This year it will happen on Thursday September 22, at 14:21 Universal (Greenwich) time.
At this instant, Earth will have its north pole tilted maximally forward along its direction of travel. That is the very definition of the autumn equinox. It’s really a more fundamental definition than the usual one, that this is the time when night (nox) and day are equal.
Yet it takes a few seconds to remember that this is what is happening, because it feels a little paradoxical – sounds more like spring than autumn. Our part of our planet is leading the way – which sounds like a breakthrough into more light, but is the opposite.
It makes sense when you think about it for a moment more. The north pole is leaning forward in anticipation of the time when it will be leaning most away. Earth is entering the half of its orbit where its northern hemisphere is tilted away, and that is the wintry half. Picture someone stooping forward to enter a darker room. Or picture the planet as a spinning skater, always leaning at 23 degrees in relation to the ice. Now she’s sideways to the Sun; in December she’ll be head-away-from, feet-toward, the Sun.
At this moment of balance between the summer and winter halves of the year, the Sun enters, appropriately, the constellation of the “Scales,” Libra. Or did, two thousand years ago; so it is still regarded as doing so, astrologically. Because of the gradual change called precession, spinning Earth is now oriented a different way, and the Sun appears back among the stars of Virgo.
Ecliptic-based chart of the Sun’s motion in September, from Astronomical Calendar 2014. The Sun is shown for each date at its true size, or twice size at the 1st, 11th etc. of the month. On Sep. 22 it crosses the celestial equator, and the 180° line of longitude, anciently the beginning of the sign Libra; it won’t reach the starry region of Libra till Oct. 20.