Periapsis is the point in an orbit at which the orbiting body is closest to the focus of the conic section, typically a star or planet. In polar coordinates, it corresponds to the minimum radial distance from the focus.
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Periapsis is derived from Greek words meaning 'near' and 'high point.'
In elliptical orbits, periapsis is where the orbiting object travels fastest due to Kepler's second law.
For Earth's orbit, this point is specifically called the perihelion.
The formula $r_{min} = \frac{a(1-e)}{1+e}$ can be used to find periapsis distance in polar coordinates, where $a$ is the semi-major axis and $e$ is the eccentricity.
In conic sections such as ellipses, hyperbolas, and parabolas, periapsis refers to different nearest points relative to their respective foci.
Review Questions
What is periapsis in terms of polar coordinates?
How does Kepler's second law relate to periapsis?
What formula would you use to calculate the distance of periapsis for an elliptical orbit?
Related terms
Apogee: The point in an orbit farthest from Earth.
Eccentricity: A parameter that determines the amount by which an orbit deviates from a perfect circle.
Semi-Major Axis: Half of the longest diameter of an ellipse; a key parameter in determining orbital size.