Let's not waste any time. I've worked on this definition , for years so here it is:
Equinox: The Spring or Autumn quarter begins as both the North and South poles are momentarily poised on the Earth's 'terminator' the globe's boundary line between night and day. As a consequence, only at these "times of the year are the hours of night and day equal.
That's it. The great mystery' at last revealed! It's not the definition you will find anywhere else; yet is correct and I feel it's the best, for a number of reasons.
Do you understand it though? I've tried to keep it as simple, extremely brief and to the point as possible but yet still focusing on the nature of the actual physical event, the reason, rather than trying to reach a. definition of it by listing the effects it causes. The cause of the equinox is something which I feel is never really addressed in the definitions you find in popular or technical reference works. You can expect to find whenever you look, over and over, definitions which rely on Spherical Astronomy and it's subjective or 'geocentric' view for describing the workings of the world.
"equinox: one of the two opposite points at which the sun crosses the celestial equator, when the days and nights are equal... " - Encyclopedia Britannica World Language edition of Funk & Wagnal's Standard Dictionary of the Finish Language, 1959
"Crosses the celestial equator..."? What we're being told here is that if we were to plot the Sun's apparent position against the background stars on a map of the heavens we would find that on the equinox the sun has progressed to the point: where it is crossing the equator of that map, only then standing precisely half way between the two celestial poles. This is all the conventional explanation has to tell us: the position of the Sun relative to the background starts, if we could see them; but, you know what?.. we can't.
Should one really need a working knowledge of the elements of Spherical Astronomy to unlock a simple 'definition' in the dictionary? It seems to me that the choice has always been made for us here and as such, it reflects the arrogance of the academic mind which insists that we must 'learn it the hard way' (i.e., as they did; often they don't understand it well enough themselves to explain it simply, to others...). Although geocentric Spherical Astronomy painstakingly interprets the sky and its motions as seen from an observer's viewpoint on the ground, this seemingly 'persona!' mode of modeling the world was the very culprit behinds Western Culture's hundreds of years of resistance to the simple truths of the solar system which Copernicus finally forced upon us. Although it may be argued that, philosophically, we are at the center of the universe, geometrically we are not. In this neck of the great cosmic woods, there's no doubt whatsoever that the Sun is at the center. Whether it's in Nature or in our personal lives, the subjective viewpoint usually limits our ability to interpret the world accurately
The Wondrous Angle
Everything 'seasonal' relating to the year: the seasons themselves, the solstices and equinoxes, varying length of the daylight hours, the varying times of sunrise and sunset, the elevation of the Sun at noon, and... other matters, perhaps even the initial development of Life on Earth leading to our very existence... are inextricably linked to one magical and, we shall consider here, sacred thing...
the Angle: 23.43493° (hereafter '23.5°)
Chances are you may already know that 23.50 is the amount of tilt the Earth's axis maintains as we make our yearly sweep around the Sun. Most folks who remember that much still don't understand the next specifically important fact about the tilt, an extra fact which would lead to a more correct and revealing picture in one's mind... Try adding this to your notion of the Earth as always being slightly 'cock-eyed' as it swings around the Sun:
At any given moment, the Earth's axis will always be found to be parallel to any of its previous positions.
A minor deviation in the above rule of thumb shows the axis completes only one whole 'wobble' out of parallelness over a period of approximately 26,000 years!