You know that the orbit of one body around another, such as the
Earth around the Sun, is an ellipse.  In this example, you
know that the Sun is at one focus of the ellipse.  You might
well ask, "Which focus?  An ellipse has two foci -- which
does the Sun occupy?"
I think the problem is an unconscious inference that the location of the ellipse determines the location of the foci, and thus of the Sun.  Of course it is actually the other way around: the locations of the Sun and Earth determine the location of the ellipse and its foci.
This is in contrast to something like a teeter-totter, which has two positions that can be occupied.  The difference is that a teeter-totter already exists before anyone sits on it, while an elliptical orbit exists only because two bodies are there.
To my Space and Science home page
August 22, 2002