Mercury A to Z: Resonance

Hello, friends!  Welcome back to the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, and in today’s post R is for:


There are many weird coincidences in the Solar System, most of which really aren’t coincidences.  It’s usually just the force of gravity doing its thing.  Sometimes objects in space keep tugging on each other, gravitationally, pulling each other into these oddly specific mathematical relationships which astrophysicists call “resonances.”

As an example, for every three complete orbits of Neptune, Pluto completes exactly two (a 3:2 orbital resonance).  Another example: for every thirteen complete orbits of Venus, Earth completes exactly eight (a 13:8 orbital resonance).  And then there are the resonating moons of Jupiter.  For every four complete orbits of Io, Europa completes exactly two orbits, and Ganymede completes exactly one (a 4:2:1 orbital resonance).

Earth’s Moon is locked into a different kind of resonance, called a spin-orbit resonance.  For every orbit the Moon completes around the Earth, the Moon rotates (spins) exactly one time (a 1:1 spin-orbit resonance).  This happens because the Moon is not perfectly spherical.  The Moon bulges a little at the equator, kind of like a football (or a rugby ball, for readers outside the U.S.).  As the Moon circles around the Earth, Earth’s gravity keeps pulling one of the pointy ends of this football-shaped Moon towards the Earth.  This is why you always see the same side of the Moon whenever the Moon appears in the sky.

Mercury is also locked into a spin-orbit resonance, specifically a 3:2 spin-orbit resonance.  So for every two orbits around the Sun, Mercury rotates (spins) exactly three times.  This happens for two reasons:

  • Like the Moon, Mercury is slightly football shaped.
  • Unlike the Moon, Mercury’s orbit is highly eccentric (non-circular).

Since Mercury is slightly football-shaped, gravity tries to keep one of Mercury’s pointy ends pointed toward the Sun.  However, Mercury’s highly eccentric orbit means that Mercury’s orientation, with respect to the Sun and with respect to the Sun’s gravity, changes.  This changing orientation creates torque, which causes Mercury to spin.

So every time Mercury reaches perihelion (the point when Mercury is closest to the Sun) one end of the football-shaped planet points directly toward the Sun.  As Mercury moves away from perihelion, gravitational torque sets the planet spinning.  Mercury spins very slowly, only managing to turn 180° before reaching perihelion again, at which time gravity once again forces the football-shaped planet to point directly toward the Sun.  This has probably been happening for billions of years now, and it will probably keep happening for billions of years to come.


I want to recommend this very short video I found on YouTube.  I’ve read a lot about this topic.  Most sources I looked at get very technical very fast.  This video was, by far, the clearest and most straightforward explanation of Mercury’s spin-orbit resonance that I could find.

I also want to recommend this article from  Some exoplanets may be locked into Mercury-like spin-orbit resonances, and for exoplanets orbiting red dwarf stars, a spin-orbit resonance has implications for potential alien life.