Fun Fact 4/4/17: Equinoxes and Equiluxes

Fun Fact for April 4th, 2017:

Most people say that the equinox is the day where day and night are of equal length, but there are two problems with this statement. First of all, the equinox isn’t a day, but rather a single instant in time. What people call the vernal equinox is the day on which the actual equinox lies. The true definition of the equinox is the moment in time when the sun is directly overhead the equator, a moment which happens only twice a year due to earth’s tilted axis of rotation. The other problem is that the day of the equinox doesn’t have a day and night of equal length, and it isn’t even the closest day of the year of having equal day and night. The reason is that day officially begins once the first bit of the sun peeks over the horizon, and doesn’t officially end until the sun completely dips below the horizon. Since the sun isn’t a single point in the sky, the time between the first bit of the sun peeking over the horizon and the sun being halfway above the horizon and the time from the sun halfway through setting to fully under the horizon adds time to an otherwise 12-hour day, causing the imbalance that we observe on equinox days. The real day with equal day and night is known as the equilux, and they happen a few days before the vernal equinox and a few days after the autumnal equinox. The exact date of the equilux varies depending on your geography and position along the earth, but regardless, it’s interesting to think about how so many simple things in our life may have been misnamed or described incorrectly.