On Sunday, September 22, 2024, at exactly 8:43 a.m. EDT, the Sun will cross the celestial equator. You won’t notice it, and there won’t be anything to see, but it’s an important moment of the year on Earth – and especially for skywatchers.

The geometry of the equinox may mean an increase in the intensity of the geomagnetic storms that cause the appearance of the Northern Lights (also called aurora borealis).

Here is everything you need to know about the September 2024 equinox:

September equinox declared

Equinox is Latin for “equal night.” It marks the point in the Earth’s orbit around the sun when there are approximately 12 hours of day and night across the entire planet. This is possible because the Earth’s axis is sideways to the sun, so every place receives the same amount of sunlight.

There are two such moments every year – at the end of March (the northern equinox, or spring) and the end of September (the southern equinox, or autumn). They mark the day when the midday sun crosses the equator – once north, once south – and are a global event that happens simultaneously for everyone on Earth.

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

The tilted Earth axis

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 the way it affects light and heat from the sun. During our planet’s 365-day orbit around the sun, this tilt constantly changes the proportion of sunlight that falls on any part of the planet. This results in four astronomical seasons – spring, summer, autumn 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, the equinox occurs halfway in between.

Technically speaking, everything from Earth’s perspective is determined by the Sun’s apparent maximum altitude in the sky. This determines the length of the day and therefore how much heat each 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 dominated by the Earth’s magnetic field.

At the equinox, the Earth’s axis is oriented sideways to the Sun. The magnetic fields of the Earth and the solar wind are briefly aligned according to the Russell-McPherron effect, creating cracks that can accelerate charged particles, resulting in more intense auroral phenomena.

Appointments for your calendar

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 signal the start 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, although few connect their existence with the 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 holiday in the Christian calendar.
• March 20, 2025: March equinox (northern)
• 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 festival of the first harvest of the season.

I wish you clear skies and big eyes.