[ODE] tanget vector

[jnilson_99@yahoo.com]

Hi,

I've been using ode to build a demo that zooms in on
the surface on the earth from around 10 miles in space
down to ground level.

Ode works great, no problems there. However since my
problem is calculus related I thought someone might
know a quick solution.

 I have a problem finding the tanget plane to the
point on the surface of the earth where the user
"touches down". 

Is there a simple way of calculating this plane given
the point (in cartesian or polar coordinates).

My first guess is to find the first derivative of the
sphere equation: X^2 + Y^2 + Z^2 - r = 0. Then plug in
my point to get the plane.

Is this the righ approach?

thanks,

john

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