Mercury's Spin-Orbit Resonance (Animation)

This is an animation we created for our episode about the planet Mercury, but we thought it might be helpful on its own. In the animation, the green light is a simple way to mark when a full rotation has happened. The astronaut is simply there to help you see Mercury's rotation. The yellow line is the zenith of the sun's light to help you mark the solar days.

Mercury is tidally locked with the sun in a 3:2 spin-orbit resonance, meaning that it rotates on its axis exactly 3 times for every two trips around the sun. Three sidereal days every two years.

A year on Mercury (the time it takes to orbit the sun once) is only 88 days. A single solar day (the time it takes for the sun to be in the same position in the sky on Mercury) takes 176 earth days. So its solar day is twice as long as its year. And while most planets in our solar system have fairly circular orbits, Mercury has an elliptical orbit. At its closet, or perihelion, Mercury is 29 million miles from the sun (47 million km) and at its farthest, or aphelion, it’s as far as 43 million miles (70 million km). As this happens, Mercury’s spin (its sidereal day) is constant, but the position of the sun in Mercury's sky speeds up, slows down, and even goes backwards sometimes!

So here are the highlights about Mercury's unique orbit:

Mercury’s orbit is highly elliptical.
Its year is 88 earth days.
Its Solar Day is 176 earth days.
Its Sidereal Day is 88 earth days.
Its tidally locked in a 3:2 spin-to-orbit resonance.

We hope this animation is helpful!

Watch our explanation episode about Mercury:    • Planet Mercury: The closest planet to...