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posted: 9/22/2017 1:00 AM

Happy Equinox! Here's how quickly the days are getting shorter where you live

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  • Sunset over the southern part of the Atlantic Ocean. Photo taken from the International Space Station. Johnson Space Center/NASA.

    Sunset over the southern part of the Atlantic Ocean. Photo taken from the International Space Station. Johnson Space Center/NASA.

 
Christopher Ingraham
The Washington Post

Today marks the fall equinox, ushering in the year's best season on a day that lasts almost exactly 12 hours no matter where you are on the globe.

Many of us in the Northern Hemisphere start noticing the days get shorter right about now, although in truth this has been happening for months - ever since June 21, the year's longest day.

But for most of us, the speed at which the daylight is dying is faster now than at any other time of the year.

Plotted over a year (in this case, from March to March), the length of daylight looks like one of our old friends from trigonometry class - the sine curve. It peaks on June 21 - when daylight hours are longest - and bottoms out on Dec. 21.

The height and depth of the curve vary by a location's latitude - how far north it is. In places close to the equator, such as Quito, Ecuador, the curve essentially flattens out to a straight line because days last roughly 12 hours all year long. Farther north in places such as Juneau, Alaska, the curve gets very steep - days are extremely long in the summer and extremely short in the winter.

Even farther north - above the Arctic Circle, in towns such as Barrow, Alaska - the curve gets blown out completely. The sun never sets for part of the summer, and it never rises in the depths of winter.

One cool thing to note: the lines all converge around Sept. 22 and March 20 - the fall and spring equinoxes, respectively. On those days, every place on earth gets the exact same amount of sunlight, because as NASA explains, "At an equinox, the Earth's terminator - the dividing line between day and night - becomes vertical and connects the north and south poles."

This is all caused by the earth's 23.5-degree tilt as it rotates the sun.

The daily change in the amount of daylight differs dramatically by latitude at this time of year. On the equator, the rate of change is essentially zero - the day will be about 12 hours long today, and 12 hours long tomorrow, too. But as you trek north up the globe, that rate changes.

Miami, for instance, is losing about 1 1⁄2 minutes of daylight now, every single day. Washington's losing 2 1⁄2 minutes. Where I live in Red Lake Falls, Minn., we're losing nearly 3 1⁄2 minutes of light each day.

As you go by the Arctic Circle, the change in daylight becomes extreme. Barrow, Alaska, is losing nearly 10 minutes a day. In the now-abandoned settlement of Etah, Greenland, the daylight is dying at a rate of more than 15 minutes a day. Winter is coming.

For the Southern Hemisphere, you'd essentially reverse this trend - head south away from the equator and the days are getting longer.

Again, this all goes back to axial tilt - without that 23.5-degree offset we'd have no seasons. The weird thing about this tilt is that it changes slightly over 40,000-year periods, varying between 22.1 and 24.5 degrees. Our current tilt is somewhere in the middle of that range and headed toward the low end of it, which scientists believe will make the difference between the seasons feel somewhat less extreme - if we're still around to notice it.

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