Periapsis
The smallest radial distance of an Ellipse as measured from a Focus. Taking 
in the equation of anEllipse
gives the periapsis distance
Periapsis for an orbit
around the Earth 
is calledperigee, and periapsis for an orbit around the Sun iscalled perihelion. See also Apoapsis, Eccentricity, Ellipse, Focus- Crucial Point
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