How Mercury and Starlight Overthrew Newton
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Every object pulls on every other object. The pull grows with their masses and weakens with the square of the distance between them. This one law explains the orbits of the planets and why things fall on Earth.
Mercury’s perihelion moves about 38 arcseconds per century more than Newton’s law predicts from the pull of the known planets.
Newton’s law isn’t wrong. Unseen matter inside Mercury’s orbit, a planet or a ring of small bodies, would explain the difference. That’s how the unseen Neptune explained the odd motion of Uranus.
Photographs from the eclipses of 1901, 1905, and 1908 show no planet inside Mercury’s orbit big enough to explain its motion. Whatever moves Mercury’s perihelion, it isn’t a planet, or anything else bright enough to photograph.