The last of summer isn’t for another three weeks, but it’s taking a shockingly large chunk of its daylight before it goes. Between now and the autumnal equinox on Sept. 22, Wyoming will lose more than an hour of daylight.
“We lose an average of 55 minutes a month after the equinox,” said Max Gilbraith, planetarium coordinator at the University of Wyoming. “In the next 18 days, we will lose about an hour and 10 minutes of daylight.”
The loss of daylight will go hand-in-hand with the first bites of winter. Cooler nights, lower humidity, and the first snowfall of the season are all possible before summer’s officially over.
“The first half of September is the period of the year when we lose daylight the fastest,” Gilbraith said.
On The Sun’s Straightaway
This year’s summer solstice fell on June 20. “Solstice” is a Latin word that translates to “the sun stands still,” referencing how the sun reaches and stays at its highest or lowest point in the sky, depending on the season.
After the summer solstice, the amount of daylight has been decreasing. Gilbraith compared the rapid loss in September to the Earth “hitting the straightway” as it laps in its orbit around the Sun.
“As we approach the equinoxes, the Earth is moving at its fastest, south or north,” he said. “When you're at the apex of the next turn, you're moving the fastest. That’s where the sun is right now: on the straightaway, down to the next turn.”
The axis of the Earth is also staying fixed, mostly around the star Polaris. That means it’s tilted in a way that would compensate for the lost daylight.
The first half of September is the unfortunate sweet spot where the Northern Hemisphere of the Earth is speeding past the sun while its axis doesn’t shift. Thus, daylight is lost at an incredible rate, sometimes as much as three minutes a day.
“If you toss a ball into the air, it’s fast when it leaves your hand, goes to zero when it reaches the top, and then it'll be fastest again as it goes past your hands,” Gilbraith said. “That's a way to think about the motion of the Earth and the sun in the sky.”
Sunrise, Sunset
On Sept. 1, the sun rose over Cheyenne at 6:26 a.m. and set at 7:31 p.m. By Sept. 22, the autumnal equinox, the sun will rise at 6:47 a.m. and set at 6:55 p.m. Over an hour of daylight will be lost between the two dates.
And that’s a southern spot in Wyoming. The northern half will lose more daylight faster over the same period.
In Cody, the sun rose at 6:39 a.m. and set at 7:52 p.m. on Sept. 1. On Sept. 22, it will rise at 7:03 a.m. and set at 7:12 p.m., a loss of 1 hour and 4 minutes, with an average of three minutes of daylight lost every day from Sept. 6 through the end of the month.
“The two weeks before and after the equinox are the fastest that we’ll lose daylight,” Gilbraith said. “We will lose around three hours of daylight from Sept. 21 to Dec. 21, but the rate of loss during this one-month period is just a little extra.”
You’ll Feel It
The loss of daylight will be accompanied by noticeable changes in Wyoming’s weather. When you lose over an hour of daylight in two weeks, there’s going to be an impact.
“September is the month when average temperatures start to drop rather quickly, coinciding with the decreasing amount of daylight hours,” said Cowboy State Daily meteorologist Don Day. “First, we notice it being cooler and drier at night. And when nights are longer, temperatures in that dry air go up and down a little bit quicker.”
Day said the loss of daylight is noticeable, but its impacts aren’t immediate. Once we reach the autumnal equinox, Wyomingites won’t need to be told that fall has fallen.
“It’s cumulative, but there’s always a delay,” he said. “Once you start to get longer nights and shorter daylight hours in the Arctic Circle and northern areas of Canada, the colder air really starts to build at night. When it's been dark enough for long enough in those northern latitudes, they start to build up significant cold air that can spill south.”
Day said the post-equinox period, from Sept. 22 to the beginning of October, is when the continental U.S. will really feel the loss of warmth and daylight. That’s when he anticipates Wyoming will get its first “cold shots” of the fall.
“I'm expecting a cool down, but not a big shot of cold, in mid-September,” he said. “But, oh boy, it certainly looks like there’s potential for a bigger shot of colder temperatures between the third and fourth weeks of September that could even bring snow.”
Day’s long-range forecast for Winter 2025-2026 called for cold spells in the fall like Wyoming hasn’t experienced for several winters. The rapid loss of daylight will exacerbate any cold spells that penetrate into the Rocky Mountain Region.
“This winter is analogous to Winter 2019-2020,” he said. “We had a couple of hard freezes and snowstorms that affected Wyoming in the Fall of 2019. That was quite impactful, and we’ve been seeing a similar weather pattern in the Pacific Ocean that ended up leading to what we saw in 2019.”
Darkest Before The Dawn
Wyoming will continue losing sunlight until the winter solstice on Dec. 21. After that, there’ll be a slow but steady increase in daylight in the first three months of 2026.
Unfortunately, it means that summer’s almost over in the Cowboy State. So, get it while it’s good.
“If you were to say that summer is over after your location hits 32 degrees for the first time, then we're looking at two to three weeks left of summer,” Day said. “That doesn't mean you can't have a few days of summer-like weather in October, but we’ll have our first freeze before then.”
Below is how much daylight some Wyoming communities will lose in September, measured from sunrise to sunset:
Afton
Sept. 1: 6:48 a.m. and 7:57 p.m.
Sept. 30: 7:20 a.m. and 7:06 p.m.
Loss: 1 hour, 18 minutes
Casper
Sept. 1: 6:30 a.m. and 7:39 p.m.
Sept. 30: 7:01 p.m. and 6:47 p.m.
Loss: 1 hour, 23 minutes
Cheyenne
Sept. 1: 6:26 a.m. and 7:31 p.m.
Sept. 30: 6:55 a.m. and 6:42 p.m.
Loss: 1 hour, 18 minutes
Cody
Sept. 1: 6:39 a.m. and 7:46 p.m.
Sept. 30: 7:19 a.m. and 7:06 p.m.
Loss: 1 hour, 29 minutes
Evanston
Sept. 1: 6:50 a.m. and 7:55 p.m.
Sept. 30: 7:34 a.m. and 5:36 p.m.
Loss: 1 hour, 20 minutes
Gillette
Sept. 1: 6:25 a.m. and 7:37 p.m.
Sept. 30: 6:59 a.m. and 6:43 p.m.
Loss: 1 hour, 28 minutes
Jackson
Sept. 1: 6:47 a.m. and 7:57 p.m.
Sept. 30: 7:19 a.m. and 7:05 p.m.
Loss: 1 hour, 24 minutes
Lusk
Sept. 1: 6:23 a.m. and 7:31 p.m.
Sept. 30: 6:54 a.m. and 6:40 p.m.
Loss: 1 hour, 22 minutes
Riverton
Sept. 1: 6:38 a.m. and 7:47 p.m.
Sept. 30: 7:10 a.m. and 6:55 p.m.
Loss: 1 hour, 24 minutes
Sheridan
Sept. 1: 6:30 a.m. and 7:43 p.m.
Sept. 30: 7:05 a.m. and 6:49 p.m.
Loss: 1 hour, 29 minutes
Thermopolis
Sept. 1: 6:37 a.m. and 7:47 p.m.
Sept. 30: 7:09 a.m. and 6:54 p.m.
Loss: 1 hour, 26 minutes
Wamsutter
Sept. 1: 6:38 a.m. and 7:44 p.m.
Sept. 30: 7:08 a.m. and 6:54 p.m.
Loss: 1 hour, 18 minutes
Andrew Rossi can be reached at arossi@cowboystatedaily.com.
