Mercury's Spin-Orbit Resonance

MERCURY'S SPIN-ORBIT RESONANCE
(animation may take a few moments to load)

Viewed from the Sun, Mercury exhibits a peculiar motion indeed! Over eons, its spin (rotation) and orbit have become synchronized by gravitational forces such that Mercury rotates on its axis three times for every two orbits. This has the effect of making Mercury appear—when viewed from the Sun—to stop spinning and even spin very slightly backwards each time it moves through perihelion. At the planet's surface this same synchronized motion results in very curious days indeed! The Sun can appear to stop in Mercury's sky! It also makes parts of Mercury experience double sunrises! This phenomenon occurs nowhere else in the Solar System, and can be examined easily in Celestia: (1.6.x) (1.4.1).

Though Mercury's "3-to-2" spin-orbit ratio appears to be unique, relationships between a body's rotation and orbit are not uncommon in the Solar System. Astronomers call this kind of dynamic linking a "resonance", and we do not have to look far to see other examples. For instance, over eons gravity has similarly coupled the lunar motions into a 1-to-1 resonance, so that the same side of the Moon always face Earth! Other related gravitational resonances have synchronized the orbital periods of the inner three Galilean moons of Jupiter.

Note: the eccentricity of the orbit shown in the above animation has been greatly exaggerated for emphasis. Below you can examine the actual proportions of Mercury's orbit.

And don't forget: with our View of the Inner Planets you can always check out Mercury's current position.

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