In this diagram, an orbital plane (yellow) intersects a reference plane (gray). For objects in solar orbit, the reference plane is usually the plane of the ecliptic. The intersection is called the line of nodes, as it connects the center of mass with the ascending and descending nodes. The Vernal Point, (?) is the heliocentric longitude of 0, the angular position of Earth's northern vernal equinox. Heliocentric longitudes count up in a prograde direction (counterclockwise, when viewed from the north side of the ecliptic plane).