What actually happens on the fall equinox?
The leaves have been changing, the temperature has been dropping, and the pumpkin-flavored coffee drinks are flowing.
Those are clues that fall has arrived. But if you look at a calendar, there's a specific date in late September when the season actually changes: The fall equinox.
"The fall equinox is one of two times during the year where the Earth is not tilted relative to the sun. So, we're sort of straight up and down relative to sun," said Thaddeus LaCourseir, the planetarium production coordinator at the Bell Museum.
During an equinox, either fall or spring, sunlight evenly hits both the northern and southern hemispheres as the sun is pointed directly at the Earth's equator.
"Since we're not at a tilt to the sun, we have about equal day and equal night," he said.
It's how equinox got its name. It uses the Latin words "aequus", meaning equal, and "nox," meaning night.
Based on that definition, you might believe there are exactly 12 hours of day and 12 hours of night on the equinox, but it's not that simple.
"The sun is not just like a single little point of light. It is a ball. And because of that, and because of the Earth's atmosphere, light from the sun refracts through the atmosphere," said LaCoursier.
Those variables lead to more than 12 hours of sunlight during the fall equinox, but not by much.
LaCoursier said the daylight is roughly 12 hours and six minutes. The extra time can be explained at dawn and dusk. People see sunlight over the horizon before the sun fully rises and for a few minutes after it sets.
After the fall equinox occurs, the sun is then pointing toward the southern hemisphere, marking the start of fall for the northern hemisphere.
But meteorologists like WCCO's Lisa Meadows have their own way of tracking the seasons.
"Astronomical fall is actually when the Earth rotates around the sun, its position around the sun, whereas meteorological fall has to do just really just making it easy when it comes to record keeping," Meadows said. "Meteorological fall starts on Sept. 1, and it continues to go through the end of November."
The methodology is helpful since the equinox date varies each year.
Fall's opening day is Sept. 21-24 but mainly lands on the 22nd or 23rd.
According to CBS News Philadelphia, "the equinox dates vary because of the difference between how the Gregorian calendar defines a year (365 days) and the time it actually takes for Earth to complete its orbit around the sun (about 365 and 1/4 days)."
"Since those dates could change just by about a day or so each year, we just keep it the same, each meteorological calendar, just to keep it easy with record keeping," said Meadows.
The records allow meteorologists to compare current temperatures to the averages gathered in the most recent decade, giving viewers better context on the type of weather they're experiencing.
