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Weather Whys – Autumnal Equinox

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ABILENE, Texas (KTAB/KRBC) – Farewell flip-flops and hello to pumpkin spice – yesterday marked the beginning fall!

While meteorological fall began on the first of this month, astronomical fall officially began Wednesday, September 22 at 2:21pm CDT. You may be wondering, “Why exactly at 2:21pm? That seems oddly specific.” And you would be right, but I promise there is a reason, and it all has to do with the sun.  

Fall “officially” begins on the autumnal equinox. The equinox occurs when the sun sits directly above the equator and Earth’s axis is parallel to the sun, and according to NASA that happened for the Big Country at 2:21pm. The word “equinox” comes from a Latin word meaning “equal” and nox, meaning “night”.  People living very close to the equator have roughly a 12-hour day and night split year-round, so they won’t notice much difference. However, people who live close to one of the poles go through extreme swings in day and night ratios year-round. So, on the days that fall on the equinoxes, they get to enjoy that 12/12 split with all of the world. Except there is one catch… it isn’t perfectly equal worldwide.  

You actually can get a little more daylight than darkness on the equinox, it all depends on where you are on the planet and how sunlight is measured. The first item that complicates the equation is that the sun is a sphere and not a point. Most almanacs regard sunrise as when the leading edge of the sun first touches the eastern horizon, and sunset as when the sun’s trailing edge touches the western horizon. This way of measuring sunlight alone provides and extra 2.5 to 3 minutes of daylight at mid-latitudes.  

Another aspect to keep in mind is atmospheric refraction. The Earth’s atmosphere acts like a lens or a prism, uplifting the sun about half a degree from its true geometrical position. In simpler terms – when your eye sees the sun on the horizon, it’s actually just below the horizon geometrically. This refraction done by the atmosphere advances the sunrise and delays the sunset, adding about 6 more minutes of daylight at mid-latitudes. And to add even more variation, atmospheric refraction can vary depending on atmospheric conditions such as temperature, humidity, and barometric pressure. All of this adds up to a non-equal split of day and night at the equinox. 

Equinoxes occur twice each year, not necessarily on the same day, rather they happen around March 20 and September 22. The shifting of these dates is due to Earth’s year not being exactly 365 days. There is an extra quarter of a day that we accumulate each year, this is why we get an extra day at the end of February every four years. This extra day, as well as the orientation of Earth towards the sun, causes the date of the equinoxes to shift. The planet isn’t following a track on its orbit throughout the year, there are shifts that occur, tweaking the timing of the equinox as well. 

As mentioned, September brings the beginning of fall in the Northern Hemisphere. While at the same time, the Southern Hemisphere shifts into spring. The opposite is true in March, when the northern half of the planet enters spring and the southern half descends into the cooler months of autumn. Equinoxes are not unique to Earth. Every planet in the solar system has a time when the planet’s orbit and tilt with respect to the sun result in both hemispheres receiving roughly equal amounts of sunlight and solar radiation.

Equinoxes are not a “new” occurrence either. People have been tracking the sun’s movement for thousands of years. Most of the time, they pair the equinoxes with cultural and religious celebrations and traditions. They used these solar changes to know when to plant or harvest crops, as they marked the beginning of seasons.

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