At exactly 8:43 a.m. EDT on Sunday, September 22, 2024, the sun will cross the celestial equator. You won’t notice it and there won’t be anything to see, but it’s a big moment of the year on Earth, especially for stargazers.

The geometry of the equinox can mean an increase in the intensity of the geomagnetic storms that cause displays of the northern lights (also called aurora).

Here’s everything you need to know about the September equinox of 2024:

The September Equinox Explained

Equinox is Latin for equal night. It marks the moment in the Earth’s orbit around the sun when there are approximately 12 hours of day and night for the entire planet. This is possible because the Earth’s axis is tilted sideways to the sun, giving every location an equal amount of sunlight.

There are two of these moments each year: late March (the northern or vernal equinox) and late September (the southern or autumnal equinox). They mark the daya when the midday sun crosses the equator, one heading north and the other heading south, and it is a global event that happens simultaneously for everyone on Earth.

The March equinox marks the beginning of astronomical spring in the Northern Hemisphere, and today’s September equinox marks the beginning of astronomical autumn. In the Southern Hemisphere, it’s the other way around.

The tilted axis of the Earth

The equinoxes and solstices (which occur in late December and late June) are the result of the 23.5 degree tilt of the Earth’s axis and how it affects the light and heat from the sun. During our planet’s 365-day orbit around the sun, this tilt continually changes the amount of sunlight that falls on a part of the planet. This causes four astronomical seasons: spring, summer, fall, and winter.

While a solstice occurs when the Earth’s axis is tilted so that one of the planet’s hemispheres receives the most sunlight and the other the least, an equinox is the point halfway between the two.

Technically, as seen from Earth, it is all determined by the apparent maximum height of the sun in the sky. That is what determines the length of the day and therefore how much heat a place on Earth receives.

Aurora effect

What might the equinox 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 the Earth’s magnetosphere, the region of space where the Earth’s magnetic field dominates.

It is during the equinoxes that the Earth’s axis tilts sideways to the Sun. The Earth’s magnetic fields and the solar wind are briefly aligned, according to the Russell-McPherron effect, creating cracks that allow charged particles to accelerate, leading to more intense displays of aurora.

Dates for your diary

Today’s equinox marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. It is one of four signposts that herald the beginning of a new astronomical season, but there are also four signposts between each solstice and equinox that mark the midpoints. Some of these days are well-known, though few connect their existence to the Earth’s journey around the Sun:

• September 22, 2024: September Equinox (Southern)
• 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: Labor Day, a traditional spring holiday in the Northern Hemisphere.
• June 20, 2025: June Solstice
• August 1, 2025: Lammas, a traditional pagan festival celebrating the first harvest of the season.

I wish you clear skies and big eyes.