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Sep 22, 2009 11:39 pm US/Central
Good Question: Why Did Fall Start At 4:18?
(WCCO)
There was no trumpet sounding or parade down West 7th Street but at precisely 4:18 p.m., summer ended and autumn began. Why 4:18?
"There is a special time at which the sun crosses the equator. It is above for half the year, it is below for half the year. And at some point it crosses and we can define that as precisely as you want," said Dr. Larry Rudnick, an astronomy professor at the University of Minnesota.
Rudnick said the equinox creates close to the same amount of daylight and darkness in day; 12 hours of each.
"It's not precise, because we still can see the sun's rays when it's below the horizon, but it's basically a time of equal day and equal nights which is what equinox means," he said.
The U.S. Naval Observatory tracks the "official" time the sun crosses the equator of the Earth, but it is not the same time, nor is it the same date every year.
"It does change a little bit. The earth spins on its axis 24 hours, but it doesn't take an even number of those 24-hour days to go all the way around the sun," he explained.
That's why we have leap years, and that's why when you check the calendar, the sun crosses the equator at different times every year. One orbit of the earth is not equal to 365 days. So, in 2010, Minnesota's fall will start at 10:09 p.m. on the September 22.
"Does it really matter when the earth crosses the sun?" asked WCCO reporter Jason DeRusha.
"Not at all, it's just fun," laughed Rudnick.
(© MMIX, CBS Broadcasting Inc. All Rights Reserved.)
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