At exactly 08:43 EDT on Sunday, September 22, 2024, the sun will cross the celestial equator. You won’t notice it happen, and there’s nothing to see, yet it’s an important moment of the year on Earth—and particularly for sky-watchers.
The geometry of the equinox can mean an uptick in the intensity of the geomagnetic storms that cause displays of the northern lights, also known as aurora.
Here’s everything you need to know about 2024’s September equinox:
September’s Equinox Explained
Equinox is Latin for equal night. It marks the moment of Earth’s orbit around the sun when there are roughly 12 hours of day and night for the entire planet. That’s possible because Earth’s axis is side-on to the sun, giving every location equal amounts of sunlight.
There are two of these moments each year—in late March (the northward or vernal equinox) and late September (the southward or fall equinox). They mark the daya when the midday sun crosses the equator—one going north and the other south—and it’s a global event occurring simultaneously for everyone on Earth.
March’s equinox marks the beginning of astronomical spring in the Northern Hemisphere, and today’s September equinox signals the start of astronomical fall or autumn. In the Southern Hemisphere, it’s the opposite.
Earth’s Tilted Axis
Equinoxes and solstices (which occur in late December and late June) result from the 23.5-degree tilt of Earth’s axis and the way that it affects light and heat from the sun. During our planet’s 365-day orbit of the sun, that tilt constantly changes the proportion of sunlight that falls on any part of the planet. This causes four astronomical seasons—spring, summer, autumn and winter.
While a solstice occurs when Earth’s axis is titled such that one of the planet’s hemispheres receives the maximum sunlight and the other its minimum, an equinox is the halfway point between them.
Technically, as seen from Earth, it’s all determined by the apparent maximum height of the sun in the sky. That’s what determines the length of the day and, therefore, how much heat any place on Earth receives.
Aurora Effect
What does the equinox possibly have to do with the northern lights? The northern lights are caused by charged particles from the sun, such as the solar wind, interacting with Earth’s magnetosphere, the area of space where the magnetic field of Earth dominates.
It’s at the equinoxes that the Earth’s axis is side-on to the sun. The magnetic fields of Earth and the solar wind are briefly aligned, according to the Russell-McPherron Effect, leading to cracks through which charged particles can accelerate, causing more intense displays of aurora.
Dates For Your Diary
Today’s equinox marks the beginning of autumn or fall in the Northern Hemisphere and spring in the Southern Hemisphere. It’s one of four waymarkers that signal the start of a new astronomical season, but there are also four waymarkers between each solstice and equinox that mark the mid-points. Some of those days are well known, though few connect their existence as being linked to Earth’s journey around the sun:
- September 22, 2024: September equinox (southward)
- October 31, 2024: Halloween and All Souls’ Day.
- December 21, 2024: December solstice
- February 2, 2025: Groundhog Day and Candlemas, a holy day in the Christian calendar.
- March 20, 2025: March equinox (northward)
- May 1, 2025: May Day, a traditional spring holiday in the northern hemisphere.
- June 20, 2025: June solstice
- August 1, 2025: Lammas, a traditional pagan celebration of the first harvest of the season.
Wishing you clear skies and wide eyes.
