Tidal locking is when a smaller body orbits a larger one and rotates on its axis in the same amount of time it takes to complete one orbit. The Moon is tidally locked to Earth, but its elliptical orbit causes slight variations. Hyperion, Saturn’s moon, is irregularly shaped and has a chaotic orbit. Pluto and its moon Charon are tidally locked and rotate in sync. Some binary stars and planets may also be tidally locked.
Tidal locking, also known as captured rotation or tidal locking, is a physical phenomenon in astronomy in which a smaller body orbiting another rotates on its axis in about the same time it takes to complete one orbit around the bigger body. This causes one side of the orbiting satellite to always face the body it is orbiting. One of the most obvious examples of this is the way the Moon orbits the Earth in about 27 days and completes one revolution around its axis in the same amount of time.
While the Moon’s orbit is tidally locked, that’s not exactly the case. This is largely because the Moon’s orbit around the Earth is elliptical in shape, not perfectly circular. When the moon is at its apogee, or at its furthest distance from earth of 252,499 miles (406,357 kilometers), its revolution is slightly faster than its orbit. This reveals an additional 8° longitude of its western hemisphere.
When it is at its perigee, or closest distance from earth of 221,699 miles (356,790 kilometers), its revolution is slightly slower than its orbit. This reveals 8 degrees longitude of its eastern hemisphere. The moon is also about 5° outside the plane of the Earth’s ecliptic, or the direct line that the earth takes in orbit around the sun, which reveals an additional 7° of the surface of polar latitude during an orbit around the Earth.
While most of the moons in our Solar System are currently thought to be lock-locked around their parent bodies, one notable exception is the moon Hyperion, which orbits the planet Saturn. Hyperion is an irregularly shaped moon that is the closest object in space to the massive Titan, Saturn’s largest moon, which is larger in size than the planet Mercury. Titan and Hyperion are locked in orbital resonance, affecting each other’s orbits around Saturn in such a way that, for every four orbits of Saturn that Titan makes, Hyperion makes three.
The Cassini spacecraft made measurements of Hyperion’s orbit in close flybys of the moon in 2005. The mission determined that Hyperion is spinning between 4.2 and 4.5 times faster than what a synchronous speed would be for it. Hyperion’s orbit is described as chaotic because it changes in its revolution around its axis, meaning it has no defined equators or poles. Its position around Saturn at any time, therefore, is unpredictable.
When two bodies in space share close proximity to each other and similar physical dimensions, both tend to share synchronous orbits around each other. This is true of the dwarf planet Pluto and its largest moon Charon, which is only 12,000 miles from Pluto. The moon Charon is 790 miles (1,270 kilometers) in diameter, making it just over half the size of Pluto itself at 1,440 miles (2,320 kilometers) across.
Both Pluto and its moon Charon rotate on their respective axes in about 6.3 days, always keeping the same side of the surface facing each other. This is a phenomenon that the earth will one day do with the moon as well. These unique features have led to the Pluto-Charon system being labeled a dual planet.
Systems other than planets and moons can also display tidally locked. Some binary stars in the Milky Way galaxy, two stars locked in orbit around each other, are also known to be tidally locked. Canada’s Microvariability & Oscillations of STars (MOST) space telescope, launched in 2003, is designed to investigate this.
The star Tau Bootis, about 50 light-years from Earth, was discovered by MOST to be lock-locked with Tau Bootis b, a massive planet about 7-8 times the size of Jupiter that orbits Tau Bootis. Because it is 100 times closer to its parent star than Jupiter is to the sun, Tau Bootis b orbits its sun every 3.3 days, and the same side of the star’s surface always faces the planet. Scientists theorize that many stars may, in fact, be involved in such a tidal lock with large planets orbiting nearby. However, these planets are likely to be in decaying orbits, as their proximity to the stars suggests.
Protect your devices with Threat Protection by NordVPN
