Vernal Equinox (Spring Is Here!)

_{***Warning- The following blog entry contains a distinct Northern Hemisphere bias. If you are from the Southern Hemisphere...I'm sorry***}

"O, wind, if winter comes, can spring be far behind?" - Percy Bysshe Shelley

It's been a long winter for most of us and there were times when it felt like spring may never come. With spring right around the corner, I thought I'd science it up a little with some interesting facts about the Vernal Equinox.

The Sun's Path Through the Sky

Over the course of a year, the Sun takes a slightly different path across the sky every day. Ancient astronomers noticed that these daily changes in the Sun's arc across the sky were periodic. The arc of the Sun moved higher and higher every day, producing longer and longer days, till a certain date. Then the arc of the Sun's path in the sky gradually moved lower and lower, the days shortening, till a certain date. Then the pattern repeated. The varying length of days due to this phenomenon clearly were responsible for the seasons and thus were of supreme importance for agrarian societies. As a result, the days when the Sun took its highest arc and lowest arc, as well as the days when the Sun's arc was halfway in between these extremes, were noted as important and named.

We know now that this peculiar periodic behavior of the Sun is a result of the Earth's axial tilt (23.4º) as the Earth moves around the Sun. As the Earth orbits the Sun, the axial tilt points toward the Sun, perpendicular to the Sun, away from the Sun, and perpendicular to the Sun again. These orbital configurations correspond to the highest arc, halfway, lowest arc, and halfway respectively. Today we use the terms Summer Solstice, Autumnal Equinox, Winter Solstice, and Vernal Equinox to denote when these events take place.

The term Solstice comes from the Latin solstitium meaning "sun stands still", which makes sense since it corresponds to the dates when the arc of the Sun in the sky stops heading North (or South) and reverses course. As this reversal is happening, the Sun's arc seems to stay where it is for a couple of days (thus the name). The term Equinox comes from the Latin aequus (equal) and nox (night). Ver is the Latin word for spring and Autumnus is the Latin word for Autumn.This name makes sense as well, since the halfway point between the extremes of the Sun's arc correspond to days when the night and day are (nearly) equal in length for all latitudes.

Vernal Equinox

So here we are, approaching the Vernal Equinox (March 20th) after a brutal winter. The days have gotten longer and longer since the Winter Solstice on December 21st. You may be wondering why then the coldest days tend to be in January in the Northern Hemisphere when the shortest day is in December. We have the moderating effects of the oceans to thank for that. 70% of the Earth is covered in water and water is harder than dirt and rocks to heat up and cool down. The stored heat of the ocean delays the effects of the shorter days, thus the hysteresis.

So after the Vernal Equinox we will start having longer days then nights and temperatures will continue to rise. The snow will melt and the migrating birds will return. Plants will start budding, flowers blooming, and Spring will take hold in the North (I can only hope).

A Few More Details

As I described the solstices and equinoxes above, you might be forgiven for thinking that they were equally separated in time over the course of a year. So each season would be 365/4=91.25 days. Certainly from the orbital picture this appears to makes sense. However, here are the actually lengths of each season:

Winter: 88 days (Winter Solstice-December 21)
Spring: 92 days (Vernal Equinox-March 20)
Summer: 93 days (Summer Solstice-June 21)
Autumn: 89 days (Autumnal Equinox-September 21)

So what's going on? The reason some seasons are longer than others is because the Earth's orbit around the Sun is not perfectly circular, but rather slightly elliptical. A feature of elliptical orbits is the orbital speed will vary. In the Earth's case, at aphelion the Earth is orbiting the Sun more slowly than at perihelion. Since aphelion occurs during the Summer in the northern hemisphere, that season is stretched out by a couple of days (Thanks Kepler! (law two)).

It's also understandable if you are under the impression that the the Vernal Equinox always has fallen in March. That too is not true. In fact, around 4000 BC it fell sometime in June. This is due to something known as the precession of the equinoxes. Remember when I said that the four important points in the orbits corresponding to the equinoxes and solstices were when the Earth's axis was pointed towards, perpendicular to, away from, and perpendicular to the sun? Well the problem is that the direction the Earth's axis points changes over time. Thus the locations in the Earth's orbit (which corresponds to the month) when the axis is pointing towards, away, or is perpendicular are different as the direction the axis points changes. Think of it this way. If you've ever seen a spinning top, you know the handle that you use to spin will trace little circles as the top is spinning. The axis of the top is rotating (precessing). The Earth does the same thing, except it takes about 26,000 years for the Earth's axis to trace out one circle.

So 13,000 years ago we'd be approaching the Autumnal Equinox, not the Vernal Equinox. Every year the equinoxes and solstices shift roughly 10 minutes, so whereas this year the Vernal Equinox will occur March 20th at 4:57 pm, in 2018 it will occur on March 20th at 4:15 pm. You may have heard the term "Age of Aquarius". This term comes from the fact that as the Earth precesses, the constellation from which the Sun appears to rise on the Vernal Equinox changes. Since there are 12 zodiac constellations, and precession lasts about 26,000 years for one cycle, each sign is said to represent a 2150 year long "age".

Conclusions

So there you have it. The Vernal Equinox will come and go and Spring officially will begin! I know there are articles about the Equinoxes everywhere this time of year. I hope I provided a some extra tidbits to make this one worth reading.

Also, my Alma mater, UAlbany Great Danes, will get destroyed by play the Florida Gators tomorrow in the first round of the NCAA Basketball Tournament, so I just wanted to say "Go Purple!".