First Day of Fall 2015 (Northern Hemisphere) - An equinox is an astronomical event in which the plane of Earth's equator passes the center of the Sun. Equinoxes occur twice a year, around 21 March and 23 September.The equinoxes are the only times when the subsolar point (the place on Earth's surface where the center of the Sun is exactly overhead) is on the Equator, and, consequently, the only times when the Sun is at zenith over the Equator. The subsolar point crosses the equator, moving northward at the March equinox and southward at the September equinox. The equinoxes are the only times when the solar terminator is perpendicular to the Equator. As a result, the northern and southern Hemispheres are equally illuminated.
At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and ecliptic longitude = 0°) and the autumnal point (RA = 12h 00m 00s and ecliptic longitude = 180°). However, the axes of an equatorial or ecliptic coordinate system may be defined so as to be aligned with the ecliptic and vernal equinox at a fixed point in time (or aligned with an average); therefore due to the Earth's axial precession and changes in orbital parameters, the Sun position during equinoxes in an equatorial or ecliptic coordinate system may slightly differ from the aforementioned idealized values.
