The argument of the perihelion is one of the orbital elements describing a body in a Sun-centered orbit (e.g., a planet). It is the angle between the ascending node and the perihelion, measured in the orbital plane. Adding it to the longitude of the ascending node gives the longitude of perihelion of the body in its orbit around the Sun.
An argument of perihelion of 0° means that the body will be at perihelion right when it crosses the ecliptic from South to North. An argument of perihelion of 90° means that the body will be at perihelion at its Northmost distance from the ecliptic.
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