In astronomy and astrology, an equinox is defined as the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator.
The word "equinox" comes from the Latin for equal night. The equinoxes occurring with the sun at 0° Aries and Libra (late March and September, respectively, on the common calendar) are the two occasions each year when the day and the night are of equal duration. For measuring the length of a day, sunrise is the moment when the sun is half-above the horizon and sunset is the moment when the sun is half-under the horizon. Using this definition, the length of the day (and the night) is precisely 12 hours at an equinox.
In the northern hemisphere, the Aries equinox is known as the vernal (spring) equinox and the Libra equinox is the autumnal (autumn) equinox. In the southern hemisphere, these names may be transposed. For simplicity, this article will refer to the equinoxes by the zodiacal names used above.
On the common calendar, the vernal equinox typically falls on either March 20 or 21 and the autumnal equinox on September 22 or 23 – the Gregorian dates vary because some years are leap years, shifting the calendar by a day or so relative to the seasons. The Thelemic Calendar, for obvious reasons, does not have the problem of varying dates for the equinoxes. Because the Earth's orbit is elliptical, the dates on which the equinoxes fall do not divide the year into equal halves.
The equinoxes can also be interpreted as virtual points in the sky. Although, during full daylight, stars other than the sun are overwhelmed by sunlight, making it hard to see where the sun is compared to other celestial bodies, the sun does have a position (as seen from the earth) relative to the other stars. As the Earth moves around the sun, the apparent position of the sun relative to the other stars moves in a full circle over the period of a year. This circle is called the ecliptic, and is also the plane of the Earth's orbit projected against the whole sky. The other bright planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.
Another virtual circle in the sky is the celestial equator, or the projection of the plane of the Earth's equator against the whole sky. Because the Earth's axis of rotation is tilted relative to the plane of Earth's orbit around the sun, the celestial equator is inclined to the ecliptic. Twice a year, the sun, making its progress around the ecliptic, crosses the plane of the Earth's equator. These two points are the equinoxes. The time at which the sun passes through each equinox point can be calculated precisely—so the equinox is actually a particular moment, rather than a whole day.
Books You Might Enjoy:
Johannes Kepler - Concerning The More Certain Fundamentals Of Astrology
Aleister Crowley - Rodin In Rime
Aleister Crowley - The Equinox Vol I No Viii
Sepharial - Astrology And Marriage
Aleister Crowley - The Equinox Vol I No X