Stability of the Solar System 2:1 orbit/orbit resonance. This particular resonance has important consequences because Europa's gravity perturbs the orbit of Io. As Io moves closer to Jupiter and then further away in the course of an orbit, it experiences significant tidal stresses resulting in active volcanoes. Europa is also in a 2:1 resonance with the next satellite Ganymede. The planet Mercury is especially susceptible to Jupiter's influence because of a small celestial coincidence: Mercury's perihelion, the point where it gets closest to the Sun, precesses at a rate of about 1.5 degrees every 1000 years, and Jupiter's perihelion precesses only a little slower. At
Orbit of the Moon the Moon's orbit around Earth, its relative timing varies greatly. In some places on Earth, there is only one high tide per day, whereas others such as Southampton have four, though this is somewhat rare. The notional tidal bulges are carried ahead of the Earth–Moon orientation by the continents as a result of Earth's rotation. The eccentric mass of each bulge exerts a small amount of gravitational attraction on the Moon, with the bulge on the side of Earth closest to the Moon pulling in a direction slightly forward along the Moon's orbit (because Earth's rotation has carried the bulge