What Is The Autumn Equinox? Sun Passes Directly Over Earth's Equator Marking The Official Start Of Autumn In The Northern Hemisphere

An Autumn days view

Today is officially the start of autumn and the beginning of longer nights and shorter days that will lead us into winter.

Meteorologists use a special biannual event called the equinox to mark the change in seasons. It occurs when the sun passes directly over the Earth's equator and creates a day and night that are the same length.

The equinox happens on or around March 22 and on or around September 22 each year. In the southern hemisphere, the seasons are flipped and today's equinox marks the start of longer days and shorter nights.

Why does the equinox happen?

The Autumn Equinox is marked by a yearly celestial alignment

The equinox occurs because of the tilt of the Earth in relation to the sun. This is what causes the seasons.

The Earth's tilt is 23.5 degrees relative to the plane of its orbit and means that, although one revolution of the planet takes 24 hours - it's different depending on the time of year.

During the summer time, the Northern hemisphere is tilted towards the sun. So we get longer days as more light falls on this part of the surface.

In the winter time it's the Southern hemisphere that gets the majority of the light.

"Equal night"

On the autumnal equinox, the Earth hits the turning point in its orbit where neither the North nor the South poles are tilted towards the sun.

It means the amount of daylight and night time is the same at all points on the Earth's surface.

The word equinox is Latin for "equal night".