September Equinox There are two equinoxes every year - in September and March - when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the equator, so the equinox in September is also known as the "autumnal (fall) equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "spring (vernal) equinox". Why is it called equinox? On the equinox, night and day are nearly exactly the same length - 12 hours - all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night". [smg id=4569] The Earth's position during the September equinox (ill. not to scale). However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight The September equinox occurs the moment the sun crosses the celestial equator - the imaginary line in the sky above the Earth's equator - from north to south. This happens either on September 22, 23, or 24 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis tilts neither away from nor towards the Sun, like the illustration shows.