What if Gru Stole the Moon

In Despicable Me, Gru famously steals the Moon. The film treats this primarily as a logistics problem, but the consequences would extend far beyond astronomy. The Moon is responsible for most of Earth's tidal forces, and its sudden disappearance would immediately alter coastlines, ecosystems, shipping routes, and perhaps the global economy itself.

The tidal force exerted by a body scales approximately as $M/r^3$, where $M$ is the body's mass and $r$ its distance from Earth. Although the Sun is vastly more massive than the Moon, it is also much farther away. Using masses of $1.99 \times 10^{30}\,\mathrm{kg}$ for the Sun and $7.35 \times 10^{22}\,\mathrm{kg}$ for the Moon, together with distances of $1.496 \times 10^{11}\,\mathrm{m}$ and $3.84 \times 10^{8}\,\mathrm{m}$, respectively, the Moon contributes roughly twice the tidal forcing of the Sun. If Gru removed the Moon, solar tides would remain, but total tidal amplitudes would fall dramatically.

To estimate the effect, I examined NOAA harmonic constituent data for the San Diego tide station. NOAA models tidal height as the sum of dozens of sinusoidal constituents:

where $A_i$ is the constituent amplitude, $\omega_i$ its frequency, and $\phi_i$ its phase. The largest constituents in San Diego are lunar, including M2, K1, N2, and O1.

Using NOAA's harmonic constituents, I computed tidal height both under normal conditions and under a hypothetical "Gru scenario" in which all lunar constituents are removed while solar constituents remain.[^2] At $t=2$ hours, the predicted tidal height falls from 6.55 feet to 1.52 feet, a reduction of approximately 5 feet.

h_t2 = calculate_tidal_height(t_hours=2.0)
height_at_2 = calculate_solar_tidal_height(t_hours=2.0)

print(h_t2)
print(height_at_2)
print(h_t2 - height_at_2)

Output:

6.5525
1.5191
5.0334

A single tidal snapshot does not capture the entire phenomenon, but it illustrates the scale of the change. Many coastlines would experience substantially weaker tides. Estuaries, tidal flats, and shallow harbors would be transformed almost immediately.

How much would this cost?

A rough Fermi estimate can be constructed from maritime trade. Suppose:

• 10% of world shipping experiences delays due to reduced tidal ranges.
• Global maritime trade is approximately 80% of all trade, which is on the order of $26 trillion per year.[^1]
• Reduced tidal access imposes a 0.5% efficiency cost on affected traffic.

Then:

or roughly $13 billion annually.

This figure is surprisingly modest. The direct economic cost of weaker tides may only amount to a few billion dollars per year. The larger losses would likely come from ecological disruption. Tidal marshes, mudflats, estuaries, and coastal food chains depend on regular tidal exchange. Their destruction would affect fisheries, tourism, biodiversity, and coastal resilience in ways that are difficult to price.

The conclusion is somewhat unexpected. Gru's theft of the Moon would not immediately collapse civilization. Ports would adapt, shipping companies would reroute traffic, and governments would dredge channels. The direct economic damages might be measured in tens of billions rather than trillions.

The true value of the Moon lies elsewhere. It shapes ecosystems, stabilizes Earth's long-term climate, and has done so for billions of years. Those services are difficult to quantify. By comparison, the $13 billion shipping estimate may represent only a tiny fraction of the Moon's actual worth.

No wonder they wanted Gru to put it back.

[^1]: UNCTAD estimated global trade at approximately $33 trillion in 2024. Since roughly 80% of world trade by volume is transported by sea, maritime trade represents on the order of $26 trillion annually. https://maritimemag.com/en/unctad-predicts-record-33-trillion-in-world-trade-in-2024/

[^2]: Tidal heights were computed using NOAA harmonic constituent data for station 9410170 (San Diego, California). NOAA models tides as the sum of harmonic constituents with measured amplitudes, frequencies, and phase offsets. The analysis script removes lunar constituents while retaining solar constituents in order to estimate the residual solar tide after the Moon's disappearance. Source code: https://github.com/TheMerjin/website/blob/master/.py/sandiego_height.py. Harmonic constituent data: https://tidesandcurrents.noaa.gov/harcon.html?unit=0&timezone=0&id=9410170