**Author’s note: Everything written here is true for those who live in the Northern Hemisphere. While some things written here are true for anyone living in the Southern Hemisphere (AKA the land where Christmas happens during summer) the dates will be off by 6 months.
Fall is upon us! The summer heat is slowly dissipating (thank goodness!), harvests are about to be in full swing, Halloween, falling leaves, and everything that comes with the changing of the season. Tomorrow is often regarded as the official beginning to the fall season, but do you know why? Tomorrow marks the autumnal equinox. So what is an equinox?
You may have noticed that ever since the first day of summer (usually June 21), the days have slowly gotten shorter and shorter. In fact, we’ve every day since June 21 has had about 2 minutes less sunshine than the day before it. Prior to June 21, we were actually gaining the amount of time in the sun by about the same amount each day. This makes June 21 our summer solstice. It is the longest day on Earth for those living in the Northern Hemisphere. December 21 is our winter solstice: our shortest day of the year although these dates may shift a few days depending on the year due to leap years.
On either side of these two days, right in the middle, sit our vernal and autumnal equinoxes: usually March 21 and September 22. It is on these days that everywhere in the world experiences approximately the same amount of daylight hours as nighttime hours. The word equinox actually comes from the Latin word aequinoctium which itself is derived from the words aequus (equal) and noctis (night). Vernal is the Latin word for spring while autumnal is the Latin word for autumn.
In short during the winter and spring season, every day has a little bit more sunlight than the day before, and in summer and fall every day has a little less. Still following me? So why does this happen?
Well to answer that question, you have to know how our planet orbits the sun. You hopefully know that the Earth is constantly spinning and constantly revolving around the sun. But the Earth is actually tilted on its axis about 23.5 degrees. Because of this, different parts of the planet are facing the sun for different amounts of time throughout the year. This is what causes seasons.
The celestial equator is the imaginary line that dissects the Earth into the Northern and Southern Hemispheres and sits above the equator. For two days a year, the sun follows the path of this line as the Earth orbits and spins around it. These days are the equinoxes. Since the Earth’s axis is perfectly perpendicular to the sun on these days, all of the Earth receives the same amount of sunlight.
The autumnal equinox has long been noticed and celebrated by humans. It is especially important as it marks the beginning of the harvest season. Some historians have even speculated that ancient monuments such as Stonehenge were built specifically to align with the sun during the equinoxes. Early calendars were built specifically to account for the equinox and solstices and observations of the lengths of days were the building blocks early scientists used to help us understand the Earth’s place and movements in our solar system.
The days will only get shorter from here until we reach the winter solstice in December, so enjoy the sunlight while you can because after tomorrow, it will more often be night that not!