Astronomers, who like to take an extraterrestrial view of things, might describe an equinox as a point where the ecliptic crosses the celestial equator. Earthlings, less accurately, think of it as an instant when the sun, moving northwards or southwards in the course of its annual cycle of the seasons, has reached a point on its apparent journey when it is directly over the equator. Or thirdly, you can think of it as one of the two occasions every year when day and night are of the same length everywhere in the world - as indeed the term equinox or "equal night" suggests.
By two of these definitions an equinox occurs today - the autumnal equinox, at around teatime, if my calculations are correct. But strangely, if you check the times of sunrise and of sunset, you will find that daytime lasts for about 12 hours and 12 minutes, and the night a mere 11 hours 48. The night is about 25 minutes shorter than the day, which seems to ride a coach and four through any assertion that the nox is equi.
It all depends, however, on what you mean by "day" and "night". The concept of the equinox works well when viewed from outer space, but here on the surface of our planet we have two complications to contend with - both concerning the definition of sunrise and sunset.
First, the term "sunrise", by convention, means the time at which the upper limb - or the upper edge - of the solar disc peeps above the horizon. This means that "sunrise" takes place when the centre of the sun is still about a quarter of a degree below the horizon. And the Earth's atmosphere adds a further complication. Differences in the density of the air at various levels in the atmosphere cause the sun's rays to be refracted, or bent, in such a way as to exaggerate the apparent height of the sun; in fact, this phenomenon allows us to see the sun some minutes before its upper limb is geometrically level with the horizon.
This combination of convention and illusion gives us a spuriously early sunrise, and lengthens the "day" by some five or six minutes. Sunset is similarly delayed by a corresponding amount. As a result, that which we call our "day" is artificially extended by about 12 minutes - time which, of course, must be borrowed from the night. Looked at from our distorted vantage point on Earth, it is not until about two days after the autumnal equinox that our "day" shortens enough for the 24 hours to be neatly divided into equal halves.