Strictly speaking, at the equinox, the Sun's ecliptic longitude is zero. Its latitude will not be exactly zero, since Earth is not exactly in the plane of the ecliptic. Its declination will not be exactly zero either. The mean ecliptic is defined by the barycenter of Earth and the Moon combined, so the Earth wanders slightly above and below the ecliptic due to the orbital tilt of the Moon. The modern definition of equinox is the instant when the Sun's apparent geocentric longitude is 0° (northward equinox) or 180° (southward equinox). See the adjacent diagram.