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Posted by Paul R. Martin on April 16, 2002 04:41:25 UTC

Hi Scott,

***In an ellipse, geometrically speaking, there are 2 foci - Am I correct?***

Yes, there are 2 foci.

***The Sun is at one "side" of the ellipse, it is one foci in the ellipse - Am I correct?***

Yes, the Sun is located at one of the foci.

***Our orbit around the Sun is not a circle because we do not travel at equal distances away from the Sun (Our perihelion and aphelion points are proof of this) - Am I correct?***

Yes, you are correct.

***So if our orbit is not circular but rather elliptical, then what is the other focus of the ellipse?***

The other focus is simply a point in space with nothing special located there. It shouldn't be a concern that there is nothing there. Think about the orbit itself. As you said, the orbit is an ellipse. What if I asked you, What is the ellipse? (similarly to you asking me, What is the focus?). Well, except for one point on the ellipse which is occupied by the Earth at some moment, there is nothing special along the rest of the ellipse. You wouldn't be able to find the ellipse out there in space, and there is nothing special about the region of space on or near the ellipse. The ellipse is nothing more than a mathematical specification of a set of points which constitute the locus of points that will be occupied by the Earth as it traverses its orbit. Similarly, the second focus (and even the first one for that matter) is simply a mathematical specification of a point in space that happens to have a certain relationship to the set of points on the ellipse. There is nothing special about that second focal point. If you took a spaceship out there and investigated it, you would find nothing special there. Just as the explorers didn't find a barber pole or any other distinguishing feature when they discovered the south pole.

(BTW I may have made an error in my previous answer to you. I said something like you wouldn't find anything special within a million miles of that second focus. Since the earth's orbit is very nearly circular, it might be that the two foci are less than a million miles apart, so I guess the Sun could be within that region. Not important, but I was a little sloppy. Sorry.)

If this doesn't answer your question, Scott, then you will have to tell me what your specific concern is. I must be completely missing what is bothering you about this. Let me know.

Warm regards,


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