Jim,
See if this model helps you understand about "center of mass".
*) Imagine two light bulbs (equal light output, to simulate equal masses), separated by 2 meters. Call one of them A, the other B, and the midpoint between them C (which simulates the center of mass of two equal mass stars orbiting each other). You could call this midpoint a "center of radiation".
*) Now imagine using 3 telescopes to act as sensors. As you move around in the space surrounding the light bulbs, telescope A' is kept parallel to a radial line from light bulb A, telescope B' is kept parallel to a radial line from light bulb B, and scope C' is kept parallel to a radial line from midpoint C.
Arrange the 3 sensors in front of you as you move around relative to the 2 light bulbs and observe the amount of light received by each scope and the spatial orientation of each scope.
(
Some definitions -
1) "Close" means less than 10 times the separation distance between the bulbs
2) "Far" means more than 100 times the separation distance between the bulbs.
3) "Real close" means less than the separation distance between the bulbs.
4) "Real far" means far enough away that the two bulbs appear to be one, except possibly with your most powerful telescope.
)
When you are close to the light bulbs, each of the sensors will generally have visibly different levels of light falling on them.
When you are real close the visual differences betweent the sensors will generally be very large, except for certain special positions. And the orientation of the 3 sensors will generally be different from each other, except for certain special positions.
When you are far away from the light bulbs, each of the sensors will generally receive light levels that are NOT visually different from each other. Sensitive instruments would most likely be able to detect differences, however.
When you are real far away each of the sensors will appear to have the same levels of light falling on them, even if measured with sensitive light measuring devices. The orientation of all three sensors will appear to be the same (they will be parallel to each other) and there will be no special places where light levels or orientation differ.
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Note, in particular, that in the far case and especially in the real far case the light falling on sensor C' is the same as the light falling on sensors A' and B'. Despite the fact that no light comes from midpoint C. Not a single itty-bitty wave.
In the close and real close cases the light falling on sensor C' is frequently less than the light falling on sensors B' and A'. From many locations around the light bulbs sensor C' will detect no light at all while sensors B' and A' are detecting lots of light.
Regards,
LB
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