What Is the Autumnal Equinox?

What Is The Autumnal Equinox?

By Tim Lewis | Expert Astronomer at Cal-a-Vie


Fall is literally on our doorstep. I love the fall for its crispness with the arrival of cooler weather, the winter sky that beholds the photogenic Milky Way, longer nights that provide a sense of relaxation as I wind down and the sense of change for my personal life.


This year, we will be graced by Fall on September 22, 3:20 PM EDT in the Northern Hemisphere-- the astronomical beginning of the fall season also known as the autumnal equinox. The Autumnal Equinox is the date and time at which a point on the Earth's equator is the closest point on Earth to the Sun (or, put another way, it is the date and time when the Sun is directly over the Equator).  As a result, it is when day and night are roughly equal in length; hence, the term Equinox, from the Latin aequus, meaning “equal,” and nox, “night.”  After September 22nd, our nights will get longer, and days will get shorter until December 21st which is Winter Solstice (more on that later).


Of course, since the onset of organized agriculture, Fall has really heralded the “harvest” which made it’s arrival a very important piece of knowledge. In order to do this well, humans have built astronomical observatories to correctly determine the various seasonal changes. Stonehenge, Majorville, Brodgar, El Karnak, Easter Island, Angkor Wat, and Machu Picchu are just a few of such ancient structures that have been used by our ancestors to determine the main astronomical cycles Earth goes through as it circles the Sun. Speaking of Observatories, if you haven’t been to our Observatory for Star Gazing here at Cal-a-Vie, it’s a must at your next visit.


For you astronomy lovers out there, you may be asking -- why do we have these equinoxes and solstices? It has to do with the Earth’s tilt on its axis relative to its orbital plane around the Sun. All of the planets have some degree of tilt, and in Earth’s case, that tilt is roughly 23.5 degrees. As the Earth circles the Sun, the relation of that tilt relative to the location of the Sun causes the position of the Sun in our sky to appear to oscillate; moving toward the North in the summer, and toward the South in the winter. As the Sun’s position changes in our sky, this changes the length of day and night on any given day. The Solstices occur on the days when the axis tilt is aligned with the Sun, and the Equinoxes occur on the days when the axis tilt is aligned with the direction of the Earth’s travel. 


So, get ready for this lovely Fall; a time of vibrant color, the smell of leaves, a freshness about the air and embrace the change, improvement, and commitment to a new season for you.


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