You are talking of the concept of [center of mass][1], which is widely used in astronomical and general kinematics problems.

In physics, the center of mass, of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero. The distribution of mass is balanced around the center of mass and the average of the weighted position coordinates of the distributed mass defines its coordinates. Calculations in mechanics are often simplified when formulated with respect to the center of mass.

How good an approximation this is in case of the solar system depends on the distance between the solar system and the nearest heavy stellar mass. Consider that [the force][2] goes as

![gravitation][3]

once the distance is large enough the internal structure is irrelevant .It is only relevant for close encounters .

The center of mass of the solar system is not in the center of the sun, by the way; this also is irrelevant for very large distances. [1]: http://en.wikipedia.org/wiki/Center_of_mass [2]: http://en.wikipedia.org/wiki/Gravitational_force [3]: https://i.sstatic.net/rgBhE.png

You are talking of the concept of center of mass, which is widely used in astronomical and general kinematics problems.

In physics, the center of mass, of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero. The distribution of mass is balanced around the center of mass and the average of the weighted position coordinates of the distributed mass defines its coordinates. Calculations in mechanics are often simplified when formulated with respect to the center of mass.

How good an approximation this is in case of the solar system depends on the distance between the solar system and the nearest heavy stellar mass. Consider that the force goes as

once the distance is large enough the internal structure is irrelevant .It is only relevant for close encounters .

The center of mass of the solar system is not in the center of the sun, by the way; this also is irrelevant for very large distances.