The autumnal equinox or September equinox falls on September 22nd, 2017, on this day the day night will be almost equal in most locations. Here are some facts we all need to know about the equinox and signs of the equinox in nature and the first day of astronomical fall (autumn) in the Northern Hemisphere.
There are two types of equinoxes each year – one falls in September and the other in March. This happens when the Sun shines directly on the equator and the length of day and night are nearly same. However, the autumnal fall equinox in the Northern Hemisphere is known as spring vernal equinox in the Southern Hemisphere and vice versa.
The September equinox is on or around September 22nd. Sometimes it may fall on the 23rd or 24th, while the first equinox of the year falls on March, it takes place on or around March 21st every year. During these two equinoxes the Sun shines directly on the equator and the length of day and night is almost nearly equal – but not exactly.
Did you know that the September equinox marks the second the sun crosses the celestial equator? The slightly imaginary line in the sky above the Earth’s equator is seen from North to South and vice versa in March.
In the above picture, you can see the equinoxes and solstices that mark the start of astronomical seasons. While the equinoxes start in spring and fall, solstices start astronomical summer and winter.
Earth's Axial Tilt
The Earth's axis is slanted at an angle of about 23.4° this is in relation to the ecliptic, which is the imagined plane created by Earth's path around the Sun. However, on any other day of the year, both the Southern Hemisphere and the Northern Hemisphere tilts a little towards the Sun. Then on the two equinoxes, the tilt of Earth's axis will be perpendicular to the Sun's rays, like the above illustrations show.
Why is it Called "Equinox"?
On the day of the equinox, both night and day are nearly closely the same length – 12 hours – all over the world. That is the reason it's called an "equinox," the word is derived from a Latin aequus which means equal and nox meaning night.
However, even if this is usually accepted, it isn't entirely true. In reality, equinoxes don't have accurately 12 hours of daylight.
Why is it called the First Day of Fall?
While in the Northern Hemisphere, the fall equinox marks the first day of fall - autumn is what it is known as in astronomical seasons. There is also another, and more common definition of when the season’s start, specifically meteorological definitions, which are based on typical temperatures rather that astronomical events.