What Is This Vernal Equinox Thing?

There is some confusion regarding the solar event that happens this week.

I thought I might try to clear that up. 

True or False: The vernal equinox is when spring begins.

Not exactly. The beginning of spring depends on where you live, and it is more a cultural phenomenon than a meteorological one. Tradition in Ireland has spring beginning with St Brigid’s Day on 1 February. What we might consider warm, spring-like weather may not happen in Ireland for months yet (if ever), but their season of spring lasts from 1 February to 1 May. Similarly, if you’ve been reading this blog for a few weeks, you know that those peoples who celebrate Chinese New Year say that spring begins with that New Year, the Spring Festival, also in February.

In North America, we say the season of spring begins on the vernal equinox. But that’s just a convention and has little to do with either the season or the equinox. There won’t be spring weather for much of the country for a long while — where spring weather even happens. Back home in New Mexico, we have, um, different seasons. I call them Wind, Hot, Rain, and Cold. Not equally spaced and somewhat interwoven. No spring whatsoever. The current season is Wind with a side of Cold.

Is this planting season? I’m betting if you look out your window, you’ll see this isn’t true. (It was 8°F a couple days ago in my garden — not ideal planting weather.) Planting is not as tied to spring as the popular symbology would have us believe; it’s not at all tied to the equinox. Planting is dependent on what you are growing and where you are growing it. For example, spring doesn’t actually exist in tropical regions where there is little year-round difference in temperature. There the growing season is tied to annual rains more often than not, and the rains usually come after the summer solstice. For farmers in New England, spring begins when the maple sap is running, and it’s grain (maize) planting season when the oak leaves are as big as a squirrel’s ear. Obviously, this doesn’t happen on the same date each year, and it’s rare that planting begins as early as the equinox even for frost-tolerant garden vegetables like peas, radishes and cabbages.

You will be forgiven for thinking that there should be flowers at this time given the popular floral images associated with the equinox. But flowers won’t happen in my garden until later in April. Even where it’s warm enough, flowers won’t be showing their lovely faces just yet. Most plants have internal clocks that count down to an average annual safe date to wake up. For some this is dependent on day length. Trees tend to hold off until there are over twelve hours of sunlight in a day. For other plants it’s critical to avoid freezing or to wake when the rains begin. They do take cues from the current weather, but they mainly just count days until, on average, the conditions they need will be occurring. Now, when I say “count”, that is how humans perceive it. We don’t actually know how a seed or a tree keeps time — only that they do.

So the equinox does not bring flowers nor probably green leaves. What from the popular bag of spring symbols does it bring? For starters, rabbits are indeed busy at this time. They begin mating sometime in late February and are well into the swing of things by March — males trying to get near the females, females boxing away any unwanted advancements, baby bunnies seemingly sprouting from the earth under the hedges. Rabbits and hares are truly mad every March.

Another popular equinox symbol that does, in fact, happen at this time of year is egg laying. Most of us know that chickens lay eggs only when their eyes are stimulated by over 12 hours of daylight. (It might be the other way around: not stimulated by over 12 hours of darkness, but that’s not really important.) So chickens don’t lay many eggs between the autumnal equinox and the vernal equinox without humans shining artificial lights in their eyes. The return of eggs in the coop is a highly welcome thing! I know chicken keepers who post photos of the first egg of the year online (ahem). Hey, it’s a big deal.

Similarly, other birds are also working on creating the next generation around the equinox. They’ve been engaged in the mating dance for several weeks by now. If there is enough food, there are probably nests with eggs in the trees around your home. This is why, if you start feeding birds in the autumn, it’s important to keep feeding them in the early spring. They’re depending on that food; there aren’t other sources yet. When mom starts to lay, she needs quite a bit of energy; so she’ll normally wait until there are insects and buds aplenty. But if she thinks there are full bird feeders nearby, she’ll lay sooner than the rest of the garden has awoken from winter sleep. Starting early means she can get in more than one breeding cycle in the warm months, so there is selective pressure to do that if possible. And your feeder is what makes it possible.

True or False: The vernal equinox is the day when day and night are equal length.

True, right? Nope. Equinox does in fact mean “equal night”, but this is not that day. There is no one day that day and night are equal except at the equator — where day and night are always equally twelve hours in length. From this, you can probably guess that latitude affects day length. Actually, you know this already. You know it’s “day” for weeks at the poles at the summer solstices. Well, similarly, the day when day and night are equal length varies with latitude.

Let’s look at the Northern Hemisphere since I’m talking about the vernal equinox (which is six months away for the Southerners). The sun’s apparent path through the sky moves north between the winter solstice and the vernal equinox. At the equator, as I said, day length is always twelve hours. As the sun moves north (or more properly as the northern part of the planet becomes more tilted toward the sun as it travels around the sun on its tilted rotation axis) northern day length increases along the sun’s apparent path. So Florida will see equal day and night before Vermont — two days sooner as a matter of fact. 

At New England latitudes, day and night are closest to equal (12 hours each) on St. Patrick’s Day — 3-4 days before the equinox. By this year’s equinox on 20 March (at 5:37am), day will be nearly ten minutes longer than night.

True or False: The vernal equinox is when the sun rises and sets directly due east and west.

Again, false. This too is dependent on latitude, but much less so. At the equator, the sun always appears to rise over the horizon directly due east. In the subtropical regions, the sun rises due east for several days around the vernal equinox. Up north where I live, the sun rises at 90°E on March 18th and 19th. By the equinox, it’s one degree north of east. Not that you can see the difference, but still, the equinox is not when the sun rises and sets exactly due east and west.

True or False: The vernal equinox is when the sun’s apparent path, the ecliptic, intersects the celestial equator, an imaginary ring around the Earth set directly above the planetary equator. Or in uglier terms, the vernal equinox occurs when the apparent ecliptic longitude of the Sun is 0°.

Yes. This is true. It’s also really hard to visualize. Or much care about.

But this brings up a dilemma for those interested in the exact timing of astronomical events. As you can see there is some confusion over exactly what event takes place on the equinox. If for some reason you need to know exactly when the sun will rise in the east or when day and night are equal length, use an almanac for your geographical location. It will very likely not be on the equinox. If you do need to know when the sun will be at the cross-roads of the ecliptic and the celestial equator, however, that is the recorded time — and it’s a very short time! — of the vernal equinox.

However, if you are celebrating spring and renewal and the warming of your home, you can go ahead and do that when it happens in your locality — from when the sap starts running until whenever you figure summer has begun. Time is relative. Make of it what you will!

©Elizabeth Anker 2021