The equinoxes (from the Latin aequinoctium (aequus nocte), “equal night”) are the times of the year in which the Sun is located in the plane of the celestial equator. On that day and for an observer on the Earth’s equator, the Sun reaches the zenith (the highest point in the sky relative to the observer, which is just above his head, that is, at 90 °). The parallel declination of the Sun and the celestial equator then coincide. It occurs twice a year: on 20 or 21 March and 22 or 23 September of each year,
On the dates of the equinoxes, the day lasts approximately the same as the night in all places on Earth. Although the word equinox refers to equality, this is not so because of the size of the sun (relative to its central point), and atmospheric refraction, which cause differences in the length of the day in different latitudes. In the equinox happens the change of annual station opposite in every hemisphere of the Earth. As an astronomical reference, equinox is synonymous with the first point of Aries (also: Aries point): point of the celestial sphere of right ascension, and null declination. It is the point where the Sun in its apparent annual movement through the ecliptic passes from south to north of the celestial equator, and its decline changes from negative to positive. Also called this point or node Vernal Equinox.
For example, sidereal time is measured from the local meridian to the equinox of March in the retrograde direction, and the straight ascension of a body in the celestial sphere is taken from the Aries point to the hourly circle of the object, in the direct sense. Now, the equinox is not a fixed point (neither equinox, of course), but is progressively moved by precession and nutation. The first assumes an angular displacement of about 50.3 “each year. Truee equinox is the intersection of the ecliptic with the true equator which is moved by precession and nutation.
Middle equinox or middle equinox date. The nutation is dispensed with. The equinox moves uniformly due only to precession. The equinoxes occur when the Sun is at the first point of Aries or at the first point of Libra. The first is the point of the celestial equator where the Sun in its annual apparent movement through the ecliptic passes from south to north with respect to the equatorial plane, and its decline goes from negative to positive. In the first point of Librasucede the opposite: the Sun appears to pass from north to south of the celestial equator, and its decline goes from positive to negative.
Actually none of the equinoxes are in the constellation that names them, due to precession: the first point of Aries is in Pisces, and the first point of Libra is in Virgo. The equatorial coordinates of each equinox are: a) for the vernal equinox, right ascension and zero declination; b) for the first point of Libra, straight ascension, 12 hours, and null decline.
Specifically the equinoctial point that we see located towards the constellation of Pisces is to 8 degrees of the border with the one of Aquarius.