December Solstice Opens Astronomical Winter With 2016’s Longest Night

Summary: In the Northern Hemisphere, the 2016 December solstice opens astronomical winter with 2016’s longest night.

December solstice: Alexander Novati, Public Domain, via NASA Jet Propulsion Laboratory SciJinks

In the Northern Hemisphere, the 2016 December solstice, which occurs Wednesday, Dec. 21, at 10:44 Coordinated Universal Time (5:44 a.m. Eastern Standard Time), opens astronomical winter with 2016’s longest night.

Solstices represent opposite seasons in the Northern and Southern Hemispheres because of the tilt of the Earth’s axis. The December solstice opens astronomical winter in the Northern Hemisphere and astronomical summer in the Southern Hemisphere.

The Earth’s north-to-south axis, which currently tilts by 23.43711 degrees from an upright position, leans away from the sun for the December solstice. The season of winter occurs in the Northern Hemisphere, which is tilted away from the sun. During the December solstice, the North Pole is tilted farthest from the sun.

The season of summer occurs in the Southern Hemisphere, which tilts closer to the sun and receives the greatest amount of incoming solar energy. During the December solstice, the South Pole tilts closest to the sun.

The December solstice is also known as the southern solstice. The Southern Hemisphere is closer to the sun than the Northern Hemisphere during this astronomical event.

The December solstice marks the sun’s southernmost annual reach. During the December solstice, the subsolar point, where the sun’s rays angle perpendicularly to the Earth’s surface, is directly over the Southern Hemisphere’s Tropic of Capricorn. Also known as the Southern Tropic, the Tropic of Capricorn is currently placed at 23 degrees 26 minutes 13.6 seconds south latitude.

The Southern Hemisphere knows the December solstice as the summer solstice. The Southern Hemisphere receives its greatest number of hours of sunlight and experiences its longest days for the year. South of the equator, day lengths exceed 12 hours.

In the Northern Hemisphere, the December solstice is called the winter solstice. The Northern Hemisphere receives its greatest number of sunless hours and experiences its longest nights for the year. North of the equator, day lengths tally less than 12 hours.

Polar days south of the Antarctic Circle, currently placed at 55 degrees 33 minutes 46.4 seconds south latitude, last more than 24 hours. Beginning with the Southern Hemisphere’s spring equinox in September, Antarctica experiences 24 hours of midnight sun continuously until the hemisphere’s autumnal equinox in March.

Astronomical polar nights are varying periods of continuous night that stretch from the North Pole southward to the Arctic Circle, currently placed at 66 degrees 33 minutes 46.4 seconds north latitude. The Northern Hemisphere’s period of astronomical polar nights begin with the setting of the midnight sun at the North Pole around the autumnal equinox in September, intensifies by the December solstice and recedes by the spring equinox in March.

The nearly end-of-year solstice can take place on any of four dates around two-thirds of the way through the Gregorian calendar year’s twelfth and last month. The solstice may fall on December 20, 21, 22 or 23. December 20 and 23 solstices occur less frequently than December 21 and December 22 solstice. A December 20 solstice last occurred in 1796 and will not recur until 2080. A December 23 solstice last happened in 1903 and will not recur until 2303.

In the 21st century, which spans Jan. 1, 2001 to December 31, 2100, Dec. 21 is the typical southern solstice date, with 82 of the century’s 100 occurrences. Dec. 22 claims 13 southern solstices, and Dec. 20 has five southern solstices.

The December solstice occurs at the same instant everywhere on Earth. The solstice’s occurrence, however, sends opposite signals to Earth’s Northern and Southern Hemispheres. In the Northern Hemisphere, the December solstice opens astronomical winter with 2016’s longest northern night. In the Southern Hemisphere, the December solstice opens astronomical summer with 2016’s longest southern day.

The takeaway for the 2016 December solstice is that the astronomical event, occurring globally at the same time, opens opposite astronomical seasons in the Northern and Southern Hemispheres.

Blue Marble Monthlies map of day and night sides of Earth at instant of December solstice, 10:44 UTC Dec. 21, 2016; Earth imagery from NASA Blue Marble Terra/MODIS cloudless Earth and Black Marble night lights images: John Walker/Earth and Moon Viewer, Public Domain, via Fourmilab Switzerland

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

December solstice: Alexander Novati, Public Domain, via NASA Jet Propulsion Laboratory SciJinks @ http://scijinks.jpl.nasa.gov/solstice/

Blue Marble Monthlies map of day and night sides of Earth at instant of December solstice, 10:44 UTC Dec. 21, 2016; Earth imagery from NASA Blue Marble Terra/MODIS cloudless Earth and Black Marble night lights images: John Walker/Earth and Moon Viewer, Public Domain, via Fourmilab Switzerland @ https://www.fourmilab.ch/cgi-bin/Earth

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