Weather FAQ: What is the vernal equinox and why is spring starting Monday (again)?

Spring will finally be here – by every way of measuring its arrival.

Monday is the vernal equinox, the starting point of spring, as determined by people who base their seasons on the Earth’s position relative to the sun and stars. (Or, more generally, who base their calendar on the sky.)

On Monday, at 5:28 a.m., the sun will be positioned such that it shines directly on the equator. The northern and southern hemispheres will receive exactly the same amount of the sun’s rays. Night and day will be almost equal length. This is an important milestone if you’re into traditional calendars or pagan rituals.

Why is Monday important?

In many parts of the ancient world, four important dates delineated the seasons: the summer solstice (when spring gives way to summer), the autumnal equinox (when summer gives way to fall), the winter solstice (fall to winter) and vernal equinox (winter to spring). These have been important markers on mankind’s journey through time. In the olden days, before our iPhones told us everything, the solstices and equinoxes helped people to know things like when to plant crops or bust out the short pantaloons.

So, um, why are those particular dates important? Why do they separate the seasons?

On just two days a year – the equinoxes – the sun is exactly above the equator. That was a reliable way to mark spring and fall. And for only twice a year –the solstices – the sun hits a maximum high or minimum low point in the noon sky. These were also also deemed a reliable way to mark seasonal transitions.

Why are there only two days when the sun in directly above the equator?

The Earth’s equator is on a tilted axis relative to the sun — the axis is actually tilted 23.5 degrees. That means that as the Earth rotates around the sun, its northern and southern hemispheres trade places in receiving more light from the sun. At the equinoxes, the axis is neither inclined toward nor away from the sun.

Why do they call today the vernal equinox?

No idea. Next question.

No, seriously – there has to be good reason to call it the vernal equinox, right?

You sure about that? #Fakenews! (Googling … Googling …) OK, according to Merriam-Webster, vernal is a derivation of the Latin term for “of spring.”

The term equinox has something to do with Latin, right?

Correct. “Equinox” is derived from the Latin term aequinoctium, which combines equal (aequus) and night (nox).

But day and night won’t be exactly the same length today, will they?

Already covered that. No, day and night will not be exactly the same length. There is another designation, equilux, sometimes used to refer to a day when the amount of light and dark are equal, a day when the Force is in proper balance. (Okay, Disney would never allow the Force to achieve proper balance because that would make for a boring movie and, c’mon, there is no way Jar Jar Binks could be a Sith lord.)

So spring starts with the equinox? But didn’t spring start awhile back?

Yes. Kind of. We’ve had spring-like weather for much of the last few months, what with this winter being the warmest on record in Central Texas and all. So it’s felt like spring almost all winter along. We also had wildflowers blooming – a harbinger of spring – back in February.

But didn’t spring arrive March 1?

That also happened. March 1 was “meteorological spring.” Meteorologists prefer a calendar in which the seasons start on the same days every year. It helps for record keeping, among other reasons. But the Earth, sun and stars do not quite conform to the western calendar — thus the vernal equinox does not fall on the same day every year. The vernal equinox follows celestial trends, at the expense of syncing precisely with the western calendar. That’s why the vernal equinox is often said to usher in “astronomical spring.”

(Side note: commentators mean something different when they say Tony Romo’s salary is “astronomical.”)