This Month in Science – The Equinox and Changing Seasons

September 22, 2020, is the Autumnal Equinox. It marks the beginning of the fall season. On the equinox, the Earth experiences almost equal amounts of daylight and darkness across the globe. “Equinox” comes from the Latin words aequus, meaning “equal”, and nox, meaning “night”. An equinox occurs twice a year, in the spring and the fall. On these two days, the planet experiences about 12 hours of daylight and 12 hours of night-time. Equinoxes mark the changing of the seasons.

The Earth’s position relative to the sun is not upright, but tilted on its axis at a 23.5 degree angle. This tilt affects the amount of light and warmth each hemisphere receives from the Sun during Earth’s orbit. The equinox occurs twice a year when the Earth is positioned in such a way that its axis is tilted neither towards, nor away from, the Sun. At this time, the Sun is positioned in line with Earth’s equator. The shadowed line that separates night from day runs through the North and South Poles, dividing the globe in half. Even though equinox means “equal night”, the Earth’s atmosphere bends light in such a way that the line does not pass perfectly through the halfway point, so one hemisphere still receives slightly more light than the other.

We experience changing seasons because of the Earth’s tilted axis. Without tilt, each latitude would only have one season, experiencing the same amount of heat and light from the Sun throughout the year. In the Northern Hemisphere, the Autumnal Equinox marks the beginning of fall; the Sun begins to shine less on the Northern Hemisphere and more on the Southern. On the northern half of the planet, days continue to get shorter and the temperatures cool off from summer highs, while on the southern half, days get longer and warmer. Seasons also change after a Summer or Winter Solstice. On the solstice, the Earth’s tilt toward the Sun is at its maximum. In the summer, the Northern Hemisphere experiences the longest day and shortest night of the year. On the Winter Solstice, the opposite is true, with the shortest day and longest night.

Humans have been tracking the changing of the seasons for thousands of years. Archeological evidence indicates that many ancient civilizations constructed large monuments to track the position of the Sun over time. At Machu Picchu, the Incans placed the Intihuatana rock so that its shadow disappears completely at noon on the equinox. At Chichen Itza, the Mayans constructed the Kukulcán pyramid to display a serpentine shadow down the northern facing steps on the day of an equinox.  At Chaco Canyon, New Mexico, the Anasazi positioned three giant slabs of rock so that on the equinox a dagger of light would shine onto the halfway point of a spiral carved into a rock wall behind the slabs. These structures served as a sort of ancient calendar to determine the position of the Sun throughout the year and alert these civilizations to the changing seasons.

To this day, cultures around the world celebrate the equinox. The Chinese Harvest Moon Festival is celebrated on the day of the full moon closest to the Autumnal Equinox. The tradition started during the Shang Dynasty (1600-1046 BCE) to celebrate a successful harvest of rice and wheat. Japanese Buddhists celebrate Higan on both the Autumnal and Vernal (Spring) equinox. On Higan, the Buddhists return to their homeland to remember their ancestors. The equinox not only marks the changing seasons, but is also a time of celebration for many cultures across the globe.

To learn more, watch this video about The Equinox.

Previous Columns

August: Perseid Meteor Shower
July: Plastic Free July
June: Sargasso Sea
May: Getting to Mars
April: Earth Day 50th Anniversary
March: Ghost Trees
February: Fire
January 2020: Biodiversity of the Cape Fear
December: Snowflakes
November: Carolina Bays
October: The Longleaf Pine
September: Jellyfish
August: What’s in Your Childhood Chemistry Set?
July: What Happened to the Dinosaurs?
June: Hurricane Season
April: Quantum Levitation
March: Venus Flytrap
February: A Shifting Magnetic Field
January 2019: Giant Ground Sloth


On the equinox, the Earth experiences almost equal amounts of light and darkness.
Credit: NASA images by Robert Simmon, using data ©2010 EUMETSAT

The Earth is tilted on its axis at a 23.5 degree angle. Twice a year, this tilt results in an equinox.

On the equinox, the line between night and day runs through North and South Poles, dividing the planet almost perfectly in half.
Credit: Illustration by Przemyslaw, Wikimedia. CC-BY-SA-2.0

On the equinox at Chichen Itza, shadow and sunlight work together to form a serpent that slithers down the north facing steps.
Credit: PlayadelCarmen Blog
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