Hill sphere سپهر ِ هیل sepehr-e Hill
*Fr.: sphère de Hill*
The spherical region around a → *secondary* in which the
secondary's gravity is more important for the motion of a particle about
the secondary than the tidal influence of the → *primary*.
The radius is described
by the formula: *r* = *a* (*m*/3*M*)^{1/3}, where,
in the case of the Earth, *a* is the semi-major axis of the orbit around the Sun,
*m* is the mass of Earth, and *M* is the mass of the
Sun. The Hill sphere for the Earth has a radius of 0.01
→ *astronomical unit*s (AU).
Therefore the Moon, lying at a distance of 0.0025 AU, is well within the Hill
sphere of the Earth. Named for George William Hill (1838-1914), an American astronomer who
described this sphere of influence; → *sphere*. |