On September 22, 2020, we marked the autumnal equinox—a celestial event you've likely heard of. But what precisely is an equinox, and how does it shape our seasons? As experts in celestial mechanics from the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) explain, this phenomenon underpins the rhythmic change of seasons on Earth.
An equinox occurs when the Sun crosses Earth's equatorial plane, shifting its direct light from one hemisphere to the other. This happens twice yearly—in spring and fall—triggering a six-month reversal in climatic conditions: the hemisphere tilted toward the Sun enters warmer seasons, while the other cools.
Precisely at 1:30:40 p.m. UTC on Tuesday, September 22, 2020 (3:30:40 p.m. French legal time), per IMCCE calculations, the 2020 autumn equinox arrived. Summer concluded after 93 days, 15 hours, 46 minutes, and 58 seconds. At that instant, the Sun's geocentric latitude was -0.18″, right ascension 11h 59m 59.995s, and declination -0.16″—positioning it at the zenith over the Equator.
In the Northern Hemisphere, this signals autumn's onset with cooler temperatures and shortening days. In the Southern Hemisphere, it heralds spring's warmth.
Dr. James O'Donoghue, planetary scientist at the Japan Aerospace Exploration Agency (JAXA), crafted an insightful animation illustrating equinoxes, solstices, and their seasonal impacts. Watch it below:
Equinoxes and solstices stem from Earth's 23.5° axial tilt relative to the Sun. In winter, the Northern Hemisphere tilts away, receiving less direct sunlight; in summer, it leans toward the Sun for more intense rays. This tilt drives our seasonal cycle, as vividly shown in the animation.
During solstices—the extremes of tilt—one hemisphere maximizes exposure while the other minimizes it. On December 21, the Northern Hemisphere gets under nine hours of daylight, versus over fifteen in the south. Summer heat intensifies as sunlight traverses a thinner atmospheric layer, per O'Donoghue.
"Equinox" derives from Latin aequinoctium, meaning "equal night." Here, day and night each last twelve hours worldwide because Earth's axis aligns neither toward nor away from the Sun. Sunrays strike both hemispheres evenly, rising due east and setting due west—unlike other days.
On the equator at 1:30 p.m. UTC that day, shadows vanished briefly as the Sun reached zenith. Earth's 107,000 km/h orbital speed ensures this moment passes swiftly.
Related: What colors are sunsets on other planets?
Annual equinoxes span March 19-21 (Northern spring) and September 21-24 (Northern autumn) due to Earth's 365-day, 5-hour, 46-minute orbit. Leap years (366 days) prevent seasonal drift by aligning calendar and solar years.
