The differential gravitational force exerted by one celestial body on another, caused by the variation in gravitational strength across the affected object. This force is responsible for ocean tides on Earth and can tear apart objects that get too close to massive bodies.
From 'tidal' (relating to tides) and 'force' from Latin 'fortis' (strong). The connection between tides and gravitational forces was first understood by Newton in the 17th century, though the term 'tidal force' became common in the 20th century as astronomers applied the concept beyond Earth's oceans.
Tidal forces are the reason you'd be 'spaghettified' if you fell into a black hole - your feet would experience stronger gravity than your head, stretching you into a long, thin shape! These same forces are slowly stealing energy from Earth's rotation, making our days longer by about 2 milliseconds per century and causing the Moon to spiral away from us.
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