= Argument of periapsis
{title2=$\omega$}
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= Argument of pericentre
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= Argument of pericenter
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The <argument of periapsis> is the angle in the <orbital plane> from the <ascending node> to <periapsis>, measured in the direction of motion. It differs from the <longitude of periapsis> $\varpi=\Omega+\omega$. For an inclined orbit this sum is a conventional longitude, not generally the azimuth of the projected periapsis. The argument is undefined for a circular orbit and the node-based description is singular when the orbit and reference plane coincide.
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