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Posted by David Williams on May 28, 2006, 9:28 pm
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-> For the Earth-Moon system, that barycenter is about 3000 miles from the
-> center of the Earth. But if you place a satellite (say a shuttle
-> mission) in LEO, it doesn't orbit the barycenter but just the Earth's
-> center. More precisely it's orbiting a point just off the Earth's
-> center towards the barycenter. Else the shuttle 200 miles altitude
-> orbit would have an orbital path that (assuming diameter of the Earth is
-> 8000 miles here) that would peak 3200 miles altitude above the Earth's
-> surface and bottom out 800 miles below the Earth's surface. Obviously
-> that doesn't happen.
What do we mean by "orbiting"?
When the shuttle is directly between the earth and the moon, it is
pulled by the moon's gravity so that the radius of curvature of its
orbit is increased, compared with the way it would be if the moon were
not present. So the centre of curvature of the shuttle's orbit is a bit
*beyond* the earth's centre, i.e. on the opposite side from the moon.
When the shuttle's position is such that the line joining it to the
barycentre is perpendicular to the earth-moon line, the centre of
curvature of the orbit is the direction of the barycentre, but is a
bit *closer* to the shuttle.
dow
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