marks the end of Summer with Fall arriving in Los Angeles on Sept 23, 2015 at 1:22a
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.
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 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". However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight