Geologic Column: Rebirth on the vernal equinox

by John Copeland
Wednesday, February 15, 2017

March is about rebirth. Credit: Public Domain.

March is a particularly tempestuous month. The Anglo Saxons called it “Hrethmonath,” which translates as “rough month.” The name referred to March’s blustery winds that often blow with a force unfelt all winter, and which make March a great month for kite flying and a common time for tornadoes to strike.

During the Roman Empire, March’s winds often carried the sound of blaring war trumpets, as the Roman Legions went on the offensive at this time of year. In fact, March is named for Mars, the Roman god of war. And, of course, Julius Caesar was killed on the Ides of March.

But wind and war aside, March is really more about rebirth. On March 20, we celebrate the vernal, or spring, equinox in the Northern Hemisphere. Seasonal beginnings are determined by astronomical mechanics. The vernal equinox marks the moment the sun crosses the celestial equator moving from south to north. It happens worldwide at the same instance: This year, it’s at 10:28 a.m., Coordinated Universal Time.

However, there are different ideas about how to define seasons. Astronomers count the vernal equinox as the start of spring, which then ends on the summer solstice, when astronomical summer begins. Meteorologists, on the other hand, mark the beginning of spring in the Northern Hemisphere three weeks before the vernal equinox, on March 1, and end it on May 31.

We didn’t always understand that the timing of the equinoxes and solstices occurs as a product of Earth’s yearly orbit around the sun. Today, we know they relate to Earth’s 23.4-degree tilt on its axis and its motion in orbit. Because of the axial tilt, Earth’s Northern and Southern hemispheres each take a turn in the sun during the year, variously receiving the sun’s most direct light and warmth. Our biannual equinoxes happen when Earth’s axial tilt and its orbital location combine in such a way that the planet is tilted neither away from, nor toward, the sun, meaning both hemispheres are illuminated equally.

The word equinox derives from the Latin words “aequus” (“equal”) and “nox” (“night”). Hence, on the equinox, the length of day and night are also essentially equal, though the equality is short-lived as the balance tips in favor of more daylight in the Northern Hemisphere in March and more in the Southern Hemisphere in September.

On the first day of spring, we are halfway between the winter solstice and the summer solstice — 89 days from each. However, there are 94 days between the summer solstice and the autumn equinox. This imbalance arises because Earth does not orbit the sun at a constant speed, so the seasons are not equal in length. I don’t know about you, but I prefer a longer summer than a longer winter.

Curiously, the equinoxes themselves are not fixed in time. They occur about six hours later every year, equaling one full day every four years, an offset that is reset with each leap year. Our calendar is good, but not perfect.

To our forebears, watching the sunrise or gazing at the moon and stars were sources of mystery, but also continuity as they used celestial movements to mark time. In the millennia before automobiles, electric lights, televisions, computer tablets, smartphones, and clocks, our skywatching ancestors recognized that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted throughout the year. The shifts signaled the start of the growing season, or the onset of winter, for example. The ability to predict the seasons was key to survival in ancient times.

Researchers have discovered that many cultures around the world constructed sites that were astronomically aligned on either the equinoxes or the solstices. In Europe, many of the prehistoric Neolithic monuments, standing stones and stone circles, like Stonehenge, are in tune with the equinoxes, aligned with either the rising sun or rising moon. It is probably no coincidence that early Egyptians built the Great Sphinx to directly face the rising sun on the vernal equinox.

On the spring equinox, many ancient cultures celebrated the start of the new year. Just as dawn is the time of new light, our ancestors considered the vernal equinox the time of new life. For many cultures, the spring equinox signaled the return of weather that favored the planting of new crops, and, for this reason, humans have associated the spring equinox with our ability to perpetuate ourselves. It is understandable why we have chosen to celebrate such a momentous occasion, and why it has come to represent “rebirth” in a variety of contexts.

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