What Is an Equinox?

Multicolored pumpkins, brown pinecones, red apples, and yellow leaves spread on a wooden table surface in front of a yellow background In the fall, leaves change color and apples and pumpkins are ripe. BrianAJackson/iStock/Getty Images Plus

Meteorological fall arrived on September 1. This is harvest season for many people in the Northern Hemisphere. Apples are ready to pick at orchards and pumpkin patches await visitors. Are you raking leaves yet? Are you enjoying delicious, warm apple cider as the temperature begins to cool?  The first day of astronomical fall in the Northern Hemisphere officially arrives September 22 when the Sun crosses the celestial equator going north to south, which is called the autumnal equinox. During an equinox the tilt of Earth’s axis and Earth’s orbit around the Sun are positioned such that the axis isn’t tilting one hemisphere toward or away from the Sun. Direct sunlight shines on the equator, so the length of day and night is nearly equal for both hemispheres.

Black and white image of Earth with half illuminated by sunlight and half in darknessDuring an equinox, the Sun shines directly on Earth’s equator, resulting in nearly equal amounts of daylight and darkness. NASA


How do different cultures celebrate the fall equinox?

Chichen Itza is a popular place in Mexico to celebrate an equinox. Chichen Itza is a Mayan city on the Yucatan Peninsula with a centerpiece called El Castillo pyramid. The pyramid is the temple of the god Kukulcan—the Feathered Serpent. The temple design is nine terraced platforms with a staircase centered on each of the four sides. The head of the Feathered Serpent is sculpted at the bottom of the northwest stairs. Around each equinox, the afternoon Sun casts a series of the platforms’ triangular shadows on the wall of the staircase. As the shadows creep higher, the body of a snake appears to be crawling down the pyramid. The site draws numerous people who celebrate with music and dancing.

Limestone pyramid with a triangular shadow pattern cast on one side of the pyramid where the remaining sunlight resembles the body of a snake connected to the carved head of a serpent at the base of the pyramidAround the equinoxes, sunlight and shadows combine to look like a snake crawling down the El Castillo pyramid.

In the pagan culture, the fall equinox is called Mabon. Mabon is the second of three harvest festivals and a time to give thanks for the summer and welcome the coming darkness.

The fall season is also central to the story of Persephone in Ancient Greek mythology. Persephone was the daughter of Demeter, the harvest goddess. Every fall Persephone had to return to the underworld to spend three months with Hades. During the three months, Demeter stopped the plants from growing until Persephone came back.

Cover of "How Can We Predict When the Sky Will Be Dark?" teacher guide

First-grade students have an opportunity to explore the annual pattern of daylight in a three-dimensional learning module from the Smithsonian Science for the Classroom series titled, How Can We Predict When the Sky Will Be Dark? They will also read a story about festivals related to daylight in Sky Patterns from the Smithsonian Science Stories Literacy Series.

Students, educators, and caregivers can also watch this free video series from the SSEC’s Distance Learning website that guides students in making observations of the Sun’s shadow.