September in the Northern Hempisphere brings back-to-school time, the last poolside splashes, and the first falling leaves as autumn slowly creeps in. This year, the autumnal equinox arrives September 22, 2021, signifying the official end of summer and the slow transition to cooler, shorter days. But unlike the summer and winter solstices that occur three months before and after fall, the autumnal equinox marks a different moment in Earth’s journey around the sun. Here’s what happens during the autumnal equinox.
How Earth’s Seasons Change
The Northern Hemisphere begins its slow descent from summer’s heat to winter’s chill in mid-September thanks to two planetary features: Earth’s oblong orbit around the sun and its tilted axis. The planet spins at a 23.5-degree angle, not directly on the northern and southern poles the way it appears to us on maps. Combined with its annual rotation around the sun, Earth’s tilted poles allow one hemisphere to receive more sunlight than the other for half the year.
Those shifts toward and away from the sun lead the separate hemispheres to experience different weather; without the shifts, there’d be no major seasonal changes on any part of the planet. The Northern Hemisphere experiences cooler temperatures and winter when Earth’s northern pole is angled away from the sun, while at the same time, the Southern Hemisphere gets more sunlight and moves toward summer. But the angles don’t change quickly; it takes months for the hemispheres to trade spots, which allows seasons to gradually wax and wane.
The Difference Between a Solstice and an Equinox
Solstices and equinoxes mark different points along Earth’s route around the sun and the transition from more to less sunlight. As one of the poles gets closer to the sun, the amount of daylight hours grows until reaching its maximum — in the Northern Hempisphere, we call this the summer solstice or the longest day of the year. Half a year later, when the northern pole is angled its furthest from the sun, the Northern Hemisphere experiences the day with the longest hours of night, also called the winter solstice.
Spring and fall are transitions between these two extremes, and with them come equinoxes. Meaning “equal night” in Latin, equinoxes mark the time when day and night are nearly the same length of time (approximately 12 hours of sunlight and 12 of darkness). That’s because following the summer solstice, days become slowly shorter and nights gradually longer until the winter solstice arrives. After the winter solstice, daylight hours increase, becoming momentarily equal with hours of night when the vernal (spring) equinox arrives in March.
Like solstices, equinoxes last only a day and are calculated down to the minute, though they aren’t perfectly equal — in some regions there’s a small, 10-minute difference based on how the sun’s rays reach Earth’s atmosphere. This year, the autumnal equinox in the Northern Hemisphere occurs at 3:20 p.m. ET on September 22.
When Does Autumn Really Begin?
While most calendars mark mid-September as the beginning of autumn, you may have heard that fall begins a few weeks earlier. How is it possible to have two first days of fall? There are actually two commonly used calendars for determining seasonal changes: astrological and meteorological.
Astrological seasons are based on Earth’s tilt, transitioning throughout the year based on the equinoxes and solstices. But meteorological seasons align with the Gregorian calendar, syncing the start and end dates with set three-month spans of time. Meteorological calendars break down the year into four distinct seasons with a nearly equal number of days. Instead of determining seasons by the number of daylight and nighttime hours, meteorological seasons are based on historical temperature records and weather patterns. Because seasons are determined by date (and don’t fluctuate in number of days like astrological seasons), scientists and researchers can more easily compile data about weather changes and trends.
In the Northern Hemisphere, meteorological autumn begins September 1 and ends November 30 each year. But if you’re looking to knock out a lengthy autumnal bucket list with apple picking, seasonal drinks, and leaf peeping, you’re free to keep honoring fall for a few additional weeks, since astronomical winter won’t begin until December 21, 2021.
Popular Equinox Myths, Debunked
While most modern cultures give little thought to the autumnal equinox, societies of the past noted the first day of fall during harvest season preparations. Over time, some unattributed equinox lore has stuck around from centuries past — the most common tall tale says it’s possible to balance a raw egg on end only during the equinox. Fortunately for party tricksters, this myth has been disproven by scientists who note that the planet’s position and hours of sunlight have no impact on gravity. So long as you have a steady hand, it’s possible to balance an egg any day of the year.
And that myth about your shadow disappearing entirely during the equinox? It’s mostly untrue, unless you live within the Tropic of Cancer, close to the equator. Because of Earth’s axis tilt, sunlight always hits us at an angle, creating a shadow. But during the equinox, those who live within this zone can be momentarily shadowless at precisely noon, when the sun is directly overhead and casts shadows directly downward. The phenomenon lasts only minutes and isn’t possible anywhere else — a momentary treat to honor Earth’s journey around the sun.
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