20 March 2015
The Vernal Equinox (for 2015) occurs at 2245 GMT Friday, and there’s a good chance that just about everything else you were taught about it is wrong. Don’t say it’s the first day of spring, because that’s true only in a traditional sense, and most certainly not a scientific one, and if you live in the Southern Hemisphere it’s wrong on both accounts! The quarter of the year between the coldest quarter and the hottest (in the northern hemisphere) actually begins in early March (Meteorological Spring) in most regions. When it comes to keeping weather and climate records, the period from March 1 to May 31 is considered ‘spring’.
What about the fact that the days and nights are equal over the world! Sorry, but no. This is a very common misconception, and if you look at the sunrise and sunset for today in your location, you’ll see that I’m right. The days are already longer than the nights in the Northern Hemisphere. The reason for this is that we see the sun after it has set. You can thank the atmosphere for that, because it bends the light rays over the horizon and adds about 6 minutes to the day. If you live on or very near the equator, the day is ALWAYS longer than the night because of this.
There are some additional reasons the days and nights are not equal, and one of them has to do with our definition of sunrise and sunset. The top part of the sun at sunrise, is nearer the bottom at sunset (think about that for a second) and the width of the sun’s disk (as viewed from Earth) takes a bit of time to go over the horizon. There are even more reasons, but they get very complicated and involve geometry, trigonometry, orbital alignments, and well… you get the idea. If you want to dive deeper the U.S. Naval Observatory website has a ton of info about this subject and related topics here.
You can see the change of seasons when looking at the GOES satellite imagery movie with one image for every day of the year, and NASA Goddard Space Flight Center did just that for you below:
I should mention that while the sun’s rays will be coming nearly straight down (at midday) on the equator today, the equinox is defined as when the sun passes a certain point in the sky (the First Point of Aries) with respect to distant background stars. For most folks on the equator, you will not likely measure the sun reaching the exact zenith at Noon. Close enough for government work though, and twilight will be very short at low latitudes because the sun will be going down at nearly a 90 degree angle. The twilight is always very short in the tropics for this reason actually, and one of my better memories is watching the sky slowly darken over 4 hours one summer evening just south of the arctic circle in Greenland.
There is one last thing I should mention, that being the story about balancing an egg on its end at the time of the equinox, and that’s something you know is not true right?
It’s True! You can, if you’re careful and nimble, stand an egg on its end on the day of the equinox, and furthermore you can do it any other day of the year as well. So, nothing left to say except welcome to Autumn (for all my friends south of the equator)!