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The Moon is Earth's only natural satellite.It orbits around Earth at an average distance of 384,400 km (238,900 mi), about 30 times the width of Earth. The Moon faces Earth always with the same side, because tidal forces between Earth and the Moon have synchronized the Moon's rotation period with its orbital period (lunar month) at 29.5 Earth days.
The actual distance between the Moon and Earth can change as quickly as 75 meters per second, [20] or more than 1,000 km (620 mi) in just 6 hours, due to its non-circular orbit. [21] There are other effects that also influence the lunar distance.
Considering the Earth–Moon system as a binary planet, its centre of gravity is within Earth, about 4,671 km (2,902 miles) [24] or 73.3% of the Earth's radius from the centre of the Earth. This centre of gravity remains on the line between the centres of the Earth and Moon as the Earth completes its diurnal rotation.
Although the Moon's Hill sphere extends to a radius of 60,000 km (37,000 mi), [6] the gravity of Earth intervenes enough to make lunar orbits unstable at a distance of 690 km (430 mi). [7] The Lagrange points of the Earth-Moon system can provide stable orbits in the lunar vicinity, such as halo orbits and distant retrograde orbits.
The moon is the namesake of Europa, in Greek mythology the daughter of the Phoenician king of Tyre. ... with an orbital radius of about 670,900 km.
The Moon probably has a liquid core of about 20% of the Moon's radius. [11] The radius of the lunar core-mantle boundary is determined as 381 ± 12 km. [46] The polar flattening of the lunar core-mantle boundary is determined as (2.2 ± 0.6) × 10 −4. [46] The free core nutation of the Moon is determined as 367 ± 100 yr. [46]
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1,880,000 km (26.3 times the 71,492 km radius of Jupiter itself). [3] This is significantly larger than the orbital radius—1,070,000 km—of the next-closest Galilean satellite, Ganymede.
If this is the case, the radius of the core (372 km) is about 64% of the radius of the moon, and its mass is around 56% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Ariel is about 0.3 GPa (3 kbar). [28] The current state of the icy mantle is unclear.