On Mercury One Day Lasts Two Years

Mercury has the most noncircular or eccentric orbit of any nondwarf planet in the solar system. This eccentricity has trapped Mercury in a 3:2 spin-orbit resonance, where it rotates three times for every two revolutions. When nearest Sol at perihelion, Sol’s tidal forces are greatest, Mercury’s spin and orbit (or rotation and revolution) match, and Sol momentarily stands still in Mercury’s sky.

For long times, Mercury’s orbit precesses due to the gravity of Jupiter, the oblateness of Sol, and spacetime curvature, first described by Albert Einstein’s theory of General Relativity. For short times, as the animation shows, one solar day lasts two years!

Mercury spins 3 times for every 2 orbits, and 1 day lasts 2 years

Mercury spins 3 times for every 2 orbits, and 1 day lasts 2 years

About John F. Lindner

John F. Lindner was born in Sleepy Hollow New York and educated at the University of Vermont and Caltech. He is a professor of physics and astronomy at The College of Wooster. He has enjoyed multiple yearlong sabbaticals at Georgia Tech, University of Portland, and University of Hawai'i. His research interests include nonlinear dynamics, celestial mechanics, and variable stars.
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