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Winter arrives Wednesday morning. Five maps that explain the solstice

December 20, 2016 3:56 PM
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Winter arrives Wednesday morning. Five maps that explain the solstice

It’s certainly felt like winter in much of the country these last few weeks. And if you’re a weather enthusiast, winter already began Dec. 1. However, if you prefer the traditional, astronomy-based definition, then winter officially begins with the solstice, at 5:44 a.m. eastern on Dec. 21.

For those of us in the Northern Hemisphere, the winter solstice marks the the shortest day and longest night of the year.

This map projection of the Earth shows how dark the Northern Hemisphere is at the time of the winter solstice, 5:44 a.m.

Because the Earth’s axis of rotation is tilted 23.5 degrees, each hemisphere receives different amounts of sunlight as our planet orbits the sun.

The December solstice occurs when the sun’s direct rays reach their southernmost position with respect to Earth’s equator. At that moment, the sun shines directly overhead at 23.5 degrees south latitude, along the Tropic of Capricorn (this year, the sun will be straight overhead in southern Africa when the solstice arrives).

Since the sun’s direct rays reach their southernmost point with respect to Earth’s equator, the December solstice brings us the southernmost sunrise and sunset of the year. In the Northern Hemisphere, we see the sun take its lowest and shortest path across the southern sky. That means if you catch a glimpse of the sun at local noon, your shadow will be the longest of the year.

A good portion of the Lower 48 sees the sun up for 9 to 10 hours on the winter solstice, leaving about 14-15 hours of nighttime (see chart).

In Washington, D.C. the sun is up for 9 hours and 26 minutes (rising at 7:23 a.m. and setting at 4:49 p.m). That’s about an hour less daylight than in South Florida, but still an hour longer than in cities across the Northern Tier, like Minneapolis and Seattle.

Of course, our long winter night pales in comparison to Alaska, where the sun barely climbs above the horizon for 3-4 hours in much the Last Frontier. North of the Arctic Circle – at 66.5ºN latitude – the sun never rises, and darkness prevails as the Earth rotates on its axis.

Along the midsection of the Lower 48 – along a line stretching roughly from San Francisco to D.C. – there are about five and a half fewer daylight hours on the winter solstice than on the summer solstice in June.

Cities at lower latitudes, such as Atlanta and Dallas, see less variation in daylight between the longest and shortest day of the year (only about four and a half hours). Further north, in Chicago and Boston, the difference amounts to over six hours. Then there’s extreme Alaska, where nighttime is anywhere from 12 to 24 hours longer now than it was six months ago.

The shortest day of the year means dusk arrives early. While our earliest sunsets are already behind us (more on that in map no. 5), this map presents a time-zone adjusted view of sunset times in the U.S. and Canada just before the solstice.

In gray regions – which includes D.C. and the Mid-Atlantic – the earliest sunsets are between 4:30 and 5 p.m. Light yellow regions encompass the cities of Chicago, Boston and Seattle, where the sun sets between 4 and 4:30 p.m. In purple areas, the earliest sunsets are not until after 5 o’clock.

One thing we can glean from this map is the combined effect of time zones and latitude on sunset times. The color contours are slanted from northwest to southeast because the Earth’s northern hemisphere is tilted away from the sun during winter.

Pick any two points on the map within the same time zone and draw a line between them. Anytime you move north and east, sunset arrives earlier, while moving south and or west means sunset will be later.

Although the winter solstice marks the shortest daylight period in the Northern Hemisphere, it’s not the day of the latest sunrise or earliest sunset, as this final map shows. In fact, most places in the mid-latitudes see their earliest sunset two weeks before the solstice, while the latest sunrise isn’t until early January.

The succinct explanation is that solar noon – the time the sun reaches its highest point in the sky each day – moves several minutes later in December, a phenomenon caused by Earth’s planetary tilt and elliptical orbit around the sun. When the sun takes more than 24 hours to reach the same point in the sky from one day to the next, we start to see a lag between our 24-hour clocks and the sun’s apparent daily motion in the sky. This discrepancy pushes sunrise and sunset times later even as the days continue to shorten right up until the solstice.

Note from this map how the dates of the earliest sunset and latest sunrise change with distance from Earth’s equator. At higher latitudes, the earliest sunset and latest sunrise happen closer to the solstice, while in lower latitudes they can be well over a month apart.

In Washington, D.C., sunset on the solstice is already three to four minutes later compared to the earliest sunset of the year, which came around Dec. 7. Meanwhile, sunrise will continue to advance for the next two weeks, with the latest sunrise not until Jan. 5.

But even though our latest sunrise and earliest sunset don’t neatly coincide with the shortest day of the year, remember that the days will slowly start to get longer now. So if the dark days of winter are getting to you, rest assured, it only goes up from here – and that’s something to celebrate.

Source: washingtonpost.com

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