Synchronous rotation, also known as captured rotation or tidal locking, is a physical phenomenon in astronomy where a smaller body orbiting another rotates on its own axis in roughly the same amount of time it takes to complete one orbit around the larger 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 in how the Moon orbits the Earth in approximately 27 days, and completes a revolution around its own axis in the same amount of time.
While the Moon's orbit is one of synchronous rotation, it is not perfectly so. This is largely due to the fact that the Moon' s orbit around the earth is elliptical in shape, not perfectly circular. When the moon is at its apogee, or farthest distance from the earth of 252,499 miles (406,357 kilometers), its revolution is slightly faster than its orbit. This reveals an extra 8° of 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 of longitude of its eastern hemisphere. The moon also sits about 5° outside the earth's ecliptic plane, or the direct line the earth takes in orbiting the sun, which reveals an additional 7° of polar latitude surface during an orbit around the earth.
While most moons in our Solar System are believed to currently be in synchronous rotation around their parent bodies, one notable exception to this is the moon Hyperion, which orbits the planet Saturn. Hyperion is an irregularly shaped moon that is the nearest object in space to massive Titan, the largest moon of Saturn, which is larger in size than the planet Mercury. Titan and Hyperion are locked in orbital resonance, affecting each other's orbits around Saturn such that, for every four orbits of Saturn that Titan makes, Hyperion makes three.
The Cassini spacecraft took measurements of Hyperion's orbit in close flybys of the moon in 2005. The mission determined that Hyperion is rotating between 4.2 and 4.5 times faster than what would be a synchronous rate for it. Hyperion's orbit is described as chaotic because changes in its revolution around its own axis, meaning that it has no definite equator or poles. Its location around Saturn at any one time, therefore, is unpredictable.
When two bodies in space share close proximity to each other and similar physical sizes, they both tend to share synchronous orbits around each other as well. 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 a little over half the size of Pluto itself at 1,440 miles (2,320 kilometers) in diameter.
Both Pluto and its moon Charon revolve on their respective axis in approximately 6.3 days, each keeping the same side of the surface facing each other at all times. This is a phenomenon that one day the earth will also do with the moon. These unique characteristics have resulted in the Pluto-Charon system being labeled as a double planet.
Other systems besides planets and moons can also display synchronous rotation. Certain binary stars in the Milky Way galaxy, two stars locked in orbits around each other, are also known to be in synchronous rotation. Canada's Microvariability & Oscillations of STars (MOST) space telescope, launched in 2003, is designed to investigate this.
The Tau Bootis star, approximately 50 light years from Earth, was discovered by MOST to be locked in synchronous rotation with tau Bootis b, a massive planet about 7 to 8 times the size of Jupiter that orbits Tau Bootis. Since 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 surface of the star always faces the planet. Scientists theorize that many stars may, in fact, be engaged in such tidal locking with large orbiting planets that are close. It is likely that these planets are in decaying orbits, however, as their close proximity to the stars suggests.