What is the September Equinox?

On Wednesday, September 23, 2015, the day and night will be of an almost equal length, everywhere on Earth. This is the fall Equinox, marking the end of summer and the beginning of fall, in the northern hemisphere. In the southern hemisphere, this September event marks the beginning of Spring and so is called the Vernal, or Spring, Equinox.

The Equinox is actually a specific moment, not an all day event. It occurs at the exact time the Sun crosses the celestial equator; the imaginary line in the sky above the Earth’s Equator. At this instant, the Earth's rotational axis is neither tilted away from nor towards the Sun.

This year the moment of Equinox is 08:22 UTC. This means that it actually happened on September 22nd in places that at at least nine hours behind UTC, including parts of Alaska.