Understanding the Autumnal Equinox: The Beginning of Fall
The autumnal equinox, which falls on September 22nd, marks the beginning of fall in the Northern Hemisphere. This phenomenon occurs when the Earth experiences equal day and night, a moment of perfect balance between light and darkness. The vernal equinox, on the other hand, marks the beginning of astronomical spring and occurs around March 20th.
Meteorological vs. Astronomical Seasons
While the autumnal equinox marks the official beginning of fall, meteorologists consider September 1st as the start of the season. This is because meteorological seasons are based on Earth’s annual temperature cycles, with the three months between the coldest and warmest months (summer and winter) defining the season. In contrast, astronomical seasons are determined by the Earth’s tilt, either away or toward the Sun.
According to Dr. Tyler Richey-Yowell, a postdoctoral fellow at Lowell Observatory in Arizona, the term “equinox” comes from the Latin “equi,” meaning equal, and “nox” or night. He explains that the equinox occurs when neither the Northern nor the Southern Hemisphere is pointed towards the Sun, resulting in equal day and night.
NOAA’s GOES-16 satellite photo of Earth on the equinox.
(NASA/NOAA / NOAA)
The Science Behind the Autumnal Equinox
Richey-Yowell uses a simple yet effective analogy to explain the concept of the equinox, demonstrating with a pencil through a tangerine. He explains that the Earth rotates on an axis, tilted by 23 and a half degrees, which results in varying amounts of sunlight throughout the year. During the equinox, the Northern and Southern Hemispheres receive equal amounts of sunlight, resulting in equal day and night.
This graphic shows the Earth and Sun during the autumnal equinox.
(FOX Weather)
According to NASA, the autumnal equinox occurs on September 22nd at 2:19 p.m. ET or 11:19 a.m. PT, marking the moment when the center of the Sun crosses the plane of our equator. For more information on the autumnal equinox and its significance, visit Here
Image Source: www.foxweather.com
