## ellipse focus question

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rriverstone
Ensign
Posts: 16
Joined: Sat Sep 18, 2010 10:57 pm
AKA: mud
Location: New Mexico, USA

### ellipse focus question

So, I'm too ignorant to know if this person's idea is misguided, so I'm asking, to find out. The person said that, if the sun is a focus of a planetary ellipse, something else has to be a focus on the other side of the ellipse. Intuitively, this feels wrong to me, probably because I play with yo yos and the like. The person said the phantom or ghost ellipse is dark matter, but i'm thinking it's just not there. SO? :/

neufer
Posts: 18184
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

### Re: ellipse focus question

rriverstone wrote:
The person said that, if the sun is a focus of a planetary ellipse, something else has to be a focus on the other side of the ellipse. Intuitively, this feels wrong to me, probably because I play with yo yos and the like. The person said the phantom or ghost ellipse is dark matter, but i'm thinking it's just not there. SO? :/
The answer is contained within the posted diagram:
There is NOT a symmetry between the two foci:
equal areas are circumscribed in equal times ONLY as regards the sun focus.
Art Neuendorffer

rriverstone
Ensign
Posts: 16
Joined: Sat Sep 18, 2010 10:57 pm
AKA: mud
Location: New Mexico, USA

### Re: ellipse focus question

I thought so. The real focus is the sun. The secondary position in the ellipse, while it may be CALLED a "focus," does not require a body with mass, gravity, etc. for the elliptical orbit to function. Having rolled pennies in those giant funnel thingies in malls, I didn't think a 2nd object was necessary. Water goes down my bath tub drain without another hole at the other end, too.