Counting down to the Spring (Vernal) Equinox

In just one week, spring will have officially arrived in the Northern Hemisphere! Although the winter here in upstate New York has been relatively mild in terms of temperatures, the frequency of days on which a cold rain has occurred has me anxiously awaiting the increased sunshine and longer days to come in the next few months. Although we wrote a blog post last year on why we have the seasons, I thought it would be fun to look at some little known facts about the spring, or vernal equinox.

Everyone has 12 hours of daylight and darkness on the equinox…right?

In most introductory meteorology classes, this is what we teach. The solstices are the longest and shortest days of the year, and on the equinoxes everywhere on earth receives 12 hours of daylight and 12 hours of darkness. Actually, it’s a little more complicated than that. In reality, the equinox represents a moment in time, not a full day. At the equinox, the sun’s direct rays are located at the equator and the earth’s axis is neither tilted toward the sun nor away from it. However, due to a few factors, the length of day is not exactly 12 hours on the equinoxes at all locations on the earth:

  • sunrise and sunset are defined as the time at which the upper edge of the sun’s disk rises above/drops below the horizon. There is some time required, although small, for the entire disk to appear in or disappear from view.
  • the earth’s atmosphere refracts, or bends, light. This allows the sun’s light to be visible even after the disk has dropped below the horizon. The degree of refraction depends on the temperature and density of the atmosphere, and this affects the length of twilight.

Equinoxes will be slightly different in the future

The earth’s axis runs straight through the earth from the North Pole to the South Pole. It is the rotation about this axis that gives us our day and night. Just like a wobbly top, the earth’s axis wobbles slightly as well, completing one full cycle roughly every 26,000 years. Due to this wobble, the point over the earth’s surface at which the sun’s direct rays cross the equator shifts slightly to the west each year.

Holidays related to the Vernal Equinox

In many cultures, the timing of major holidays are tied to astronomical events, including the spring equinox. The date of Easter is determined by the date of the spring equinox and the phases of the moon. Many ancient cultures, including the Egyptians and Romans, celebrated holidays at the onset of spring. People of Jewish heritage all over the world celebrate the feast of Passover in the spring, although this holiday is tied to the Jewish calendar. In Iran, the festival of No Ruz celebrates hope and rebirth near the time of the vernal equinox.

Ancient structures and the equinoxes

Ancient cultures around the world were very knowledgeable about astronomy, and tracked positions of many objects in the sky. Macchu Pichu, the famous Incan landmark, is perfectly aligned with the compass so that it exactly marks the equinoxes. The ancient Mayan Pyramid of Kukulkán was built such that on the equinoxes, sunlight hits the pyramid and creates the illusion of the shadow of a snake slithering down the steps. Here in the U.S., Hovenweep National Monument is a group of six prehistoric villages thought to be built between 1200 and 1300 A.D. Located on the Utah/Colorado border, contains a ‘castle’ which was thought to be built as a solar observatory. It contains several small ‘ports’, or openings, which track the sun’s position on various days of the year, including the equinoxes.

Courtesy: Smithsonian Institute

So, how will you celebrate the equinox and the transition from winter to spring? As for myself, I plan to get outside and (hopefully) enjoy some sunshine and warm weather!

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