In a recent talk in Tamil courtesy of , I introduced #darkmatter by stealing an idea from David Morrissey.
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Say the Earth is under permanent, thick cloud cover. At daytime humans can tell there's some bright external source of light that filters through.
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At night moonlight never reaches us. Could we ever tell if there's a Moon?
Yes, exploit gravity! Note the tides!
Measure them at multiple shores and times, and fit the data to a Moon + Sun model.
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OK, any complementary evidence?
Note the slow drift of seasons across the calendar!
That's from the whirl of the Earth's spin axis, the "precession of the equinoxes". It takes ~13000 years to flip summer and winter, so historic records should track seasonal shifts.
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Then hypothesize that this axial precession comes from celestial bodies pulling on the Earth's equatorial bulge via gravity and producing a torque somewhat perpendicular to the spin axis.
Fit to a Moon + Sun model and see that the Moon is the major culprit.
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You should have figured out the Moon's mass and trajectory by now, especially with lots of tidal data.
Assuming rock-like density, get its radius.
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It now comes down to convincing the final few skeptics.
Take them to Niagara Falls on April 8, 2024. After lunch, tell them: "Don't panic, ladies and gents, but the light will begin to dim now. And for four minutes beginning 3.18 pm, we will be plunged into darkness."
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(This assumes you know the angular diameter of the Sun, which you do because the clouds part once in a while during daytime.)
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