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, SunsetOn 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....
