- published: 18 May 2015
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An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. The outer shell of that region constitutes a zero-velocity surface. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, rather than of the planet itself. One simple view of the extent of the Solar System is the Hill sphere of the Sun with respect to local stars and the galactic nucleus.
In more precise terms, the Hill sphere approximates the gravitational sphere of influence of a smaller body in the face of perturbations from a more massive body. It was defined by the American astronomer George William Hill, based upon the work of the French astronomer Édouard Roche. For this reason, it is also known as the Roche sphere (not to be confused with the Roche limit).
In the example to the right, the Hill sphere extends between the Lagrangian points L1 and L2, which lie along the line of centers of the two bodies. The region of influence of the second body is shortest in that direction, and so it acts as the limiting factor for the size of the Hill sphere. Beyond that distance, a third object in orbit around the second (e.g. a satellite of Jupiter) would spend at least part of its orbit outside the Hill sphere, and would be progressively perturbed by the tidal forces of the central body (e.g. the Sun), eventually ending up orbiting the latter.
It isn't very difficult to see why
You are the way you are
Doesn't take a genius to realise
That sometimes life is hard
It's gonna take time
But you'll just have to wait
You're gonna be fine
But in the meantime
Come over here lady
Let me wipe your tears away
Come a little nearer baby
Coz you'll heal over
Heal over
Heal over someday
And I don't wanna hear you tell yourself
That these feelings are in the past
You know it doesn't mean they're off the shelf
Because pain's built to last
Everybody sails alone
But we can travel side by side
Even if you fail
You know that no one really minds
Come over here lady
Don't hold on but don't let go
I know it's so hard
You've got to try to trust yourself
I know it's so hard, so hard
Come over here lady
Let me wipe your tears away
Come a little nearer baby