Equinox: a misnomer?
The term "equinox" means equal night, which leads many to believe that the first day of fall or the first day of spring—the equinoxes—bring exactly 12 hours of day and night. However, this is only true in some locations.
Earth takes approximately 365 days to orbit around the sun, wobbling on its axis while making this revolution. Because Earth is also tilted on its axis, there are only two days a year when the sun shines down exactly over the equator, and the day/night line – called the terminator – runs straight from north to south. On these days, day and night are approximately equal around the planet.
The exact date when day and night are exactly 12 hours though, depends on the latitude of that specific location. On the equator, day and night are nearly the same lengths all year round. Sunrise is defined as the moment when the upper edge of the sun becomes visible above the horizon and sunset is defined as the moment when the upper edge of the sun disappears below the horizon. Since the time it takes the sun to fully rise and set is subtracted from the night, equinox days are actually a little longer than 12 hours.
In Baton Rouge, September 26, 2017 marks the date that day and night are equal with a sunrise of 6:55am and a sunset of 6:55pm. Days will continue to shorten until the winter solstice on December 21. The next day with exactly 12 hours of day and night will be March 16, 2018—prior to the spring equinox on March 20.