What Is #26: The Autumnal Equinox

The 'What Is' Series

Early this year I started a new series on this blog titled "What Is". Thus far the focus is on the field of astronomy where we hear all sorts of cool science taking place in space. The goal of this series is to explain the details of different objects we hear about relatively frequently.

What is the Autumnal Equinox?

Now that I've covered the summer solstice, winter solstice, and vernal equinox, it's time to cover the autumnal equinox. The word 'equinox' indicates something is equal. That something is the spread of sunlight across the northern and southern hemisphere. On the autumnal equinox, also called the first day of fall in the northern hemisphere, the Sun sits directly above the equator, providing an equal amount of light to both hemispheres. One day before the autumnal equinox the Sun sits just north of the equator and the northern hemisphere receives more light. One day after the autumnal equinox and the Sun has shifted to sit above a spot just south of the equator, giving the southern hemisphere more light. The vernal equinox and the autumnal equinox are the only two days of the year each hemisphere receives the same amount of light, thus the term 'equinox'.

What causes this? The reason we have the summer/winter solstice, and the spring/fall equinox, is due to Earth's tilt of about 23.5 degrees. Because Earth is titled with an axis always pointing in the same direction, the angle the Sun's light hits the Earth changes.

The next autumnal equinox occurs on September 23, 2019. Although the autumnal equinox is always near September 21, it can shift a day or two depending on the exact time it takes place. On the autumnal equinox, the Sun is working its way lower in the sky each day in the northern hemisphere until it reaches its lowest point on the winter solstice in December.

Earlier in this series on the summer/winter solstice, I noted that these days are not typically the hottest and coldest days of the year. Likewise, the vernal equinox and autumnal equinox are not typically equal in temperature. The vernal equinox is coming off winter and is typically colder than the autumnal equinox that is coming off summer. 

Now all of you readers are experts when it comes to the solstices and equinoxes. Go out and impress your friends at the next party! :-)