The fall equinox occurs on Tuesday, marking a significant astronomical event where light and darkness are nearly equally divided throughout the day. This event signifies the onset of fall in the Northern Hemisphere while ushering in spring in the Southern Hemisphere.

During the equinox, a unique phenomenon occurs at the equator, with the sun positioned directly overhead at noon. This is one of the rare moments when both the north and south poles receive sunlight simultaneously.

Equinoxes have held cultural significance across the globe for centuries. In Japan, the equinoxes are recognized as public holidays, reflecting the importance of this transition in seasons. At the ancient Mayan archaeological site of Chichen Itza in Mexico, visitors gather to witness a remarkable solar phenomenon where the sun casts a shadow resembling a serpent descending a pyramid, known as El Castillo.

Understanding the Equinox

The equinox occurs as Earth travels along its orbit around the sun, tilted at an angle that results in varying sunlight distribution across the planet throughout the year. On the equinox, Earth's axial tilt is such that it does not favor either hemisphere; consequently, both the Northern and Southern hemispheres share an almost equal amount of sunlight. During this time, the sun rises close to true east and sets nearly at true west.

The term “equinox” derives from two Latin words meaning “equal” and “night,” reflecting the balance of day and night. Although the days and nights are almost equal in duration during the equinox, slight variations can occur depending on one's geographical location.

In the Northern Hemisphere, the fall equinox typically occurs between September 21 and 24; this year, it falls on September 22 at 8:05 p.m. Eastern Time. Conversely, the vernal equinox in spring occurs between March 19 and 21. The precise time of the equinox coincides with the moment the sun is directly overhead at the equator.

The Significance of Solstices

In contrast to equinoxes, solstices are the times when Earth is tilted most significantly either toward or away from the sun. This stark axial tilt results in the greatest differences in sunlight exposure between the hemispheres and leads to the most unequal durations of day and night.

The Northern Hemisphere's summer solstice occurs when the upper half of the Earth leans toward the sun, resulting in the longest day and shortest night of the year. This solstice typically occurs between June 20 and 22. The winter solstice, on the other hand, is characterized by the Earth's upper half being tipped furthest away from the sun, creating the shortest day and the longest night, and falls between December 20 and 23.

It is also important to distinguish between meteorological and astronomical seasons. While astronomical seasons are based on Earth’s orbital movements around the sun, meteorological seasons are defined by weather patterns. Meteorologists categorize the year into three-month seasons based on annual temperature cycles: spring starts on March 1, summer on June 1, fall on September 1, and winter on December 1.

___

The Associated Press’ health and science coverage receives financial support from the AP Fund for Journalism and private foundations. AP is solely responsible for all content. Find AP’s standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org.

The Associated Press