World
What If Earth Had Two Moons?
How would a second moon change the tides, in a back-of-the-envelope way?
One Moon already dominates Earth's tides. A second body of lunar-ish mass would add another stretch, scaled by its mass and by one over distance cubed. Put a half-Moon at 1.2 lunar distances and the extra tide-raising is tens of percent, not a new ocean sprayed into space. Put a full Moon twice as far away and the extra is small. None of these sliders is a three-body integration. Two moons plus the Sun plus Earth's equatorial bulge is a research problem; this page adds two numbers and says so. Orbits can be chaotic, moons can trade eccentricity, and a close massive companion can threaten the original Moon. If you want a film of colliding worlds, you will have to watch a film. If you want the tidal ratio, stay here.
Change this
Results update as you move the controls.
1 is another Moon; 0.5 is half a Moon.
1 is today's mean lunar distance.
Stacked is a simplified peak-on-peak add. Not a sky simulation.
Your result
1.29×
simplified lunar tide-raising vs today
not a three-body simulation
- Moon 2 mass
- 3.67 × 10^22
- Moon 2 distance
- 461,280 km
- Moon 2 / Moon 1 tide term
- 0.29
- Combined if stacked
- 1.29×
- Model
- M / d³, two bodies
0.5 × lunar mass
1.2 lunar distances
What does that mean?
Moon 2 has 50% of the Moon's mass at 1.2 lunar distances (461 km). Its tide-raising term is 0.29 times the present Moon. Stacked on the original, the simplified lunar tide-raising becomes 1.29× today's lunar term. That is a stronger ocean, not a three-body movie. Mutual eclipses, chaotic exchanges and a possible ejection are all off-stage.
At a glance
- Today's Moon1.00
- Second moon0.29
- Stacked total1.29
Compare
| This is | a tidal-force ratio |
|---|---|
| This is not | a stable three-body orbit |
Compare scenarios
Tides add, phases argue
Tide-raising acceleration at Earth's surface from a companion is about 2 G M R / d³. The ratio of two such terms is (M₂/M₁) / (d₂/d₁)³. When both moons line up with Earth, a simplified model lets the stretches add. When they sit at right angles the pattern is messier. This control labelled 'alignment' is a switch between 'add the peaks' and 'quote the second moon alone,' not a sky chart.
Nights would be busier
Two large moons mean more than one kind of night: double full, mixed quarters, mutual eclipses if the geometry allows. The extra moonlight is a lighting design problem, not a climate one, unless you also change tides enough to mix the seas differently. Solar eclipses, already a coincidence of apparent size, would gain a second occasional actor. We do not predict the next totality. We only note that the sky stops being a one-satellite story.
Stability is someone else's computer
The real Earth–Moon system is already a three-body problem with the Sun. Adding a fourth massive actor can eject, swap or crash worlds. Hill spheres, resonances and tidal decay all matter. This experiment will not pretend it ran that integrator. If the second moon is a tenth of a lunar mass and comfortably beyond the Roche limit — and 0.5 lunar distances is still far outside that limit — a bound orbit is at least thinkable. 'Thinkable' is not 'stable for a gigayear.'
How we calculated this
The original Moon is NASA's lunar mass at the mean Earth–Moon distance. Moon 2 is a mass ratio times that mass, at a distance ratio times that distance. Extra tide-raising is massRatio / distanceRatio³. If alignment is 'stacked,' that extra is added to 1; if 'separate,' both terms are reported without summing. Solar tide is omitted here (see the Moon-disappeared experiment). Not a three-body simulation, not a Roche-limit destroyer, not an eclipse almanac.
Go further
A curated rabbit hole from this question. Each link is a real experiment, not a random suggestion.
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