In astronomy, the argument of periapsis (ω) is a way of talking about the orbit of a planet, asteroid or comet. It is also known as the argument of perihelion or the argument of perifocus. It is the angle (starting from the center of the orbit) between an orbiting body's periapsis and its ascending node. Periapsis is the point when the orbiting object comes the closest to the thing it is orbiting around; for example, the moon is at periapsis when it is closest to the Earth. The ascending node is one of two places where an orbiting object passes through the reference plane, an imaginary flat surface which runs through the object being orbited around. The size of the angle depends on which way the object is orbiting.