PITTSBURGH (KDKA) — We are almost to Spring!
The Vernal Equinox, which is a fancy way to say “beginning of Spring” occurs Wednesday, March 20th at precisely 5:58pm.
In science class, when you study the word “Equinox”, you usually come up with the definition “equal night”. From there, you can make the scientific assumption that you will have “equal day” as well.
However, if you look at the sunrise and sunset times for Pittsburgh, you will notice March the sunrise and sunset times on March 20th are NOT equal. Sunrise is at 7:24am and sunset is at 7:32pm. That is an 8 minute difference! So, the equinox does not have equal day and equal night, and that is why there is a “thing” called the “Equilux”.
The Equilux occurs when there is equal day. Looking deeper into the data, you will find that the equal day and equal night occur on March 17th. Yep, St. Patrick’s Day. That is the day the sun rises at 7:29am and sets at 7:29pm.
So, why doesn’t that happen on the Equinox? One of the biggest differences is how we measure sunrises and sunsets compared to the equinoxes (beginnings of Spring and Fall) and solstices (Starts of Summer and Winter). When a sunrise time is given, that is the time the top, or first part of the suns “disk”, shows on the horizon.
Sunset times are given when the top of the sun’s disk drops below the horizon.
On the other hand, when the time and date of an equinox or solstice is given, that is when the CENTER of the sun’s disk is passing over the Equator (or to the Tropic of Cancer, the sun’s furthest northern lattitude, or Tropic of Carpricorn, the sun’s most southern latitude).
To show this as a comparison, that would be like having one horse race where the winner is determined by the horse’s nose crossing the finish line first and another that determines the winner by the middle of the horse crossing the finish line. They’re measured differently, so the results will be different. Much like the equinox.
Another factor is that light bends as it passes through the Earth’s atmosphere. That bending is amplified when the light is shining on the horizon. This bending accounts for some of that time difference. Either way, the days are getting longer and our temperatures will be getting warmer, as we put Winter behind us.