The Earth is tilted by 23.5°, and at 21:18 GMT on September the 22nd, this tilt will be neither towards or away from the Sun. The north pole of the Earth is tilted in the same direction - towards the star Polaris - throughout the year (although this will change slightly over thousands of years). But bfore 21:18 GMT on September the 22nd, the north pole of the Earth was pointed slightly towards the Sun (which is why we had summer in the northern hemisphere), and after that moment, the north pole of the Earth will be tilted slightly away from the Sun, which is why we have autumn beginning in the northern hemisphere.
This means that on this day we have - to within minutes - an even length of day and night. The name "equinox" originates from Latin words for equal (aequus) and night (nox).
The date of the equinox varies each year because the Earth orbits the Sun in 365.2422 days, and not 365 days exactly! This is also why we have a leap-day every four years, allowing our calendars to catch up with the orbit of the Earth.
More information about the equinoxes can be found on the Wikipedia.
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