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. Check out the chart below, which plots the length of daylight versus the months of the year for Washington. I've adapted it from a nifty tool created by the University of Nebraska at Lincoln's astronomy department.
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 about this chart: 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 effect of that angle on earth is vividly illustrated in this video, which uses a time lapse of satellite imagery to illustrate how the line between day and night falls across the planet over the course of a year.
The daily change in the amount of daylight differs dramatically by latitude at this time of year. On the equator, as you might suspect, 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 11⁄2 minutes of daylight now, every single day. Washington's losing 21⁄2 minutes. Where I live in Red Lake Falls, Minn., we're losing nearly 31⁄2 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.
Want to know how short your day is getting? Figure out your latitude and see where it falls on the black line on that chart.
For the Southern Hemisphere, you'd essentially reverse this chart — head south away from the equator and the days are getting longer by these amounts.
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.
Correction: The 2017 fall equinox is September 22, not September 21. The 2017 Spring equinox is March 20.