Equinox examined
There is more to timing of spring\'s arrival than calendar alone
Ben Moyer
Friday brought us what we call “the first day of spring.” But announcing spring is an arbitrary declaration, as we saw by last week’s weather. The latest date I’ve seen snow flurries in our local mountains is June 2, 1977. Spring, like all the seasons, can be hard to pin down.
So, what really happened last Friday?
Friday, March 20 marked the vernal equinox, a real and annual celestial event that observant cultures have predicted, precisely, for centuries.
The vernal equinox happens on one of two days in Earth’s orbit of the sun when the length of darkness and daylight are equal (this is a little more complicated near the North and South poles). The other such day, directly opposite the vernal equinox on the calendar, is the autumnal equinox in September. The word “equinox” stems from the Latin for “equal night.”
Beginning with last Friday’s equinox, our period of daylight increases here in the Northern Hemisphere with each successive day. That trend continues until the summer solstice in June at which northern daylength begins to decline.
This unerring cycle is the result of two factors-Earth’s orbit around the sun, combined with Earth’s “tilt.”
To visualize that tilt, imagine a line drawn through the North Pole, extending through Earth’s center, then exiting at the South Pole-just like the spindle that held that spinning globe in your grade school classroom. No matter where Earth is in its orbit, that line (imagine the globe’s spindle) is always tilted at 23.5 degrees in relation to the sun. So, for example, during our summer Earth’s northern hemisphere is tilted toward the sun. But as Earth progresses around its orbit, still tilted at 23.5 degrees, it reaches an opposite point (in relation to the sun) when the Northern Hemisphere is tilted away from solar light and heat. That period marks our winter.
The equinox we just observed marks one of two points in its orbit when Earth’s tilt becomes momentarily irrelevant because neither hemisphere is tilted toward or away from the sun. But we in Earth’s north are entering our season of increasing daylength because the tilt re-exerts its influence as Earth continues its orbit beyond the equinox. This effect is exaggerated at the North Pole where the sun rose on Friday for the first time after six months of darkness. Just the opposite is occurring at the South Pole, where the sun just sank below the horizon for a six-month night.
Importantly for life as we know it, Earth’s tilt after the vernal equinox gifts the Northern Hemisphere more than a longer span of daylight alone. With each day now, the sun’s rays strike every point on the Northern Hemisphere at a more direct angle. The combination of longer daylength and more direct solar radiation warms our northern half of the planet.
Signs we associate with spring-blooms, buds, and the return of birds-appear around us because of that sustained heating.
Another way of thinking about the two equinoxes is that these are the two days of the year when, at midday, the sun stands directly above Earth’s equator. So, a person standing on the equator at midday on the equinox would stand inside their own shadow.
In Fayette County, we live at 39 degrees of latitude north of the equator. So, the sun is never directly overhead here; its daily arc across the sky is always somewhere south of overhead. The nearest the sun ever gets to directly overhead here is at midday on the summer solstice (first day of summer). On that day, the sun is directly above the Tropic of Cancer, which is 23.5 degrees (The same number of degrees as the tilt of Earth’s axis) north of the equator. After the summer solstice the sun’s direct rays will again begin their southward retreat, across the equator to the Tropic of Capricorn (23.5 degrees south of the equator).
The sun’s movement across the sky, of course, is illusion. It results from our perception here on a moving earth, which rotates at 24-hour intervals and orbits the sun at 365-day spans. But you can observe the sun’s apparent seasonal progress northward every day now if you have a place where you can view the sunrise in relation to some fixed point of reference on the horizon. If you’re attentive to the slight daily change, you’ll see the sun rise farther toward the northeast with each new dawn-until the cycle reverses, the sun turns south, and we cycle back toward winter-Not what you want to think about, I know.