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Sorry, folks: Equinox isn’t equal

On first day of fall, day lasts 12 hours, 8 minutes

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Today is the first day of fall. But forget what you learned about the change in grade school. Yes, it's the fall equinox, but no, the day and night are not of equal length.

That is a myth of the equinox, according to Patrick Wiggins, NASA solar system ambassador for Utah. Although it's not as bad as the patently bogus belief that only on the spring or fall equinox can you make an egg stand on end.

Unlike that silliness, the equal days and nights idea is grounded in reality. It's just not exactly true. A glance at the Deseret Morning News' weather graphic, provided by Accessweather.com, shows sunrise today at 7:16 a.m. while sunset is at 7:24 p.m.

That's a total of 12 hours, 8 minutes. Close, but not precisely a half day.

What then is the equinox?

Earth is tilted on its axis. The tilt causes the seasons. When the sun is beaming more directly toward the Northern Hemisphere, we experience summer here. When the tilt brings the Southern Hemisphere around to face the sun more squarely, it's summer there and winter here.

If the sun is more directly over the southern part of our planet, the days there are longer and the days here are shorter. And vice versa.

At some point, the sun seems to cross the equator, heading to the north or south. In late September, when it moves south of that imaginary line, that's the autumnal equinox, signaling the start of the cold seasons. When it moves back north in March, it's the spring, or vernal, equinox.

The sun was expected to slide over the equator at 4:47 a.m. today.

We judge daytime according to when light is streaming in from the sun. But Wiggins said sunrise starts when the sun first pokes the top of its glowing head above the eastern horizon.

Likewise, sundown is not when the great orb first touches the western horizon. Sunset is official only when the last part of the sun sinks below the horizon.

The difference between the first touch of the sun at one horizon and its last touch at the opposite side is the sun's diameter, Wiggins said. Earth's rotation requires some minutes to move that far. That means the night and day are not equally long — even if the center of the sun rises above the horizon exactly 12 hours before its center crosses the western horizon.

But there's even more difference.

You see the sun a little before it actually rises and a little after it sets.

"Before the sun is really above your line of sight, the atmosphere has kind of pulled it up a little bit," explained Wiggins. This atmospheric refraction works like a mirror, as if you were seeing a distant mirage.

The atmosphere refracts the light upward and you see the sun before you would if you were getting strictly a straight-line view.

The result — a little more daylight than nighttime during the equinox.

And Utahns should have plenty of chance to enjoy that daylight this week. Gorgeous fall weather is expected to continue, with highs in the 70s and 80s along the Wasatch Front, under generally clear skies.

E-MAIL: bau@desnews.com