Fall arrived in Utah Wednesday at 10:30 a.m. MDT. Or did autumn really show up on Sept. 10?
That depends on whether you judge the season by the invigorating snap in the air or the more abstract movement of Earth in its orbit.
First, the official definition. Yes, fall fell on Wednesday. That was when the autumnal equinox took place, the point at which our nights and days became equal in length. We are on the long slide toward winter, when the equation will turn around and the days begin to lengthen.
Jim Cobb, an amateur astronomer from East Millcreek, explains that the technical, astronomical definition of the first day of fall is "when the sun crosses the equator, moving south."
If you imagine the plane of Earth's equator extending out infinitely from this planet, our tilted orbit places the sun below the equatorial plane half of the year and above it the rest of the time. The sun shines more directly on the southern hemisphere during the winter so the days are longer there and shorter in the north.
"When it goes below the equator, its rays become more indirect" for the northern hemisphere, added Cobb.
Mark Eubank, KSL chief meteorologist, has a different way of defining fall.
"Fall is the season between summer, the hottest 90 days, and winter, the coldest 90 days," he said.
By that standard, he noted, there are two definitions: astronomical fall and meteorological fall.
In Salt Lake City, the hottest 90-day period of the year ends on Sept. 9, based on 30 years of careful record-keeping. Fall therefore starts on Sept. 10, by that reckoning.
"That's when we get this real nice weather," with temperatures that are like Baby Bear's porridge, not too hot and not too cold. Often, the days are sunny, with a few rain showers now and then.
The year's coldest 90 days start on Nov. 24. So, "meteorological fall" extends from Sept. 10 through Nov. 23.
The astronomers have brainwashed us about autumn, he joked.
The summer solstice, when days were the longest, was on June 20, according to the U.S. Naval Observatory. Since then, days have been getting shorter and nights longer. At the astronomical start of fall, on Wednesday, days and nights were of approximately equal length.
But the process of shortening days will continue until the winter solstice, Dec. 21.
By Wednesday, the weather had already started to react to the shortening daylight and the reduction in solar warming. Eubank said, "We really have been in fall for a couple of weeks."
Utah definitely is experiencing fall now. And it's comfortable weather should continue, according to the KSL expert.
Within the next few days, Salt Lake City "may be a little bit warmer than longer," Eubank said. The average high this time of year is 73, while 48 is the typical low.
Today should be close to normal, with a high of 69 and a low of 44. But the weather will be warming a bit, and by Sunday the high will be 80 and the low about 52 degrees.
E-mail: bau@desnews.com