Monday marked the beginning of astronomical autumn. What does that mean?

SALT LAKE CITY — The Sept. 22 autumn equinox provided most of the world with nearly equal durations of day and nighttime, while also marking the first day of astronomical fall. But to meteorologists, the season of chilly weather and colorful leaves started a few weeks ago. 

According to Accuweather, the sun is directly over the equator during the autumn equinox. Due to the planet’s geometry, the rays beat down evenly across much of Earth. 

Here in the northern hemisphere, many celebrate the beginning of autumn. In the southern hemisphere, though, people welcome spring into their communities. 

Autumn equinox brings astronomical fall — what about meteorological fall?

The National Oceanic and Atmospheric Administration explained that meteorologists break seasons down based on annual temperature cycles, while also factoring in the calendar. Contrarily, atmospheric scientists use the planet’s position in relativity to the sun to mark seasons. That’s why you may have heard about two start dates for autumn — or another season. 

Satellite video of the 2025 autumn equinox. (National Oceanic and Atmospheric Administration)

To meteorologists, autumn starts on Sept. 1 and lasts until Nov. 30. 

Since Earth actually completes an orbit of the sun every 365-and-one-quarter days, the dates for solstices and equinoxes differ slightly from year to year. It’s also why we have Leap Year once every four years. 

The autumn equinox happens between Sept. 21 and 23 each year. This year, according to Accuweather, it happened on Sept. 22 at 2:19 p.m. EST — that’s 12:19 p.m. MST. 

Now that the astronomical event has passed, daily sunlight duration will continue to diminish until Dec. 21, which marks the winter solstice. To astronomers, that date will also mark the end of fall and the beginning of winter. 

What’s an equinox? What’s a solstice?

Equinoxes and solstices occur twice a year each — the former mark the beginning of spring and fall while the latter designate summer and winter. 

Per the National Weather Service, equinoxes happen when Earth’s axis is not pointed toward or away from the sun. 

During solstices, the planet sits at its most tilted point — when the summer solstice happens, it marks the longest day and shortest night of the year for the northern hemisphere. The winter solstice brings the shortest day and longest night. 

More autumn reading: The science within the vibrant colors of autumn leaves