outperform the moon.
Gravitational Force vs. Tidal Force
Gravitational force is the pull that one massive object exerts on another. It follows Newton’s inverse-square law: the force is proportional to the product of the two masses and inversely proportional to the square of the distance between them. The moon exerts a gravitational force on every object on Earth — including you, your coffee cup, and the Atlantic Ocean. That force is real.
Tidal force is something different. It is not the total gravitational pull — it is the difference in gravitational pull between the near side of an object and the far side. The moon is closer to the side of the Earth facing it than to the side facing away — by about 12,740 kilometers (the diameter of the Earth). Because gravity falls off with distance, the near side of the Earth experiences slightly more pull than the far side. This differential — this gradient across the Earth — is what creates tides.
The key insight: tidal force depends on the size of the object being pulled relative to the distance to the pulling body. The larger the object, the greater the difference in gravitational pull across it. The closer the pulling body, the steeper the gravitational gradient, and the larger the differential.
The Earth is big — 12,740 km across. The gravitational differential across that distance, at the moon’s distance of about 384,400 km, is large enough to deform the ocean surface by about half a meter in the open sea. Tides are real and impressive.
A human body is about 1.8 meters tall. The gravitational differential across 1.8 meters, at a distance of 384,400 km, is vanishingly small. Let’s do the math.
The Calculation
The tidal force on an extended object can be approximated as:
F_tidal ≈ 2GMmr / d³
Where:
- G is the gravitational constant (6.674 × 10⁻¹¹ N·m²/kg²)
- M is the mass of the body creating the tidal force (the moon: 7.342 × 10²² kg)
- m is the mass of the object experiencing the force (a 70 kg person)
- r is the radius of the object (approximately 0.9 meters for a person)
- d is the distance between the centers (384,400 km = 3.844 × 10⁸ m)
Plugging these in:
F_tidal (moon on person) ≈ 2 × (6.674 × 10⁻¹¹) × (7.342 × 10²²) × 70 × 0.9 / (3.844 × 10⁸)³
≈ 2 × 6.674 × 10⁻¹¹ × 7.342 × 10²² × 63 / (5.68 × 10²⁵)
≈ 6.16 × 10⁻¹⁰ N
That is 0.000000000616 newtons. For comparison, a single mosquito weighs about 0.000002 newtons — roughly 3,000 times more force than the moon’s tidal pull on your body.
The Obstetrician Comparison
Now let’s calculate the tidal force exerted by an obstetrician — a 75 kg person — standing about one meter away from a newborn during delivery. Using the same formula:
- M = 75 kg (obstetrician’s mass)
- m = 3.5 kg (newborn’s mass)
- r = 0.25 m (newborn’s radius)
- d = 1 m (distance between centers, approximately)
F_tidal (obstetrician on newborn) ≈ 2 × (6.674 × 10⁻¹¹) × 75 × 3.5 × 0.25 / (1)³
≈ 2 × 6.674 × 10⁻¹¹ × 65.6 / 1
≈ 8.76 × 10⁻⁹ N
That’s approximately 14 times larger than the moon’s tidal force on the newborn at the moment of birth.
This is the calculation that astronomer Neil deGrasse Tyson has cited in various public contexts, and it is arithmetically correct. The obstetrician genuinely exerts a greater tidal force on a newborn than the moon does. So does the hospital building, the delivery table, the other people in the room, and almost every other moderately massive object in the immediate vicinity.
The comparison is deliberately vivid — chosen specifically because it highlights the implausibility of a tidal mechanism for astrological influence. The moon is 384,000 km away and weighs 7.3 × 10²² kg. The obstetrician is one meter away and weighs 75 kg. Distance matters so much in the tidal force calculation that the enormous mass advantage of the moon is completely overwhelmed by the obstetrician’s proximity.
Why This Doesn’t Settle Everything
The physics is clear, and the calculation is correct. But it’s worth being precise about what this calculation does and does not show.
It definitively rules out tidal mechanics as the mechanism for lunar influence on human biology. If someone claims that the moon affects human behavior through the same gravitational mechanism that creates ocean tides, that claim is physically wrong. The tidal force is approximately 10 billion times too small to produce any physiological effect on a human body.
It does not rule out all possible lunar effects on humans. As discussed in the companion article on moon effects and human behavior, there is reasonable evidence for a light-mediated effect of the lunar cycle on sleep patterns — not through gravity, but through the moon’s light affecting circadian rhythms. This mechanism is entirely separate from tidal mechanics and is not addressed by the obstetrician calculation.
It does not rule out all possible mechanisms for astrological influence. It rules out the specific proposed mechanism of tidal gravitational force. If astrology works — and the evidence reviewed elsewhere in this series is skeptical but not decisive — it would have to work through some mechanism other than tidal gravity. What that mechanism might be is unknown. Absence of a known mechanism is evidence against a claim, but it is not decisive refutation.
It addresses the moon specifically. The situation for other planets is even more extreme. Mars at its closest approach is about 600 times farther from Earth than the moon, and it’s significantly less massive. The tidal force Mars exerts on a human body is immeasurably smaller than the moon’s — smaller even than the tidal force exerted by a mosquito nearby. The argument that Mars’s gravitational influence at birth shapes personality is physically unsupportable.
Why the Argument Keeps Being Made
Given that the physics is clear and the calculation straightforward, why does the “moon moves oceans, therefore it moves us” argument persist?
Several reasons:
The intuitive logic is compelling. “The moon moves oceans, we’re mostly water, therefore the moon moves us” follows a form of reasoning — if X affects Y, and Y is related to Z, then X affects Z — that sounds valid and occasionally is valid. In this case it isn’t, because the specific mechanism that creates ocean tides (tidal force depending on the size of the body being acted on) does not scale down to human bodies. But the formal structure of the argument sounds reasonable until you examine the physics.
Tidal force is counterintuitive. Most people’s intuitive model of gravity is the simple inverse-square law — bigger mass means more force. The idea that a nearby 75 kg person could exert a comparable tidal force to the 7.3 × 10²² kg moon seems absurd until you work through the distance-cubed dependence of tidal force. The math is not difficult, but the result contradicts most people’s intuitive expectations.
Disproving the specific mechanism doesn’t settle the broader question. People who believe in lunar effects on human behavior often respond to this calculation by saying “I don’t claim the mechanism is tidal gravity — there might be other mechanisms we don’t understand.” This is a legitimate position. The problem is that invoking unknown mechanisms while the known ones fail is a pattern that can immunize any claim against falsification. Unknown mechanisms are always available; what they require is evidence beyond the failure of known mechanisms.
The impressive scale of ocean tides carries emotional weight. Watching a fifteen-meter tide in the Bay of Fundy is genuinely awe-inspiring. The moon moves that much water. It’s difficult to hold simultaneously the true facts that: (a) the tidal force on the ocean is real and large, and (b) the tidal force on a human body is negligibly small. Both are true. The emotional impression of the ocean tides persists even after the calculation shows that it doesn’t scale.
What the Physics Actually Tells Us
The tidal mechanics argument is a significant vulnerability for physical defenses of astrology, and understanding why it fails is useful beyond the immediate question of the moon. It illustrates a general principle: intuitive analogies from one physical context (ocean tides) don’t automatically transfer to a different physical context (human bodies), because the relevant physical quantity (tidal force) depends on parameters (the size of the affected body, the cube of the distance) that are dramatically different between the two cases.
The failure of the tidal mechanism is not devastating to all forms of astrological practice — as noted, there are lunar effects through other mechanisms, and the broader empirical questions about astrology are more complex than any single physical argument settles. But it does mean that the “it’s just physics, the moon moves oceans” defense of lunar astrology is not, in fact, a physical argument. It’s an analogy that fails to survive quantification.
Quantification is what makes the difference. “The moon is big and close” is true. “The tidal force on a human body from the moon is smaller than the tidal force from a nearby person” is also true, follows directly from the same physics, and completely deflates the intuitive argument. The honest position for anyone who uses lunar timing in their practice is to acknowledge that whatever the moon is doing — if it’s doing anything — it isn’t doing it through tidal gravity, and the mechanism remains genuinely unknown.
That’s not nothing. Unknown mechanisms can be real. But they require evidence beyond the failure of the known candidates, and that evidence — for the specific lunar effects claimed by astrological traditions — remains elusive.