Re: Gravity and the Sun

From: Greg Neill (gneillREM_at_OVE.THIS.netcom.ca)
Date: 08/13/04 

Date: Fri, 13 Aug 2004 00:37:40 -0400

"RP" <no_mail_no_spam@yahoo.com> wrote in message news:2o2ebdF65jscU1@uni-berlin.de...
>
> The only non-degenerate conic section in question is the ellipse.

There's nothing wrong with applying a general principle to a
particular case.

> Every ellipse with an eccentricity greater than zero has two defined
> foci. The perpendicular to the tangent to the ellipse (of nonzero
> eccentricity) always lags falls behind the focus being approached, i.e.
> from aphelion to perihelion the perpendicular will always pass between
> the foci. Thus anywhere along this leg of the orbit the Earth will have
> a component of velocity directed toward the Sun.

So? There is also a component away. You can draw an infinite
number of components that will add up to the velocity vector.
The tangent itself gives the direction of the velocity vector.
Teh tangent is always exterior to the orbit while the body is
moving under the influence of a central force.

> Old Man's argument
> applies only to an ellipse of zero eccentricity, i.e. to a circular
> orbit. Note that given an eccentricity of 1, the tangent coincides with
> the line of motion, which is always directed "at" or "directly away
> from" the Sun.

An eccentricity of 1 yields a parabolic orbit. The tangent
then certainly is not always directed at or away from the
Sun. For example, at perihelion the tangent is perpendicular
to the planet-Sun line.

> Please climb a tall building and launch yourself into
> such an orbit, turn off gravity in mid flight and see whether you hit
> the surface or no. ;)

Ha ha.

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